% TODO:
%
-% o I think it would be worth making the connection with CPS explicit.
+% o I (ADR) think it would be worth making the connection with CPS explicit.
% Now that we have explicit activation records (on the stack), we can
% explain the whole system in terms of CPS and tail calls --- with the
% one requirement that we carefuly distinguish stack-allocated objects
% from heap-allocated objects.
-
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\begin{document}
-\newcommand{\ToDo}[1]{{{\bf ToDo:}\sl #1}}
-\newcommand{\Arg}[1]{\mbox{${\tt arg}_{#1}$}}
-\newcommand{\bottom}{bottom} % foo, can't remember the symbol name
-
\title{The STG runtime system (revised)}
\author{Simon Peyton Jones \\ Glasgow University and Oregon Graduate Institute \and
+Simon Marlow \\ Glasgow University \and
Alastair Reid \\ Yale University}
\maketitle
\tableofcontents
\newpage
-\section{Introduction}
+\part{Introduction}
+\Section{Overview}{overview}
This document describes the GHC/Hugs run-time system. It serves as
a Glasgow/Yale/Nottingham ``contract'' about what the RTS does.
-\subsection{New features compared to GHC 2.04}
+\Subsection{New features compared to GHC 2.04}{new-features}
\begin{itemize}
\item The RTS supports mixed compiled/interpreted execution, so
that a program can consist of a mixture of GHC-compiled and Hugs-interpreted
code.
+\item The RTS supports concurrency by default.
+This has some costs (eg we can't do hardware stack checks) but
+reduces the number of different configurations we need to support.
+
\item CAFs are only retained if they are
reachable. Since they are referred to by implicit references buried
in code, this means that the garbage collector must traverse the whole
accessible code tree. This feature eliminates a whole class of painful
space leaks.
-\item A running thread has only one stack, which contains a mixture
-of pointers and non-pointers. Section~\ref{sect:stacks} describes how
-we find out which is which. (GHC has used two stacks for some while.
-Using one stack instead of two reduces register pressure, reduces the
-size of update frames, and eliminates
-``stack-stubbing'' instructions.)
+\item A running thread has only one stack, which contains a mixture of
+pointers and non-pointers. \secref{stacks} describes how we find out
+which is which. (GHC has used two stacks for some while. Using one
+stack instead of two reduces register pressure, reduces the size of
+update frames, and eliminates ``stack-stubbing'' instructions.)
+
+\item The ``return in registers'' return convention has been dropped
+because it was complicated and doesn't work well on register-poor
+architectures. It has been partly replaced by unboxed tuples
+(\secref{unboxed-tuples}) which allow the programmer to
+explicitly state where results should be returned in registers (or on
+the stack) instead of on the heap.
+
+\item
+
+Lazy black-holing has been replaced by eager black-holing. The
+problem with lazy black-holing is that it leaves slop in the heap
+which conflicts with the use of a mostly-copying collector.
+\ToDo{Why? I think we can still do lazy black holing without leaving
+slop (SDM)}
-\end{itemize}
+\end{itemize}
-\subsection{Wish list}
+\Subsection{Wish list}{wish-list}
Here's a list of things we'd like to support in the future.
\begin{itemize}
\end{itemize}
-\subsection{Configuration}
+\Subsection{Configuration}{configuration}
Some of the above features are expensive or less portable, so we
envision building a number of different configurations supporting
You can make the following choices:
\begin{itemize}
\item
-Support for concurrency or parallelism. There are four
-mutually-exclusive choices.
+Support for parallelism. There are three mutually-exclusive choices.
\begin{description}
-\item[@SEQUENTIAL@] No concurrency or parallelism support.
- This configuration might not support interrupt recovery.
-\item[@CONCURRENT@] Support for concurrency but not for parallelism.
-\item[@CONCURRENT@+@GRANSIM@] Concurrency support and simulated parallelism.
-\item[@CONCURRENT@+@PARALLEL@] Concurrency support and real parallelism.
+\item[@SEQUENTIAL@] Support for concurrency but not for parallelism.
+\item[@GRANSIM@] Concurrency support and simulated parallelism.
+\item[@PARALLEL@] Concurrency support and real parallelism.
\end{description}
\item @PROFILING@ adds cost-centre profiling.
only anticipate one, however.
\end{itemize}
-\subsection{Terminology}
+\Subsection{Glossary}{glossary}
-\begin{itemize}
+\ToDo{This terminology is not used consistently within the document.
+If you find something which disagrees with this terminology, fix the
+usage.}
-\item A {\em word} is (at least) 32 bits and can hold either a signed
-or an unsigned int.
+In the type system, we have boxed and unboxed types.
-\item A {\em pointer} is (at least) 32 bits and big enough to hold a
-function pointer or a data pointer.
+\begin{itemize}
-\item A {\em closure} is a (representation of) a value of a {\em pointed}
- type. It may be in HNF or it may be unevaluated --- in either case, you can
- try to evaluate it again.
+\item A \emph{pointed} type is one that contains $\bot$. Variables with
+pointed types are the only things which can be lazily evaluated. In
+the STG machine, this means that they are the only things that can be
+\emph{entered} or \emph{updated} and it requires that they be boxed.
-\item A {\em thunk} is a (representation of) a value of a {\em pointed}
- type which is {\em not} in HNF.
+\item An \emph{unpointed} type is one that does not contain $\bot$.
+Variables with unpointed types are never delayed --- they are always
+evaluated when they are constructed. In the STG machine, this means
+that they cannot be \emph{entered} or \emph{updated}. Unpointed objects
+may be boxed (like @Array#@) or unboxed (like @Int#@).
\end{itemize}
-Occasionally, a field of a data structure must hold either a word or a
-pointer. In such circumstances, it is {\em not safe} to assume that
-words and pointers are the same size.
-
-% Todo:
-% More terminology to mention.
-% unboxed, unpointed
-
-There are a few other system invariants which need to be mentioned ---
-though not necessarily here:
+In the implementation, we have different kinds of objects:
\begin{itemize}
-\item The garbage collector never expands an object when it promotes
-it to the old generation. This is important because the GC avoids
-performing heap overflow checks by assuming that the amount added to
-the old generation is no bigger than the current new generation.
+\item \emph{boxed} objects are heap objects used by the evaluators
-\end{itemize}
+\item \emph{unboxed} objects are not heap allocated
+\item \emph{stack} objects are allocated on the stack
-\section{The Scheduler}
+\item \emph{closures} are objects which can be \emph{entered}.
+They are always boxed and always have boxed types.
+They may be in WHNF or they may be unevaluated.
-The Scheduler is the heart of the run-time system. A running program
-consists of a single running thread, and a list of runnable and
-blocked threads. The running thread returns to the scheduler when any
-of the following conditions arises:
+\item A \emph{thunk} is a (representation of) a value of a \emph{pointed}
+type which is \emph{not} in WHNF.
+
+\item A \emph{value} is an object in WHNF. It can be pointed or unpointed.
-\begin{itemize}
-\item A heap check fails, and a garbage collection is required
-\item Compiled code needs to switch to interpreted code, and vice
-versa.
-\item The thread becomes blocked.
-\item The thread is preempted.
\end{itemize}
-A running system has a global state, consisting of
-\begin{itemize}
-\item @Hp@, the current heap pointer, which points to the next
-available address in the Heap.
-\item @HpLim@, the heap limit pointer, which points to the end of the
-heap.
-\item The Thread Preemption Flag, which is set whenever the currently
-running thread should be preempted at the next opportunity.
-\end{itemize}
-Each thread has a thread-local state, which consists of
+At the hardware level, we have \emph{word}s and \emph{pointer}s.
\begin{itemize}
-\item @TSO@, the Thread State Object for this thread. This is a heap
-object that is used to store the current thread state when the thread
-is blocked or sleeping.
-\item @Sp@, the current stack pointer.
-\item @Su@, the current stack update frame pointer. This register
-points to the most recent update frame on the stack, and is used to
-calculate the number of arguments available when entering a function.
-\item @SpLim@, the stack limit pointer. This points to the end of the
-current stack chunk.
-\item Several general purpose registers, used for passing arguments to
-functions.
-\end{itemize}
-\noindent and various other bits of information used in specialised
-circumstances, such as profiling and parallel execution. These are
-described in the appropriate sections.
+\item A \emph{word} is (at least) 32 bits and can hold either a signed
+or an unsigned int.
-The following is pseudo-code for the inner loop of the scheduler
-itself.
+\item A \emph{pointer} is (at least) 32 bits and big enough to hold a
+function pointer or a data pointer.
-@
-while (threads_exist) {
- // handle global problems: GC, parallelism, etc
- if (need_gc) gc();
- if (external_message) service_message();
- // deal with other urgent stuff
+\end{itemize}
- pick a runnable thread;
- do {
- switch (thread->whatNext) {
- case (RunGHC pc): status=runGHC(pc); break;
- case (RunHugs bc): status=runHugs(bc); break;
- }
- switch (status) { // handle local problems
- case (StackOverflow): enlargeStack; break;
- case (Error e) : error(thread,e); break;
- case (ExitWith e) : exit(e); break;
- case (Yield) : break;
- }
- } while (thread_runnable);
-}
-@
+Occasionally, a field of a data structure must hold either a word or a
+pointer. In such circumstances, it is \emph{not safe} to assume that
+words and pointers are the same size. \ToDo{GHC currently makes words
+the same size as pointers to reduce complexity in the code
+generator/RTS. It would be useful to relax this restriction, and have
+eg. 32-bit Ints on a 64-bit machine.}
-Optimisations to avoid excess trampolining from Hugs into itself.
-How do we invoke GC, ccalls, etc.
-General ccall (@ccall-GC@) and optimised ccall.
+\subsection{Subtle Dependencies}
-\section{Evaluation}
+Some decisions have very subtle consequences which should be written
+down in case we want to change our minds.
-This section describes the framework in which compiled code evaluates
-expressions. Only at certain points will compiled code need to be
-able to talk to the interpreted world; these are discussed in Section
-\ref{sec:hugs-ghc-interaction}.
+\begin{itemize}
-\subsection{Calling conventions}
+\item
-\subsubsection{The call/return registers}
+If the garbage collector is allowed to shrink the stack of a thread,
+we cannot omit the stack check in return continuations
+(\secref{heap-and-stack-checks}).
-One of the problems in designing a virtual machine is that we want it
-abstract away from tedious machine details but still reveal enough of
-the underlying hardware that we can make sensible decisions about code
-generation. A major problem area is the use of registers in
-call/return conventions. On a machine with lots of registers, it's
-cheaper to pass arguments and results in registers than to pass them
-on the stack. On a machine with very few registers, it's cheaper to
-pass arguments and results on the stack than to use ``virtual
-registers'' in memory. We therefore use a hybrid system: the first
-$n$ arguments or results are passed in registers; and the remaining
-arguments or results are passed on the stack. For register-poor
-architectures, it is important that we allow $n=0$.
+\item
-We'll label the arguments and results \Arg{1} \ldots \Arg{m} --- with
-the understanding that \Arg{1} \ldots \Arg{n} are in registers and
-\Arg{n+1} \ldots \Arg{m} are on top of the stack.
+When we return to the scheduler, the top object on the stack is a closure.
+The scheduler restarts the thread by entering the closure.
-Note that the mapping of arguments \Arg{1} \ldots \Arg{n} to machine
-registers depends on the {\em kinds} of the arguments. For example,
-if the first argument is a Float, we might pass it in a different
-register from if it is an Int. In fact, we might find that a given
-architecture lets us pass varying numbers of arguments according to
-their types. For example, if a CPU has 2 Int registers and 2 Float
-registers then we could pass between 2 and 4 arguments in machine
-registers --- depending on whether they all have the same kind or they
-have different kinds.
+\secref{hugs-return-convention} discusses how Hugs returns an
+unboxed value to GHC and how GHC returns an unboxed value to Hugs.
-\subsubsection{Entering closures}
+\item
-To evaluate a closure we jump to the entry code for the closure
-passing a pointer to the closure in \Arg{1} so that the entry code can
-access its environment.
+When we return to the scheduler, we need a few empty words on the stack
+to store a closure to reenter. \secref{heap-and-stack-checks}
+discusses who does the stack check and how much space they need.
-\subsubsection{Function call}
+\item
-The function-call mechanism is obviously crucial. There are five different
-cases to consider:
-\begin{enumerate}
+Heap objects never contain slop --- this is required if we want to
+support mostly-copying garbage collection.
+
+This is a big problem when updating since the updatee is usually
+bigger than an indirection object. The fix is to overwrite the end of
+the updatee with ``slop objects'' (described in
+\secref{slop-objects}). This is hard to arrange if we do
+\emph{lazy} blackholing (\secref{lazy-black-holing}) so we
+currently plan to blackhole an object when we push the update frame.
+
+% Idea: have specialised update code for various common sizes of
+% updatee, the update frame hence encodes the length of the object.
+% Specialised indirections will also encode the length of the object. A
+% generic version of the update code will overwrite the slop with a slop
+% object. We can do the same thing for blackhole objects, or just have
+% a generic version that is the same size as an indirection and
+% overwrite the slop with a slop object when blackholing. So: does this
+% avoid the need to do eager black holing?
-\item {\em Known combinator (function with no free variables) and enough arguments.}
+\item
-A fast call can be made: push excess arguments onto stack and jump to
-function's {\em fast entry point} passing arguments in \Arg{1} \ldots
-\Arg{m}.
+Info tables for constructors contain enough information to decide which
+return convention they use. This allows Hugs to use a single piece of
+entry code for all constructors and insulates Hugs from changes in the
+choice of return convention.
-The {\em fast entry point} is only called with exactly the right
-number of arguments (in \Arg{1} \ldots \Arg{m}) so it can instantly
-start doing useful work without first testing whether it has enough
-registers or having to pop them off the stack first.
+\end{itemize}
-\item {\em Known combinator and insufficient arguments.}
+\Section{Source Language}{source-language}
-A slow call can be made: push all arguments onto stack and jump to
-function's {\em slow entry point}.
+\Subsection{Explicit Allocation}{explicit-allocation}
-Any unpointed arguments which are pushed on the stack must be tagged.
-This means pushing an extra word on the stack below the unpointed
-words, containing the number of unpointed words above it.
+As in the original STG machine, (almost) all heap allocation is caused
+by executing a let(rec). Since we no longer support the return in
+registers convention for data constructors, constructors now cause heap
+allocation and so they should be let-bound.
-%Todo: forward ref about tagging?
-%Todo: picture?
+For example, we now write
+@
+> cons = \ x xs -> let r = (:) x xs in r
+@
+instead of
+@
+> cons = \ x xs -> (:) x xs
+@
-The {\em slow entry point} might be called with insufficient arguments
-and so it must test whether there are enough arguments on the stack.
-This {\em argument satisfaction check} consists of checking that
-@Su-Sp@ is big enough to hold all the arguments (including any tags).
+\note{For historical reasons, GHC doesn't use this syntax --- but it should.}
-\begin{itemize}
+\Subsection{Unboxed tuples}{unboxed-tuples}
-\item If the argument satisfaction check fails, it is because there is
-one or more update frames on the stack before the rest of the
-arguments that the function needs. In this case, we construct a PAP
-(partial application, section~\ref{sect:PAP}) containing the arguments
-which are on the stack. The PAP construction code will return to the
-update frame with the address of the PAP in \Arg{1}.
+Functions can take multiple arguments as easily as they can take one
+argument: there's no cost for adding another argument. But functions
+can only return one result: the cost of adding a second ``result'' is
+that the function must construct a tuple of ``results'' on the heap.
+The assymetry is rather galling and can make certain programming
+styles quite expensive. For example, consider a simple state transformer
+monad:
+@
+> type S a = State -> (a,State)
+> bindS m k s0 = case m s0 of { (a,s1) -> k a s1 }
+> returnS a s = (a,s)
+> getS s = (s,s)
+> setS s _ = ((),s)
+@
+Here, every use of @returnS@, @getS@ or @setS@ constructs a new tuple
+in the heap which is instantly taken apart (and becomes garbage) by
+the case analysis in @bind@. Even a short state-transformer program
+will construct a lot of these temporary tuples.
+
+Unboxed tuples provide a way for the programmer to indicate that they
+do not expect a tuple to be shared and that they do not expect it to
+be allocated in the heap. Syntactically, unboxed tuples are just like
+single constructor datatypes except for the annotation @unboxed@.
+@
+> data unboxed AAndState# a = AnS a State
+> type S a = State -> AAndState# a
+> bindS m k s0 = case m s0 of { AnS a s1 -> k a s1 }
+> returnS a s = AnS a s
+> getS s = AnS s s
+> setS s _ = AnS () s
+@
+Semantically, unboxed tuples are just unlifted tuples and are subject
+to the same restrictions as other unpointed types.
-\item If the argument satisfaction check succeeds, we jump to the fast
-entry point with the arguments in \Arg{1} \ldots \Arg{arity}.
+Operationally, unboxed tuples are never built on the heap. When
+an unboxed tuple is returned, it is returned in multiple registers
+or multiple stack slots. At first sight, this seems a little strange
+but it's no different from passing double precision floats in two
+registers.
-If the fast entry point expects to receive some of \Arg{i} on the
-stack, we can reduce the amount of movement required by making the
-stack layout for the fast entry point look like the stack layout for
-the slow entry point. Since the slow entry point is entered with the
-first argument on the top of the stack and with tags in front of any
-unpointed arguments, this means that if \Arg{i} is unpointed, there
-should be space below it for a tag and that the highest numbered
-argument should be passed on the top of the stack.
+Notes:
+\begin{itemize}
+\item
+Unboxed tuples can only have one constructor and that
+thunks never have unboxed types --- so we'll never try to update an
+unboxed constructor. The restriction to a single constructor is
+largely to avoid garbage collection complications.
-We usually arrange that the fast entry point is placed immediately
-after the slow entry point --- so we can just ``fall through'' to the
-fast entry point without performing a jump.
+\item
+The core syntax does not allow variables to be bound to
+unboxed tuples (ie in default case alternatives or as function arguments)
+and does not allow unboxed tuples to be fields of other constructors.
+However, there's no harm in allowing it in the source syntax as a
+convenient, but easily removed, syntactic sugar.
+
+\item
+The compiler generates a closure of the form
+@
+> c = \ x y z -> C x y z
+@
+for every constructor (whether boxed or unboxed).
+
+This closure is normally used during desugaring to ensure that
+constructors are saturated and to apply any strictness annotations.
+They are also used when returning unboxed constructors to the machine
+code evaluator from the bytecode evaluator and when a heap check fails
+in a return continuation for an unboxed-tuple scrutinee.
\end{itemize}
+\Subsection{STG Syntax}{stg-syntax}
-\item {\em Known function closure (function with free variables) and enough arguments.}
-A fast call can be made: push excess arguments onto stack and jump to
-function's {\em fast entry point} passing a pointer to closure in
-\Arg{1} and arguments in \Arg{2} \ldots \Arg{m+1}.
+\ToDo{Insert STG syntax with appropriate changes.}
-Like the fast entry point for a combinator, the fast entry point for a
-closure is only called with appropriate values in \Arg{1} \ldots
-\Arg{m+1} so we can start work straight away. The pointer to the
-closure is used to access the free variables of the closure.
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\part{System Overview}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-\item {\em Known function closure and insufficient arguments.}
+This part is concerned with defining the external interfaces of the
+major components of the system; the next part is concerned with their
+inner workings.
-A slow call can be made: push all arguments onto stack and jump to the
-closure's slow entry point passing a pointer to the closure in \Arg{1}.
+The major components of the system are:
+\begin{itemize}
-Again, the slow entry point performs an argument satisfaction check
-and either builds a PAP or pops the arguments off the stack into
-\Arg{2} \ldots \Arg{m+1} and jumps to the fast entry point.
+\item
+The evaluators (\secref{sm-overview}) are responsible for
+evaluating heap objects. The system supports two evaluators: the
+machine code evaluator; and the bytecode evaluator.
-\item {\em Unknown function closure or thunk.}
+\item
-Sometimes, the function being called is not statically identifiable.
-Consider, for example, the @compose@ function:
-@
- compose f g x = f (g x)
-@
-Since @f@ and @g@ are passed as arguments to @compose@, the latter has
-to make a heap call. In a heap call the arguments are pushed onto the
-stack, and the closure bound to the function is entered. In the
-example, a thunk for @(g x)@ will be allocated, (a pointer to it)
-pushed on the stack, and the closure bound to @f@ will be
-entered. That is, we will jump to @f@s entry point passing @f@ in
-\Arg{1}. If \Arg{1} is passed on the stack, it is pushed on top of
-the thunk for @(g x)@.
+The scheduler (\secref{scheduler-overview}) acts as the
+coordinator for the whole system. It is responsible for switching
+between evaluators, switching between threads, garbage collection,
+communication between multiple processors, etc.
-The {\em entry code} for an updateable thunk (which must have arity 0)
-pushes an update frame on the stack and starts executing the body of
-the closure --- using \Arg{1} to access any free variables. This is
-described in more detail in section~\ref{sect:data-updates}.
+\item
-The {\em entry code} for a non-updateable closure is just the
-closure's slow entry point.
+The storage manager (\secref{evaluators-overview}) is
+responsible for allocating blocks of contiguous memory and for garbage
+collection.
-\end{enumerate}
+\item
-In addition to the above considerations, if there are \emph{too many}
-arguments then the extra arguments are simply pushed on the stack with
-appropriate tags.
+The loader (\secref{loader-overview}) is responsible for
+loading machine code and bytecode files from the file system and for
+resolving references between separately compiled modules.
-To summarise, a closure's standard (slow) entry point performs the following:
-\begin{description}
-\item[Argument satisfaction check.] (function closure only)
-\item[Stack overflow check.]
-\item[Heap overflow check.]
-\item[Copy free variables out of closure.] %Todo: why?
-\item[Eager black holing.] (updateable thunk only) %Todo: forward ref.
-\item[Push update frame.]
-\item[Evaluate body of closure.]
-\end{description}
+\item
+The compilers (\secref{compilers-overview}) generate machine
+code and bytecode files which can be loaded by the loader.
-\subsection{Case expressions and return conventions}
-\label{sect:return-conventions}
+\end{itemize}
-The {\em evaluation} of a thunk is always initiated by
-a @case@ expression. For example:
-@
- case x of (a,b) -> E
-@
+\ToDo{Insert diagram showing all components underneath the scheduler
+and communicating only with the scheduler}
-The code for a @case@ expression looks like this:
+
+\Section{The Evaluators}{evaluators-overview}
+
+There are two evaluators: a machine code evaluator and a bytecode
+evaluator. The evaluators task is to evaluate code within a thread
+until one of the following happens:
\begin{itemize}
-\item Push the free variables of the branches on the stack (fv(@E@) in
-this case).
-\item Push a \emph{return address} on the stack.
-\item Evaluate the scrutinee (@x@ in this case).
+\item heap overflow
+\item stack overflow
+\item it is preempted
+\item it blocks in one of the concurrency primitives
+\item it performs a safe ccall
+\item it needs to switch to the other evaluator.
\end{itemize}
-Once evaluation of the scrutinee is complete, execution resumes at the
-return address, which points to the code for the expression @E@.
+The evaluators expect to find a closure on top of the thread's stack
+and terminate with a closure on top of the thread's stack.
-When execution resumes at the return point, there must be some {\em
-return convention} that defines where the components of the pair, @a@
-and @b@, can be found. The return convention varies according to the
-type of the scrutinee @x@:
+\Subsection{Evaluation Model}{evaluation-model}
-\begin{itemize}
+Whilst the evaluators differ internally, they share a common
+evaluation model and many object representations.
-\item
+\Subsubsection{Heap objects}{heap-objects-overview}
-(A space for) the return address is left on the top of the stack.
-Leaving the return address on the stack ensures that the top of the
-stack contains a valid activation record
-(section~\ref{sect:activation-records}) --- should a garbage collection
-be required.
+The choice of heap and stack objects used by the evaluators is tightly
+bound to the evaluation model. This section provides an overview of
+the most important heap and stack objects; further details are given
+later.
-\item If @x@ has a pointed type (e.g.~a data constructor or a function),
-a pointer to @x@ is returned in \Arg{1}.
+All heap objects look like this:
-\ToDo{Warn that components of E should be extracted as soon as
-possible to avoid a space leak.}
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+\emph{Header} & \emph{Payload} \\ \hline
+\end{tabular}
+\end{center}
-\item If @x@ is an unpointed type (e.g.~@Int#@ or @Float#@), @x@ is
-returned in \Arg{1}
+The headers vary between different kinds of object but they all start
+with a pointer to a pair consisting of an \emph{info table} and some
+\emph{entry code}. The info table is used both by the evaluators and
+by the storage manager and contains an @INFO_TYPE@ field which
+identifies which kind of heap object uses it and determines the
+interpretation of the payload and of the other fields of the info
+table. The entry code is some machine code used by the machine code
+evaluator to evaluate closures and raises an error for other kinds of
+objects.
-\item If @x@ is an unpointed tuple constructor, the components of @x@
-are returned in \Arg{1} \ldots \Arg{n} but no object is constructed in
-the heap. Unboxed tuple constructors are useful for functions which
-want to return multiple values such as those used in an (explicitly
-encoded) state monad:
+The major kinds of heap object used are as follows. (For simplicity,
+this description omits certain optimisations and extra fields required
+by the garbage collector.)
-\ToDo{Move stuff about unboxed tuples to a separate section}
+\begin{description}
-@
-data unpointed AAndState a = AnS a State
-type S a = State -> AAndState a
+\item[Constructors] are used to represent data constructors. Their
+payload consists of the fields of the constructor; the tag of the
+constructor is stored in the info table.
-bindS m k s0 = case m s0 of { AnS s1 a -> k s1 a }
-returnS a s = AnS a s
-@
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+@CONSTR@ & \emph{Fields} \\ \hline
+\end{tabular}
+\end{center}
-Note that unboxed datatypes can only have one constructor and that
-thunks never have unboxed types --- so we'll never try to update an
-unboxed constructor. Unboxed tuples are \emph{never} built on the
-heap.
+\item[Primitive objects] are used to represent objects with unpointed
+types which are too large to fit in a register (or stack slot) or for
+which sharing must be preserved. Primitive objects include large
+objects such as multiple precision integers and immutable arrays and
+mutable objects such as mutable arrays, mutable variables, MVar's,
+IVar's and foreign object pointers. Since unpointed objects are not
+pointed, they cannot be entered. Their payload varies according to
+the kind of object.
-When passing an unboxed tuple to a function, the components are
-flattened out and passed in \Arg{1} \ldots \Arg{n} as usual.
+\item[Function closures] are used to represent functions. Their
+payload (if any) consists of the free variables of the function.
-\end{itemize}
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+@FUN@ & \emph{Free Variables} \\ \hline
+\end{tabular}
+\end{center}
-\subsection{Vectored Returns}
+Function closures are only generated by the machine code compiler.
-Many algebraic data types have more than one constructor. For
-example, the @Maybe@ type is defined like this:
-@
- data Maybe a = Nothing | Just a
-@
-How does the return convention encode which of the two constructors is
-being returned? A @case@ expression scrutinising a value of @Maybe@
-type would look like this:
-@
- case E of
- Nothing -> ...
- Just a -> ...
-@
-Rather than pushing a return address before evaluating the scrutinee,
-@E@, the @case@ expression pushes (a pointer to) a {\em return
-vector}, a static table consisting of two code pointers: one for the
-@Just@ alternative, and one for the @Nothing@ alternative.
+\item[Thunks] are used to represent unevaluated expressions which will
+be updated with their result. Their payload (if any) consists of the
+free variables of the function. The entry code for a thunk starts by
+pushing an \emph{update frame} onto the stack and overwriting the
+thunk with a \emph{black hole} (see Black Holes, below). When
+evaluation of the thunk completes, the update frame will cause the
+thunk to be overwritten again with an \emph{indirection} to the result
+of the thunk, which is always a constructor or a partial application.
-\begin{itemize}
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+@THUNK@ & \emph{Free Variables} \\ \hline
+\end{tabular}
+\end{center}
-\item
+Thunks are only generated by the machine code evaluator.
-The constructor @Nothing@ returns by jumping to the first item in the
-return vector with a pointer to a (statically built) Nothing closure
-in \Arg{1}.
+\item[Byte-code Objects (@BCO@s)] are generated by the bytecode
+compiler. In conjunction with \emph{updatable applications} and
+\emph{non-updatable applications} they are used to represent
+functions, unevaluated expressions and return addresses.
-It might seem that we could avoid loading \Arg{1} in this case since the
-first item in the return vector will know that @Nothing@ was returned
-(and can easily access the Nothing closure in the (unlikely) event
-that it needs it. The only reason we load \Arg{1} is in case we have to
-perform an update (section~\ref{sect:data-updates}).
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+@BCO@ & \emph{Constant Pool} & \emph{Bytecodes} \\ \hline
+\end{tabular}
+\end{center}
-\item
+\item[Non-updatable (Partial) Applications] are used to represent the
+application of a function to an insufficient number of arguments.
+Their payload consists of the function and the arguments received so far.
-The constructor @Just@ returns by jumping to the second element of the
-return vector with a pointer to the closure in \Arg{1}.
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+@PAP@ & \emph{Function Closure} & \emph{Arguments} \\ \hline
+\end{tabular}
+\end{center}
-\end{itemize}
+@PAP@s are used when a function is applied to too few arguments and by
+code generated by the lambda-lifting phase of the bytecode compiler.
-In this way no test need be made to see which constructor returns;
-instead, execution resumes immediately in the appropriate branch of
-the @case@.
+\item[Updatable Applications] are used to represent the application of
+a function to a sufficient number of arguments. Their payload
+consists of the function and its arguments.
-\subsection{Direct Returns}
+Updateable applications are like thunks: on entering an updateable
+application, the evaluators push an \emph{update frame} onto the stack
+and overwrite the application with a \emph{black hole}; when
+evaluation completes, the evaluators overwrite the application with an
+\emph{indirection} to the result of the application.
-When a datatype has a large number of constructors, it may be
-inappropriate to use vectored returns. The vector tables may be
-large and sparse, and it may be better to identify the constructor
-using a test-and-branch sequence on the tag. For this reason, we
-provide an alternative return convention, called a \emph{direct
-return}.
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+@AP@ & \emph{Function Closure} & \emph{Arguments} \\ \hline
+\end{tabular}
+\end{center}
-In a direct return, the return address pushed on the stack really is a
-code pointer. The returning code loads a pointer to the closure being
-returned in \Arg{1} as usual, and also loads the tag into \Arg{2}.
-The code at the return address will test the tag and jump to the
-appropriate code for the case branch.
+@AP@s are only generated by the bytecode compiler.
-The choice of whether to use a vectored return or a direct return is
-made on a type-by-type basis --- up to a certain maximum number of
-constructors imposed by the update mechanism
-(section~\ref{sect:data-updates}).
+\item[Black holes] are used to mark updateable closures which are
+currently being evaluated. ``Black holing'' an object cures a
+potential space leak and detects certain classes of infinite loops.
+More imporantly, black holes act as synchronisation objects between
+separate threads: if a second thread tries to enter an updateable
+closure which is already being evaluated, the second thread is added
+to a list of blocked threads and the thread is suspended.
-Single-constructor data types also use direct returns, although in
-that case there is no need to return a tag in \Arg{2}.
+When evaluation of the black-holed closure completes, the black hole
+is overwritten with an indirection to the result of the closure and
+any blocked threads are restored to the runnable queue.
+
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+@BH@ & \emph{Blocked threads} \\ \hline
+\end{tabular}
+\end{center}
-\ToDo{Say whether we pop the return address before returning}
+\ToDo{In a single threaded system, it's trivial to detect infinite
+loops: reentering a BH is always an error. How easy is it in a
+multi-threaded system?}
-\ToDo{Stack stubbing?}
+\item[Indirections] are used to update an unevaluated closure with its
+(usually fully evaluated) result in situations where it isn't possible
+to perform an update in place. (In the current system, we always
+update with an indirection to avoid duplicating the result when doing
+an update in place.)
-\subsection{Updates}
-\label{sect:data-updates}
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+@IND@ & \emph{Closure} \\ \hline
+\end{tabular}
+\end{center}
-The entry code for an updatable thunk (which must also be of arity 0):
+Indirections needn't always point to a closure in WHNF. They can
+point to a chain of indirections which point to an evaluated closure.
+When revertible black holes are added, they may also point to reverted
+black holes.
-\begin{itemize}
-\item copies the free variables out of the thunk into registers or
- onto the stack.
-\item pushes an {\em update frame} onto the stack.
+\item[Thread State Objects (@TSO@s)] represent Haskell threads. Their
+payload consists of a unique thread id, the status of the thread
+(runnable, blocked, etc) and the stack. @TSO@s may be resized by the
+scheduler if its stack is too small or too large.
-An update frame is a small activation record consisting of
\begin{center}
-\begin{tabular}{|l|l|l|}
-\hline
-{\em Fixed header} & {\em Update Frame link} & {\em Updatee} \\
-\hline
+\begin{tabular}{|l|l|l|l|}\hline
+@TSO@ & \emph{Thread Id} & \emph{Status} & \emph{Stack} \\ \hline
\end{tabular}
\end{center}
-\note{In the semantics part of the STG paper (section 5.6), an update
-frame consists of everything down to the last update frame on the
-stack. This would make sense too --- and would fit in nicely with
-what we're going to do when we add support for speculative
-evaluation.}
-\ToDo{I think update frames contain cost centres sometimes}
+\end{description}
-\item
-If we are doing ``eager blackholing,'' we then overwrite the thunk
-with a black hole. Otherwise, we leave it to the garbage collector to
-black hole the thunk.
+\Subsubsection{Stack objects}{stack-objects-overview}
-\item
-Start evaluating the body of the expression.
+The stack contains a mixture of \emph{pending arguments} and
+\emph{stack objects}.
-\end{itemize}
+Pending arguments are arguments to curried functions which have not
+yet been incorporated into an activation frame. For example, when
+evaluating @let { g x y = x + y; f x = g{x} } in f{3,4}@, the
+evaluator pushes both arguments onto the stack and enters @f@. @f@
+only requires one argument so it leaves the second argument as a
+\emph{pending argument}. The pending argument remains on the stack
+until @f@ calls @g@ which requires two arguments: the argument passed
+to it by @f@ and the pending argument which was passed to @f@.
-When the expression finishes evaluation, it will enter the update
-frame on the top of the stack. Since the returner doesn't know
-whether it is entering a normal return address/vector or an update
-frame, we follow exactly the same conventions as return addresses and
-return vectors. That is, on entering the update frame:
+Unboxed pending arguments are always preceeded by a ``tag'' which says
+how large the argument is. This allows the garbage collector to
+locate pointers within the stack.
-\begin{itemize}
-\item The value of the thunk is in \Arg{1}. (Recall that only thunks
-are updateable and that thunks return just one value.)
+There are three kinds of stack object: return addresses, update frames
+and seq frames. All stack objects look like this
-\item If the data type is a direct-return type rather than a
-vectored-return type, then the tag is in \Arg{2}.
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+\emph{Header} & \emph{Payload} \\ \hline
+\end{tabular}
+\end{center}
+
+As with heap objects, the header starts with a pointer to a pair
+consisting of an \emph{info table} and some \emph{entry code}.
+
+\begin{description}
+
+\item[Return addresses] are used to cause selection and execution of
+case alternatives when a constructor is returned. Return addresses
+generated by the machine code compiler look like this:
+
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+\emph{@RET_ADDR@} & \emph{Free Variables of the case alternatives} \\ \hline
+\end{tabular}
+\end{center}
+
+The free variables are a mixture of pointers and non-pointers whose
+layout is described by the info table.
+
+Return addresses generated by the bytecode compiler look like this:
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+\emph{@BCO_RET@} & \emph{BCO} & \emph{Free Variables of the case alternatives} \\ \hline
+\end{tabular}
+\end{center}
+
+There is just one @BCO_RET@ info pointer. We avoid needing different
+@BCO_RET@s for each stack layout by tagging unboxed free variables as
+though they were pending arguments.
+
+\item[Update frames] are used to trigger updates. When an update
+frame is entered, it overwrites the updatee with an indirection to the
+result, restarts any threads blocked on the @BH@ and returns to the
+stack object underneath the update frame.
+
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+\emph{@UPDATE@} & \emph{Next Update Frame} & \emph{Updatee} \\ \hline
+\end{tabular}
+\end{center}
+
+\item[Seq frames] are used to implement the polymorphic @seq@ primitive.
+They are a special kind of update frame.
+
+\ToDo{Describe them properly}
+
+
+\end{description}
+
+\ToDo{We also need a stop frame which goes on the bottom of the stack
+when the thread terminates.}
+
+
+\Subsubsection{Case expressions}{case-expr-overview}
+
+In the STG language, all evaluation is triggered by evaluating a case
+expression. When evaluating a case expression @case e of alts@, the
+evaluators pushes a return address onto the stack and evaluate the
+expression @e@. When @e@ eventually reduces to a constructor, the
+return address on the stack is entered. The details of how the
+constructor is passed to the return address and how the appropriate
+case alternative is selected vary between evaluators.
+
+Case expressions for unboxed data types are essentially the same: the
+case expression pushes a return address onto the stack before
+evaluating the scrutinee; when a function returns an unboxed value, it
+enters the return address on top of the stack.
+
+
+\Subsubsection{Function applications}{fun-app-overview}
+
+In the STG language, all function calls are tail calls. The arguments
+are pushed onto the stack and the function closure is entered. If any
+arguments are unboxed, they must be tagged as unboxed pending
+arguments. Entering a closure is just a special case of calling a
+function with no arguments.
+
+
+\Subsubsection{Let expressions}{let-expr-overview}
+
+In the STG language, almost all heap allocation is caused by let
+expressions. Filling in the contents of a set of mutually recursive
+heap objects is simple enough; the only difficulty is that once the
+heap space has been allocated, the thread must not return to the
+scheduler until after the objects are filled in.
+
+
+\Subsubsection{Primitive operations}{primop-overview}
+
+\ToDo{}
+
+Most primops are simple, some aren't.
+
+
+
+
+
+
+\Section{Scheduler}{scheduler-overview}
+
+The Scheduler is the heart of the run-time system. A running program
+consists of a single running thread, and a list of runnable and
+blocked threads. A thread is represented by a \emph{Thread Status
+Object} (TSO), which contains a few words status information and a
+stack. Except for the running thread, all threads have a closure on
+top of their stack; the scheduler restarts a thread by entering an
+evaluator which performs some reduction and returns to the scheduler.
+
+\Subsection{The scheduler's main loop}{scheduler-main-loop}
+
+The scheduler consists of a loop which chooses a runnable thread and
+invokes one of the evaluators which performs some reduction and
+returns.
+
+The scheduler also takes care of system-wide issues such as heap
+overflow or communication with other processors (in the parallel
+system) and thread-specific problems such as stack overflow.
+
+\Subsection{Creating a thread}{create-thread}
+
+Threads are created:
+
+\begin{itemize}
+
+\item
+
+When the scheduler is first invoked.
+
+\item
+
+When a message is received from another processor (I think). (Parallel
+system only.)
+
+\item
+
+When a C program calls some Haskell code.
+
+\item
+
+By @forkIO@, @takeMVar@ and (maybe) other Concurrent Haskell primitives.
-\item The update frame is still on the stack.
\end{itemize}
-We can safely share a single statically-compiled update function
-between all types. However, the code must be able to handle both
-vectored and direct-return datatypes. This is done by arranging that
-the update code looks like this:
-@
- | ^ |
- | return vector |
- |---------------|
- | fixed-size |
- | info table |
- |---------------| <- update code pointer
- | update code |
- | v |
-@
+\Subsection{Restarting a thread}{thread-restart}
-Each entry in the return vector (which is large enough to cover the
-largest vectored-return type) points to the update code.
+When the scheduler decides to run a thread, it has to decide which
+evaluator to use. It does this by looking at the type of the closure
+on top of the stack.
+\begin{itemize}
+\item @BCO@ $\Rightarrow$ bytecode evaluator
+\item @FUN@ or @THUNK@ $\Rightarrow$ machine code evaluator
+\item @CONSTR@ $\Rightarrow$ machine code evaluator
+\item other $\Rightarrow$ either evaluator.
+\end{itemize}
+
+The only surprise in the above is that the scheduler must enter the
+machine code evaluator if there's a constructor on top of the stack.
+This allows the bytecode evaluator to return a constructor to a
+machine code return address by pushing the constructor on top of the
+stack and returning to the scheduler. If the return address under the
+constructor is @HUGS_RET@, the entry code for @HUGS_RET@ will
+rearrange the stack so that the return @BCO@ is on top of the stack
+and return to the scheduler which will then call the bytecode
+evaluator. There is little point in trying to shorten this slightly
+indirect route since it is will happen very rarely if at all.
+
+\note{As an optimisation, we could store the choice of evaluator in
+the TSO status whenever we leave the evaluator. This is required for
+any thread, no matter what state it is in (blocked, stack overflow,
+etc). It isn't clear whether this would accomplish anything.}
+
+\Subsection{Returning from a thread}{thread-return}
+
+The evaluators return to the scheduler when any of the following
+conditions arise:
-The update code:
\begin{itemize}
-\item overwrites the {\em updatee} with an indirection to \Arg{1};
-\item loads @Su@ from the Update Frame link;
-\item removes the update frame from the stack; and
-\item enters \Arg{1}.
+\item A heap check fails, and a garbage collection is required.
+
+\item A stack check fails, and the scheduler must either enlarge the
+current thread's stack, or flag an out of memory condition.
+
+\item A thread enters a closure built by the other evaluator. That
+is, when the bytecode interpreter enters a closure compiled by GHC or
+when the machine code evaluator enters a BCO.
+
+\item A thread returns to a return continuation built by the other
+evaluator. That is, when the machine code evaluator returns to a
+continuation built by Hugs or when the bytecode evaluator returns to a
+continuation built by GHC.
+
+\item The evaluator needs to perform a ``safe'' C call
+(\secref{c-calls}).
+
+\item The thread becomes blocked. This happens when a thread requires
+the result of a computation currently being performed by another
+thread, or it reads a synchronisation variable that is currently empty
+(\secref{MVAR}).
+
+\item The thread is preempted (the preemption mechanism is described
+in \secref{thread-preemption}).
+
+\item The thread terminates.
\end{itemize}
-We enter \Arg{1} again, having probably just come from there, because
-it knows whether to perform a direct or vectored return. This could
-be optimised by compiling special update code for each slot in the
-return vector, which performs the correct return.
+Except when the thread terminates, the thread always terminates with a
+closure on the top of the stack. The mechanism used to trigger the
+world switch and the choice of closure left on top of the stack varies
+according to which world is being left and what is being returned.
-\subsection{Semi-tagging}
-\label{sect:semi-tagging}
+\Subsubsection{Leaving the bytecode evaluator}{hugs-to-ghc-switch}
-When a @case@ expression evaluates a variable that might be bound
-to a thunk it is often the case that the scrutinee is already evaluated.
-In this case we have paid the penalty of (a) pushing the return address (or
-return vector address) on the stack, (b) jumping through the info pointer
-of the scrutinee, and (c) returning by an indirect jump through the
-return address on the stack.
+\paragraph{Entering a machine code closure}
-If we knew that the scrutinee was already evaluated we could generate
-(better) code which simply jumps to the appropriate branch of the @case@
-with a pointer to the scrutinee in \Arg{1}.
+When it enters a closure, the bytecode evaluator performs a switch
+based on the type of closure (@AP@, @PAP@, @Ind@, etc). On entering a
+machine code closure, it returns to the scheduler with the closure on
+top of the stack.
-An obvious idea, therefore, is to test dynamically whether the heap
-closure is a value (using the tag in the info table). If not, we
-enter the closure as usual; if so, we jump straight to the appropriate
-alternative. Here, for example, is pseudo-code for the expression
-@(case x of { (a,_,c) -> E }@:
+\paragraph{Returning a constructor}
+
+When it enters a constructor, the bytecode evaluator tests the return
+continuation on top of the stack. If it is a machine code
+continuation, it returns to the scheduler with the constructor on top
+of the stack.
+
+\note{This is why the scheduler must enter the machine code evaluator
+if it finds a constructor on top of the stack.}
+
+\paragraph{Returning an unboxed value}
+
+\note{Hugs doesn't support unboxed values in source programs but they
+are used for a few complex primops.}
+
+When it returns an unboxed value, the bytecode evaluator tests the
+return continuation on top of the stack. If it is a machine code
+continuation, it returns to the scheduler with the tagged unboxed
+value and a special closure on top of the stack. When the closure is
+entered (by the machine code evaluator), it returns the unboxed value
+on top of the stack to the return continuation under it.
+
+The runtime library for GHC provides one of these closures for each unboxed
+type. Hugs cannot generate them itself since the entry code is really
+very tricky.
+
+\paragraph{Heap/Stack overflow and preemption}
+
+The bytecode evaluator tests for heap/stack overflow and preemption
+when entering a BCO and simply returns with the BCO on top of the
+stack.
+
+\Subsubsection{Leaving the machine code evaluator}{ghc-to-hugs-switch}
+
+\paragraph{Entering a BCO}
+
+The entry code for a BCO pushes the BCO onto the stack and returns to
+the scheduler.
+
+\paragraph{Returning a constructor}
+
+We avoid the need to test return addresses in the machine code
+evaluator by pushing a special return address on top of a pointer to
+the bytecode return continuation. \figref{hugs-return-stack}
+shows the state of the stack just before evaluating the scrutinee.
+
+\begin{figure}[ht]
+\begin{center}
@
- \Arg{1} = <pointer to x>;
- tag = \Arg{1}->entry->tag;
- if (isWHNF(tag)) {
- Sp--; \\ insert space for return address
- goto ret;
- }
- push(ret);
- goto \Arg{1}->entry;
-
- <info table for return address goes here>
-ret: a = \Arg{1}->data1; \\ suck out a and c to avoid space leak
- c = \Arg{1}->data3;
- <code for E2>
+| stack |
++----------+
+| bco |--> BCO
++----------+
+| HUGS_RET |
++----------+
@
-and here is the code for the expression @(case x of { [] -> E1; x:xs -> E2 }@:
+%\input{hugs_return1.pstex_t}
+\end{center}
+\caption{Stack layout for evaluating a scrutinee}
+\label{fig:hugs-return-stack}
+\end{figure}
+
+This return address rearranges the stack so that the bco pointer is
+above the constructor on the stack (as shown in
+\figref{hugs-boxed-return}) and returns to the scheduler.
+
+\begin{figure}[ht]
+\begin{center}
@
- \Arg{1} = <pointer to x>;
- tag = \Arg{1}->entry->tag;
- if (isWHNF(tag)) {
- Sp--; \\ insert space for return address
- goto retvec[tag];
- }
- push(retinfo);
- goto \Arg{1}->entry;
-
- .addr ret2
- .addr ret1
-retvec: \\ reversed return vector
- <return info table for case goes here>
-retinfo:
- panic("Direct return into vectored case");
-
-ret1: <code for E1>
+| stack |
++----------+
+| con |--> Constructor
++----------+
+| bco |--> BCO
++----------+
+@
+%\input{hugs_return2.pstex_t}
+\end{center}
+\caption{Stack layout for entering a Hugs return address}
+\label{fig:hugs-boxed-return}
+\end{figure}
-ret2: x = \Arg{1}->head;
- xs = \Arg{1}->tail;
- <code for E2>
+\paragraph{Returning an unboxed value}
+
+We avoid the need to test return addresses in the machine code
+evaluator by pushing a special return address on top of a pointer to
+the bytecode return continuation. This return address rearranges the
+stack so that the bco pointer is above the tagged unboxed value (as
+shown in \figref{hugs-entering-unboxed-return}) and returns to the
+scheduler.
+
+\begin{figure}[ht]
+\begin{center}
@
-There is an obvious cost in compiled code size (but none in the size
-of the bytecodes). There is also a cost in execution time if we enter
-more thunks than data constructors.
+| stack |
++----------+
+| 1# |
++----------+
+| I# |
++----------+
+| bco |--> BCO
++----------+
+@
+%\input{hugs_return2.pstex_t}
+\end{center}
+\caption{Stack layout for returning an unboxed value}
+\label{fig:hugs-entering-unboxed-return}
+\end{figure}
-Both the direct and vectored returns are easily modified to chase chains
-of indirections too. In the vectored case, this is most easily done by
-making sure that @IND = TAG_1 - 1@, and adding an extra field to every
-return vector. In the above example, the indirection code would be
+\paragraph{Heap/Stack overflow and preemption}
+
+\ToDo{}
+
+
+\Subsection{Preempting a thread}{thread-preemption}
+
+Strictly speaking, threads cannot be preempted --- the scheduler
+merely sets a preemption request flag which the thread must arrange to
+test on a regular basis. When an evaluator finds that the preemption
+request flag is set, it pushes an appropriate closure onto the stack
+and returns to the scheduler.
+
+In the bytecode interpreter, the flag is tested whenever we enter a
+closure. If the preemption flag is set, it leaves the closure on top
+of the stack and returns to the scheduler.
+
+In the machine code evaluator, the flag is only tested when a heap or
+stack check fails. This is less expensive than testing the flag on
+entering every closure but runs the risk that a thread will enter an
+infinite loop which does not allocate any space. If the flag is set,
+the evaluator returns to the scheduler exactly as if a heap check had
+failed.
+
+\Subsection{``Safe'' and ``unsafe'' C calls}{c-calls}
+
+There are two ways of calling C:
+
+\begin{description}
+
+\item[``Unsafe'' C calls] are used if the programer is certain that
+the C function will not do anything dangerous. Unsafe C calls are
+faster but must be hand-checked by the programmer.
+
+Dangerous things include:
+
+\begin{itemize}
+
+\item
+
+Call a system function such as @getchar@ which might block
+indefinitely. This is dangerous because we don't want the entire
+runtime system to block just because one thread blocks.
+
+\item
+
+Call an RTS function which will block on the RTS access semaphore.
+This would lead to deadlock.
+
+\item
+
+Call a Haskell function. This is just a special case of calling an
+RTS function.
+
+\end{itemize}
+
+Unsafe C calls are performed by pushing the arguments onto the C stack
+and jumping to the C function's entry point. On exit, the result of
+the function is in a register which is returned to the Haskell code as
+an unboxed value.
+
+\item[``Safe'' C calls] are used if the programmer suspects that the
+thread may do something dangerous. Safe C calls are relatively slow
+but are less problematic.
+
+Safe C calls are performed by pushing the arguments onto the Haskell
+stack, pushing a return continuation and returning a \emph{C function
+descriptor} to the scheduler. The scheduler suspends the Haskell thread,
+spawns a new operating system thread which pops the arguments off the
+Haskell stack onto the C stack, calls the C function, pushes the
+function result onto the Haskell stack and informs the scheduler that
+the C function has completed and the Haskell thread is now runnable.
+
+\end{description}
+
+The bytecode evaluator will probably treat all C calls as being safe.
+
+\ToDo{It might be good for the programmer to indicate how the program
+is unsafe. For example, if we distinguish between C functions which
+might call Haskell functions and those which might block, we could
+perform an unsafe call for blocking functions in a single-threaded
+system or, perhaps, in a multi-threaded system which only happens to
+have a single thread at the moment.}
+
+
+
+\Section{The Storage Manager}{sm-overview}
+
+The storage manager is responsible for managing the heap and all
+objects stored in it. It provides special support for lazy evaluation
+and for foreign function calls.
+
+\Subsection{SM support for lazy evaluation}{sm-lazy-evaluation}
+
+\begin{itemize}
+\item
+
+Indirections are shorted out.
+
+\item
+
+Update frames pointing to unreachable objects are squeezed out.
+
+\item
+
+Adjacent update frames (for different closures) are compressed to a
+single update frame pointing to a single black hole.
+
+\end{itemize}
+
+
+\Subsection{SM support for foreign function calls}{sm-foreign-calls}
+
+\begin{itemize}
+
+\item
+
+Stable pointers allow other languages to access Haskell objects.
+
+\item
+
+Foreign Objects are a form of weak pointer which lets Haskell access
+foreign objects.
+
+\end{itemize}
+
+\Subsection{Misc}{sm-misc}
+
+\begin{itemize}
+
+\item
+
+If the stack contains a large amount of free space, the storage
+manager may shrink the stack. If it shrinks the stack, it guarantees
+never to leave less than @MIN_SIZE_SHRUNKEN_STACK@ empty words on the
+stack when it does so.
+
+\ToDo{Would it be useful for the storage manager to enlarge the stack?}
+
+\item
+
+For efficiency reasons, very large objects (eg large arrays and TSOs)
+are not moved if possible.
+
+\end{itemize}
+
+
+\Section{The Compilers}{compilers-overview}
+
+Need to describe interface files, format of bytecode files, symbols
+defined by machine code files.
+
+\Subsection{Interface Files}{interface-files}
+
+Here's an example - but I don't know the grammar - ADR.
@
-ind: \Arg{1} = \Arg{1}->next;
- goto ind_loop;
+_interface_ Main 1
+_exports_
+Main main ;
+_declarations_
+1 main _:_ IOBase.IO PrelBase.();;
@
-where @ind_loop@ is the second line of code.
-Note that we have to leave space for a return address since the return
-address expects to find one. If the body of the expression requires a
-heap check, we will actually have to write the return address before
-entering the garbage collector.
+\Subsection{Bytecode files}{bytecode-files}
+
+(All that matters here is what the loader sees.)
+
+\Subsection{Machine code files}{asm-files}
+
+(Again, all that matters is what the loader sees.)
+
+\Section{The Loader}{loader-overview}
+
+In a batch mode system, we can statically link all the modules
+together. In an interactive system we need a loader which will
+explicitly load and unload individual modules (or, perhaps, blocks of
+mutually dependent modules) and resolve references between modules.
+
+While many operating systems provide support for dynamic loading and
+will automatically resolve cross-module references for us, we generally
+cannot rely on being able to load mutually dependent modules.
+
+A portable solution is to perform some of the linking ourselves. Each module
+should provide three global symbols:
+\begin{itemize}
+\item
+An initialisation routine. (Might also be used for finalisation.)
+\item
+A table of symbols it exports.
+Entries in this table consist of the symbol name and the address of the
+names value.
+\item
+A table of symbols it imports.
+Entries in this table consist of the symbol name and a list of references
+to that symbol.
+\end{itemize}
+
+On loading a group of modules, the loader adds the contents of the
+export lists to a symbol table and then fills in all the references in the
+import lists.
+
+References in import lists are of two types:
+\begin{description}
+\item[ References in machine code ]
+
+The most efficient approach is to patch the machine code directly, but
+this will be a lot of work, very painful to port and rather fragile.
+
+Alternatively, the loader could store the value of each symbol in the
+import table for each module and the compiled code can access all
+external objects through the import table. This requires that the
+import table be writable but does not require that the machine code or
+info tables be writable.
+
+\item[ References in data structures (SRTs and static data constructors) ]
+
+Either we patch the SRTs and constructors directly or we somehow use
+indirections through the symbol table. Patching the SRTs requires
+that we make them writable and prevents us from making effective use
+of virtual memories that use copy-on-write policies (this only makes a
+difference if we want to run several copies of the same program
+simultaneously). Using an indirection is possible but tricky.
+
+Note: We could avoid patching machine code if all references to
+external references went through the SRT --- then we just have one
+thing to patch. But the SRT always contains a pointer to the closure
+rather than the fast entry point (say), so we'd take a big performance
+hit for doing this.
+
+\end{description}
+
+Using the above scheme, all accesses to ``external'' objects involve a
+layer of indirection. To avoid this overhead, the machine code
+compiler might provide a way for the programmer to specify which
+modules will be statically linked and which will be dynamically linked
+--- the idea being that statically linked code and data will be
+accessed directly.
+
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\part{Internal details}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+
+This part is concerned with the internal details of the components
+described in the previous part.
+
+The major components of the system are:
+\begin{itemize}
+\item The scheduler (\secref{storage-manager-internals})
+\item The storage manager (\secref{storage-manager-internals})
+\item The evaluators
+\item The loader
+\item The compilers
+\end{itemize}
+
+\Section{The Scheduler}{scheduler-internals}
+
+\ToDo{Detailed description of scheduler}
+
+Many heap objects contain fields allowing them to be inserted onto lists
+during evaluation or during garbage collection. The lists required by
+the evaluator and storage manager are as follows.
+
+\begin{itemize}
+
+\item 4 lists of threads: runnable threads, sleeping threads, threads
+waiting for timeout and threads waiting for I/O.
+
+\item The \emph{mutables list} is a list of all objects in the old
+generation which might contain pointers into the new generation. Most
+of the objects on this list are indirections (\secref{IND})
+or ``mutable.'' (\secref{mutables}.)
+
+\item The \emph{Foreign Object list} is a list of all foreign objects
+ which have not yet been deallocated. (\secref{FOREIGN}.)
+
+\item The \emph{Spark pool} is a doubly(?) linked list of Spark objects
+maintained by the parallel system. (\secref{SPARK}.)
+
+\item The \emph{Blocked Fetch list} (or
+lists?). (\secref{BLOCKED_FETCH}.)
+
+\item For each thread, there is a list of all update frames on the
+stack. (\secref{data-updates}.)
+
+\item The Stable Pointer Table is a table of pointers to objects which
+are known to the outside world and must be retained by the garbage
+collector even if they are not accessible from within the heap.
+
+\end{itemize}
+
+\ToDo{The links for these fields are usually inserted immediately
+after the fixed header except ...}
+
+
+
+\Section{The Storage Manager}{storage-manager-internals}
+
+\subsection{Misc Text looking for a home}
+
+A \emph{value} may be:
+\begin{itemize}
+\item \emph{Boxed}, i.e.~represented indirectly by a pointer to a heap object (e.g.~foreign objects, arrays); or
+\item \emph{Unboxed}, i.e.~represented directly by a bit-pattern in one or more registers (e.g.~@Int#@ and @Float#@).
+\end{itemize}
+All \emph{pointed} values are \emph{boxed}.
+
+
+\Subsection{Heap Objects}{heap-objects}
+
+\begin{figure}
+\begin{center}
+\input{closure}
+\end{center}
+\ToDo{Fix this picture}
+\caption{A closure}
+\label{fig:closure}
+\end{figure}
+
+Every \emph{heap object} is a contiguous block of memory, consisting
+of a fixed-format \emph{header} followed by zero or more \emph{data
+words}.
+
+The header consists of the following fields:
+\begin{itemize}
+\item A one-word \emph{info pointer}, which points to
+the object's static \emph{info table}.
+\item Zero or more \emph{admin words} that support
+\begin{itemize}
+\item Profiling (notably a \emph{cost centre} word).
+ \note{We could possibly omit the cost centre word from some
+ administrative objects.}
+\item Parallelism (e.g. GranSim keeps the object's global address here,
+though GUM keeps a separate hash table).
+\item Statistics (e.g. a word to track how many times a thunk is entered.).
+
+We add a Ticky word to the fixed-header part of closures. This is
+used to indicate if a closure has been updated but not yet entered. It
+is set when the closure is updated and cleared when subsequently
+entered. \footnote{% NB: It is \emph{not} an ``entry count'', it is
+an ``entries-after-update count.'' The commoning up of @CONST@,
+@CHARLIKE@ and @INTLIKE@ closures is turned off(?) if this is
+required. This has only been done for 2s collection. }
+
+\end{itemize}
+\end{itemize}
+
+Most of the RTS is completely insensitive to the number of admin
+words. The total size of the fixed header is @FIXED_HS@.
+
+\Subsection{Info Tables}{info-tables}
+
+An \emph{info table} is a contiguous block of memory, laid out as follows:
+
+\begin{center}
+\begin{tabular}{|r|l|}
+ \hline Parallelism Info & variable
+\\ \hline Profile Info & variable
+\\ \hline Debug Info & variable
+\\ \hline Static reference table & 32 bits (optional)
+\\ \hline Storage manager layout info & 32 bits
+\\ \hline Closure type & 16 bits
+\\ \hline Constructor Tag & 16 bits
+\\ \hline entry code
+\\ \vdots
+\end{tabular}
+\end{center}
+
+An info table has the following contents (working backwards in memory
+addresses):
+
+\begin{itemize}
+
+\item The \emph{entry code} for the closure. This code appears
+literally as the (large) last entry in the info table, immediately
+preceded by the rest of the info table. An \emph{info pointer} always
+points to the first byte of the entry code.
+
+\item A 16-bit constructor tag. This field is used for constructor
+info-tables only (\secref{CONSTR}), and contains an integer
+representing the tag value of the constructor, in the range $0..n-1$
+where $n$ is the number of constructors in the datatype.
+
+\item An 16-bit {\em closure type field}, which identifies what kind of
+closure the object is. The various types of closure are described in
+\secref{closures}.
+
+\item A single pointer or word --- the {\em storage manager info
+field}, contains auxiliary information describing the closure's
+precise layout, for the benefit of the garbage collector and the code
+that stuffs graph into packets for transmission over the network.
+There are three kinds of layout information:
+
+\begin{itemize}
+\item Standard layout information is for closures which place pointers
+before non-pointers in instances of the closure (this applies to most
+heap-based and static closures, but not activation records). The
+layout information for standard closures is
+
+ \begin{itemize}
+ \item Number of pointer fields (16 bits).
+ \item Number of non-pointer fields (16 bits).
+ \end{itemize}
+
+\item Activation records don't have pointers before non-pointers,
+since stack-stubbing requires that the record has holes in it. The
+layout is therefore represented by a bitmap in which each '1' bit
+represents a non-pointer word. This kind of layout info is used for
+@RET_SMALL@ and @RET_VEC_SMALL@ closures.
+
+\item If an activation record is longer than 32 words, then the layout
+field contains a pointer to a bitmap record, consisting of a length
+field followed by two or more bitmap words. This layout information
+is used for @RET_BIG@ and @RET_VEC_BIG@ closures.
+
+\item Selector Thunks (\secref{THUNK_SEL}) use the closure
+layout field to hold the selector index, since the layout is always
+known (the closure contains a single pointer field).
+\end{itemize}
+
+\item A one-word {\em Static Reference Table} field. This field
+points to the static reference table for the closure (\secref{srt}),
+and is only present for the following closure types:
+
+ \begin{itemize}
+ \item @FUN_*@
+ \item @THUNK_*@
+ \item @RET_*@
+ \end{itemize}
+
+\item \emph{Profiling info\/}
+
+\ToDo{The profiling info is completely bogus. I've not deleted it
+from the document but I've commented it all out.}
+
+% change to \iftrue to uncomment this section
+\iffalse
+
+Closure category records are attached to the info table of the
+closure. They are declared with the info table. We put pointers to
+these ClCat things in info tables. We need these ClCat things because
+they are mutable, whereas info tables are immutable. Hashing will map
+similar categories to the same hash value allowing statistics to be
+grouped by closure category.
+
+Cost Centres and Closure Categories are hashed to provide indexes
+against which arbitrary information can be stored. These indexes are
+memoised in the appropriate cost centre or category record and
+subsequent hashes avoided by the index routine (it simply returns the
+memoised index).
+
+There are different features which can be hashed allowing information
+to be stored for different groupings. Cost centres have the cost
+centre recorded (using the pointer), module and group. Closure
+categories have the closure description and the type
+description. Records with the same feature will be hashed to the same
+index value.
+
+The initialisation routines, @init_index_<feature>@, allocate a hash
+table in which the cost centre / category records are stored. The
+lower bound for the table size is taken from @max_<feature>_no@. They
+return the actual table size used (the next power of 2). Unused
+locations in the hash table are indicated by a 0 entry. Successive
+@init_index_<feature>@ calls just return the actual table size.
+
+Calls to @index_<feature>@ will insert the cost centre / category
+record in the @<feature>@ hash table, if not already inserted. The hash
+index is memoised in the record and returned.
+
+CURRENTLY ONLY ONE MEMOISATION SLOT IS AVILABLE IN EACH RECORD SO
+HASHING CAN ONLY BE DONE ON ONE FEATURE FOR EACH RECORD. This can be
+easily relaxed at the expense of extra memoisation space or continued
+rehashing.
+
+The initialisation routines must be called before initialisation of
+the stacks and heap as they require to allocate storage. It is also
+expected that the caller may want to allocate additional storage in
+which to store profiling information based on the return table size
+value(s).
+
+\begin{center}
+\begin{tabular}{|l|}
+ \hline Hash Index
+\\ \hline Selected
+\\ \hline Kind
+\\ \hline Description String
+\\ \hline Type String
+\\ \hline
+\end{tabular}
+\end{center}
+
+\begin{description}
+\item[Hash Index] Memoised copy
+\item[Selected]
+ Is this category selected (-1 == not memoised, selected? 0 or 1)
+\item[Kind]
+One of the following values (defined in CostCentre.lh):
+
+\begin{description}
+\item[@CON_K@]
+A constructor.
+\item[@FN_K@]
+A literal function.
+\item[@PAP_K@]
+A partial application.
+\item[@THK_K@]
+A thunk, or suspension.
+\item[@BH_K@]
+A black hole.
+\item[@ARR_K@]
+An array.
+\item[@ForeignObj_K@]
+A Foreign object (non-Haskell heap resident).
+\item[@SPT_K@]
+The Stable Pointer table. (There should only be one of these but it
+represents a form of weak space leak since it can't shrink to meet
+non-demand so it may be worth watching separately? ADR)
+\item[@INTERNAL_KIND@]
+Something internal to the runtime system.
+\end{description}
+
+
+\item[Description] Source derived string detailing closure description.
+\item[Type] Source derived string detailing closure type.
+\end{description}
+
+\fi % end of commented out stuff
+
+\item \emph{Parallelism info\/}
+\ToDo{}
+
+\item \emph{Debugging info\/}
+\ToDo{}
+
+\end{itemize}
+
+
+%-----------------------------------------------------------------------------
+\Subsection{Kinds of Heap Object}{closures}
+
+Heap objects can be classified in several ways, but one useful one is
+this:
+\begin{itemize}
+\item
+\emph{Static closures} occupy fixed, statically-allocated memory
+locations, with globally known addresses.
+
+\item
+\emph{Dynamic closures} are individually allocated in the heap.
+
+\item
+\emph{Stack closures} are closures allocated within a thread's stack
+(which is itself a heap object). Unlike other closures, there are
+never any pointers to stack closures. Stack closures are discussed in
+\secref{stacks}.
+
+\end{itemize}
+A second useful classification is this:
+\begin{itemize}
+
+\item \emph{Executive objects}, such as thunks and data constructors,
+participate directly in a program's execution. They can be subdivided
+into three kinds of objects according to their type: \begin{itemize}
+
+\item \emph{Pointed objects}, represent values of a \emph{pointed}
+type (<.pointed types launchbury.>) --i.e.~a type that includes
+$\bottom$ such as @Int@ or @Int# -> Int#@.
+
+\item \emph{Unpointed objects}, represent values of a \emph{unpointed}
+type --i.e.~a type that does not include $\bottom$ such as @Int#@ or
+@Array#@.
+
+\item \emph{Activation frames}, represent ``continuations''. They are
+always stored on the stack and are never pointed to by heap objects or
+passed as arguments. \note{It's not clear if this will still be true
+once we support speculative evaluation.}
+
+\end{itemize}
+
+\item \emph{Administrative objects}, such as stack objects and thread
+state objects, do not represent values in the original program.
+\end{itemize}
+
+Only pointed objects can be entered. All pointed objects share a
+common header format: the ``pointed header''; while all unpointed
+objects share a common header format: the ``unpointed header''.
+\ToDo{Describe the difference and update the diagrams to mention an
+appropriate header type.}
+
+This section enumerates all the kinds of heap objects in the system.
+Each is identified by a distinct closure type field in its info table.
+
+\begin{tabular}{|l|l|l|l|l|l|l|l|l|l|l|}
+\hline
+
+closure type & Section \\
+
+\hline
+\emph{Pointed} \\
+\hline
+
+@CONSTR@ & \ref{sec:CONSTR} \\
+@CONSTR_STATIC@ & \ref{sec:CONSTR} \\
+@CONSTR_STATIC_NOCAF@ & \ref{sec:CONSTR} \\
+
+@FUN@ & \ref{sec:FUN} \\
+@FUN_STATIC@ & \ref{sec:FUN} \\
+
+@THUNK@ & \ref{sec:THUNK} \\
+@THUNK_STATIC@ & \ref{sec:THUNK} \\
+@THUNK_SELECTOR@ & \ref{sec:THUNK_SEL} \\
+
+@BCO@ & \ref{sec:BCO} \\
+@BCO_CAF@ & \ref{sec:BCO} \\
+
+@AP@ & \ref{sec:AP} \\
+@PAP@ & \ref{sec:PAP} \\
+
+@IND@ & \ref{sec:IND} \\
+@IND_OLDGEN@ & \ref{sec:IND} \\
+@IND_PERM@ & \ref{sec:IND} \\
+@IND_OLDGEN_PERM@ & \ref{sec:IND} \\
+@IND_STATIC@ & \ref{sec:IND} \\
+
+\hline
+\emph{Unpointed} \\
+\hline
+
+@ARR_WORDS@ & \ref{sec:ARR_WORDS1},\ref{sec:ARR_WORDS2} \\
+@ARR_PTRS@ & \ref{sec:ARR_PTRS} \\
+@MUTVAR@ & \ref{sec:MUTVAR} \\
+@MUTARR_PTRS@ & \ref{sec:MUTARR_PTRS} \\
+@MUTARR_PTRS_FROZEN@ & \ref{sec:MUTARR_PTRS_FROZEN} \\
+
+@FOREIGN@ & \ref{sec:FOREIGN} \\
+
+@BH@ & \ref{sec:BH} \\
+@MVAR@ & \ref{sec:MVAR} \\
+@IVAR@ & \ref{sec:IVAR} \\
+@FETCHME@ & \ref{sec:FETCHME} \\
+\hline
+\end{tabular}
+
+Activation frames do not live (directly) on the heap --- but they have
+a similar organisation.
+
+\begin{tabular}{|l|l|}\hline
+closure type & Section \\ \hline
+@RET_SMALL@ & \ref{sec:activation-records} \\
+@RET_VEC_SMALL@ & \ref{sec:activation-records} \\
+@RET_BIG@ & \ref{sec:activation-records} \\
+@RET_VEC_BIG@ & \ref{sec:activation-records} \\
+@UPDATE_FRAME@ & \ref{sec:activation-records} \\
+\hline
+\end{tabular}
+
+There are also a number of administrative objects.
+
+\begin{tabular}{|l|l|}\hline
+closure type & Section \\ \hline
+@TSO@ & \ref{sec:TSO} \\
+@STABLEPTR_TABLE@ & \ref{sec:STABLEPTR_TABLE} \\
+@SPARK_OBJECT@ & \ref{sec:SPARK} \\
+@BLOCKED_FETCH@ & \ref{sec:BLOCKED_FETCH} \\
+\hline
+\end{tabular}
+
+\ToDo{I guess the parallel system has something like a stable ptr
+table. Is there any opportunity for sharing code/data structures
+here?}
+
+
+\Subsection{Predicates}{closure-predicates}
+
+\ToDo{The following is a first attempt at defining a useful set of
+predicates. Some (such as @isWHNF@ and @isSparkable@) may need their
+definitions tweaked a little.}
+
+The runtime system sometimes needs to be able to distinguish objects
+according to their properties: is the object updateable? is it in weak
+head normal form? etc. These questions can be answered by examining
+the closure type field of the object's info table.
+
+We define the following predicates to detect families of related
+info types. They are mutually exclusive and exhaustive.
+
+\begin{itemize}
+\item @isCONSTR@ is true for @CONSTR@s.
+\item @isFUN@ is true for @FUN@s.
+\item @isTHUNK@ is true for @THUNK@s.
+\item @isBCO@ is true for @BCO@s.
+\item @isAP@ is true for @AP@s.
+\item @isPAP@ is true for @PAP@s.
+\item @isINDIRECTION@ is true for indirection objects.
+\item @isBH@ is true for black holes.
+\item @isFOREIGN_OBJECT@ is true for foreign objects.
+\item @isARRAY@ is true for array objects.
+\item @isMVAR@ is true for @MVAR@s.
+\item @isIVAR@ is true for @IVAR@s.
+\item @isFETCHME@ is true for @FETCHME@s.
+\item @isSLOP@ is true for slop objects.
+\item @isRET_ADDR@ is true for return addresses.
+\item @isUPD_ADDR@ is true for update frames.
+\item @isTSO@ is true for @TSO@s.
+\item @isSTABLE_PTR_TABLE@ is true for the stable pointer table.
+\item @isSPARK_OBJECT@ is true for spark objects.
+\item @isBLOCKED_FETCH@ is true for blocked fetch objects.
+\item @isINVALID_INFOTYPE@ is true for all other info types.
+
+\end{itemize}
+
+The following predicates detect other interesting properties:
+
+\begin{itemize}
+
+\item @isPOINTED@ is true if an object has a pointed type.
+
+If an object is pointed, the following predicates may be true
+(otherwise they are false). @isWHNF@ and @isUPDATEABLE@ are
+mutually exclusive.
+
+\begin{itemize}
+\item @isWHNF@ is true if the object is in Weak Head Normal Form.
+Note that unpointed objects are (arbitrarily) not considered to be in WHNF.
+
+@isWHNF@ is true for @PAP@s, @CONSTR@s, @FUN@s and all @BCO@s.
+
+\ToDo{Need to distinguish between whnf BCOs and non-whnf BCOs in their
+closure type}
+
+\item @isUPDATEABLE@ is true if the object may be overwritten with an
+ indirection object.
+
+@isUPDATEABLE@ is true for @THUNK@s, @AP@s and @BH@s.
+
+\end{itemize}
+
+It is possible for a pointed object to be neither updatable nor in
+WHNF. For example, indirections.
+
+\item @isUNPOINTED@ is true if an object has an unpointed type.
+All such objects are boxed since only boxed objects have info pointers.
+
+It is true for @ARR_WORDS@, @ARR_PTRS@, @MUTVAR@, @MUTARR_PTRS@,
+@MUTARR_PTRS_FROZEN@, @FOREIGN@ objects, @MVAR@s and @IVAR@s.
+
+\item @isACTIVATION_FRAME@ is true for activation frames of all sorts.
+
+It is true for return addresses and update frames.
+\begin{itemize}
+\item @isVECTORED_RETADDR@ is true for vectored return addresses.
+\item @isDIRECT_RETADDR@ is true for direct return addresses.
+\end{itemize}
+
+\item @isADMINISTRATIVE@ is true for administrative objects:
+@TSO@s, the stable pointer table, spark objects and blocked fetches.
+
+\item @hasSRT@ is true if the info table for the object contains an
+SRT pointer.
+
+@hasSRT@ is true for @THUNK@s, @FUN@s, and @RET@s.
+
+\end{itemize}
+
+\begin{itemize}
+
+\item @isSTATIC@ is true for any statically allocated closure.
+
+\item @isMUTABLE@ is true for objects with mutable pointer fields:
+ @MUT_ARR@s, @MUTVAR@s, @MVAR@s and @IVAR@s.
+
+\item @isSparkable@ is true if the object can (and should) be sparked.
+It is true of updateable objects which are not in WHNF with the
+exception of @THUNK_SELECTOR@s and black holes.
+
+\end{itemize}
+
+As a minor optimisation, we might use the top bits of the @INFO_TYPE@
+field to ``cache'' the answers to some of these predicates.
+
+An indirection either points to HNF (post update); or is result of
+overwriting a FetchMe, in which case the thing fetched is either
+under evaluation (BH), or by now an HNF. Thus, indirections get NoSpark flag.
+
+
+\iffalse
+@
+#define _NF 0x0001 /* Normal form */
+#define _NS 0x0002 /* Don't spark */
+#define _ST 0x0004 /* Is static */
+#define _MU 0x0008 /* Is mutable */
+#define _UP 0x0010 /* Is updatable (but not mutable) */
+#define _BM 0x0020 /* Is a "rimitive" array */
+#define _BH 0x0040 /* Is a black hole */
+#define _IN 0x0080 /* Is an indirection */
+#define _TH 0x0100 /* Is a thunk */
+
+
+
+SPEC
+SPEC_N SPEC | _NF | _NS
+SPEC_S SPEC | _TH
+SPEC_U SPEC | _UP | _TH
+
+GEN
+GEN_N GEN | _NF | _NS
+GEN_S GEN | _TH
+GEN_U GEN | _UP | _TH
+
+DYN _NF | _NS
+TUPLE _NF | _NS | _BM
+DATA _NF | _NS | _BM
+MUTUPLE _NF | _NS | _MU | _BM
+IMMUTUPLE _NF | _NS | _BM
+STATIC _NS | _ST
+CONST _NF | _NS
+CHARLIKE _NF | _NS
+INTLIKE _NF | _NS
+
+BH _NS | _BH
+BH_N BH
+BH_U BH | _UP
+
+BQ _NS | _MU | _BH
+IND _NS | _IN
+CAF _NF | _NS | _ST | _IN
+
+FM
+FETCHME FM | _MU
+FMBQ FM | _MU | _BH
+
+TSO _MU
+
+STKO
+STKO_DYNAMIC STKO | _MU
+STKO_STATIC STKO | _ST
+
+SPEC_RBH _NS | _MU | _BH
+GEN_RBH _NS | _MU | _BH
+BF _NS | _MU | _BH
+INTERNAL
+
+@
+\fi
+
+
+\subsection{Closures (aka Pointed Objects)}
+
+An object can be entered iff it is a closure.
+
+\Subsubsection{Function closures}{FUN}
+
+Function closures represent lambda abstractions. For example,
+consider the top-level declaration:
+@
+ f = \x -> let g = \y -> x+y
+ in g x
+@
+Both @f@ and @g@ are represented by function closures. The closure
+for @f@ is \emph{static} while that for @g@ is \emph{dynamic}.
+
+The layout of a function closure is as follows:
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+\emph{Fixed header} & \emph{Pointers} & \emph{Non-pointers} \\ \hline
+\end{tabular}
+\end{center}
+The data words (pointers and non-pointers) are the free variables of
+the function closure.
+The number of pointers
+and number of non-pointers are stored in the @INFO_SM@ word, in the least significant
+and most significant half-word respectively.
+
+There are several different sorts of function closure, distinguished
+by their closure type field:
+
+\begin{itemize}
+
+\item @FUN@: a vanilla, dynamically allocated on the heap.
+
+\item $@FUN_@p@_@np$: to speed up garbage collection a number of
+specialised forms of @FUN@ are provided, for particular $(p,np)$
+pairs, where $p$ is the number of pointers and $np$ the number of
+non-pointers.
+
+\item @FUN_STATIC@. Top-level, static, function closures (such as @f@
+above) have a different layout than dynamic ones:
+
+\begin{center}
+\begin{tabular}{|l|l|l|}\hline
+\emph{Fixed header} & \emph{Static object link} \\ \hline
+\end{tabular}
+\end{center}
+
+Static function closures have no free variables. (However they may
+refer to other static closures; these references are recorded in the
+function closure's SRT.) They have one field that is not present in
+dynamic closures, the \emph{static object link} field. This is used
+by the garbage collector in the same way that to-space is, to gather
+closures that have been determined to be live but that have not yet
+been scavenged.
+
+\note{Static function closures that have no static references, and
+hence a null SRT pointer, don't need the static object link field. Is
+it worth taking advantage of this? See @CONSTR_STATIC_NOCAF@.}
+\end{itemize}
+
+Each lambda abstraction, $f$, in the STG program has its own private
+info table. The following labels are relevant:
+
+\begin{itemize}
+
+\item $f$@_info@ is $f$'s info table.
+
+\item $f$@_entry@ is $f$'s slow entry point (i.e. the entry code of
+its info table; so it will label the same byte as $f$@_info@).
+
+\item $f@_fast_@k$ is $f$'s fast entry point. $k$ is the number of
+arguments $f$ takes; encoding this number in the fast-entry label
+occasionally catches some nasty code-generation errors.
+
+\end{itemize}
+
+\Subsubsection{Data constructors}{CONSTR}
+
+Data-constructor closures represent values constructed with algebraic
+data type constructors. The general layout of data constructors is
+the same as that for function closures. That is
+
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+\emph{Fixed header} & \emph{Pointers} & \emph{Non-pointers} \\ \hline
+\end{tabular}
+\end{center}
+
+There are several different sorts of constructor:
+
+\begin{itemize}
+
+\item @CONSTR@: a vanilla, dynamically allocated constructor.
+
+\item @CONSTR_@$p$@_@$np$: just like $@FUN_@p@_@np$.
+
+\item @CONSTR_INTLIKE@. A dynamically-allocated heap object that
+looks just like an @Int@. The garbage collector checks to see if it
+can common it up with one of a fixed set of static int-like closures,
+thus getting it out of the dynamic heap altogether.
+
+\item @CONSTR_CHARLIKE@: same deal, but for @Char@.
+
+\item @CONSTR_STATIC@ is similar to @FUN_STATIC@, with the
+complication that the layout of the constructor must mimic that of a
+dynamic constructor, because a static constructor might be returned to
+some code that unpacks it. So its layout is like this:
+
+\begin{center}
+\begin{tabular}{|l|l|l|l|l|}\hline
+\emph{Fixed header} & \emph{Pointers} & \emph{Non-pointers} & \emph{Static object link}\\ \hline
+\end{tabular}
+\end{center}
+
+The static object link, at the end of the closure, serves the same purpose
+as that for @FUN_STATIC@. The pointers in the static constructor can point
+only to other static closures.
+
+The static object link occurs last in the closure so that static
+constructors can store their data fields in exactly the same place as
+dynamic constructors.
+
+\item @CONSTR_STATIC_NOCAF@. A statically allocated data constructor
+that guarantees not to point (directly or indirectly) to any CAF
+(\secref{CAF}). This means it does not need a static object
+link field. Since we expect that there might be quite a lot of static
+constructors this optimisation makes sense. Furthermore, the @NOCAF@
+tag allows the compiler to indicate that no CAFs can be reached
+anywhere \emph{even indirectly}.
+
+\end{itemize}
+
+For each data constructor $Con$, two info tables are generated:
+
+\begin{itemize}
+\item $Con$@_con_info@ labels $Con$'s dynamic info table,
+shared by all dynamic instances of the constructor.
+\item $Con$@_static@ labels $Con$'s static info table,
+shared by all static instances of the constructor.
+\end{itemize}
+
+Each constructor also has a \emph{constructor function}, which is a
+curried function which builds an instance of the constructor. The
+constructor function has an info table labelled as @$Con$_info@.
+
+Nullary constructors are represented by a single static info table,
+which everyone points to. Thus for a nullary constructor we can omit
+the dynamic info table and the constructor function.
+
+\subsubsection{Thunks}
+\label{sec:THUNK}
+\label{sec:THUNK_SEL}
+
+A thunk represents an expression that is not obviously in head normal
+form. For example, consider the following top-level definitions:
+@
+ range = between 1 10
+ f = \x -> let ys = take x range
+ in sum ys
+@
+Here the right-hand sides of @range@ and @ys@ are both thunks; the former
+is static while the latter is dynamic.
+
+The layout of a thunk is the same as that for a function closure.
+However, thunks must have a payload of at least @MIN_UPD_PAYLOAD@
+words to allow it to be overwritten with a black hole and an
+indirection. The compiler may have to add extra non-pointer fields to
+satisfy this constraint.
+
+\begin{center}
+\begin{tabular}{|l|l|l|l|l|}\hline
+\emph{Fixed header} & \emph{Pointers} & \emph{Non-pointers} \\ \hline
+\end{tabular}
+\end{center}
+
+The layout word in the info table contains the same information as for
+function closures; that is, number of pointers and number of
+non-pointers.
+
+A thunk differs from a function closure in that it can be updated.
+
+There are several forms of thunk:
+
+\begin{itemize}
+
+\item @THUNK@ and $@THUNK_@p@_@np$: vanilla, dynamically allocated
+thunks. Dynamic thunks are overwritten with normal indirections.
+
+\item @THUNK_STATIC@. A static thunk is also known as a
+\emph{constant applicative form}, or \emph{CAF}. Static thunks are
+overwritten with static indirections.
+
+\begin{center}
+\begin{tabular}{|l|l|l|l|l|}\hline
+\emph{Fixed header} & \emph{Pointers} & \emph{Non-pointers} \emph{Static object link}\\ \hline
+\end{tabular}
+\end{center}
+
+\item @THUNK_SELECTOR@ is a (dynamically allocated) thunk whose entry
+code performs a simple selection operation from a data constructor
+drawn from a single-constructor type. For example, the thunk
+@
+ x = case y of (a,b) -> a
+@
+is a selector thunk. A selector thunk is laid out like this:
+
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+\emph{Fixed header} & \emph{Selectee pointer} \\ \hline
+\end{tabular}
+\end{center}
+
+The layout word contains the byte offset of the desired word in the
+selectee. Note that this is different from all other thunks.
+
+The garbage collector ``peeks'' at the selectee's tag (in its info
+table). If it is evaluated, then it goes ahead and does the
+selection, and then behaves just as if the selector thunk was an
+indirection to the selected field. If it is not evaluated, it treats
+the selector thunk like any other thunk of that shape.
+[Implementation notes. Copying: only the evacuate routine needs to be
+special. Compacting: only the PRStart (marking) routine needs to be
+special.]
+
+There is a fixed set of pre-compiled selector thunks built into the
+RTS, representing offsets from 0 to @MAX_SPEC_SELECTOR_THUNK@. The
+info tables are labelled @sel_info_$n$@ where $n$ is the offset.
+
+\end{itemize}
+
+The only label associated with a thunk is its info table:
+\begin{description}
+\item[$f$@_info@] is $f$'s info table.
+\end{description}
+
+
+\Subsubsection{Byte-code objects}{BCO}
+
+A Byte-Code Object (BCO) is a container for a a chunk of byte-code,
+which can be executed by Hugs. The byte-code represents a
+supercombinator in the program: when Hugs compiles a module, it
+performs lambda lifting and each resulting supercombinator becomes a
+byte-code object in the heap.
+
+BCOs are not updateable; the bytecode compiler represents updatable
+thunks using a combination of @AP@s and @BCO@s.
+
+The semantics of BCOs are described in \secref{hugs-heap-objects}. A
+BCO has the following structure:
+
+\begin{center}
+\begin{tabular}{|l|l|l|l|l|l|}
+\hline
+\emph{Fixed Header} & \emph{Layout} & \emph{Offset} & \emph{Size} &
+\emph{Literals} & \emph{Byte code} \\
+\hline
+\end{tabular}
+\end{center}
+
+\noindent where:
+\begin{itemize}
+\item The entry code is a static code fragment/info table that returns
+to the scheduler to invoke Hugs (\secref{ghc-to-hugs-switch}).
+\item \emph{Layout} contains the number of pointer literals in the
+\emph{Literals} field.
+\item \emph{Offset} is the offset to the byte code from the start of
+the object.
+\item \emph{Size} is the number of words of byte code in the object.
+\item \emph{Literals} contains any pointer and non-pointer literals used in
+the byte-codes (including jump addresses), pointers first.
+\item \emph{Byte code} contains \emph{Size} words of non-pointer byte
+code.
+\end{itemize}
+
+
+\Subsubsection{Partial applications}{PAP}
+
+\ToDo{PAPs don't contains update frames or activation frames. When we
+add revertible black holes, we'll introduce a new kind of object which
+can contain activation frames.}
+
+A partial application (PAP) represents a function applied to too few arguments.
+It is only built as a result of updating after an argument-satisfaction
+check failure. A PAP has the following shape:
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+\emph{Fixed header} & \emph{No of arg words} & \emph{Function closure} & \emph{Arg stack} \\ \hline
+\end{tabular}
+\end{center}
+The ``arg stack'' is a copy of the chunk of stack above the update
+frame; ``no of arg words'' tells how many words it consists of. The
+function closure is (a pointer to) the closure for the function whose
+argument-satisfaction check failed.
+
+There is just one standard form of PAP. There is just one info table
+too, called @PAP_info@. Its entry code simply copies the arg stack
+chunk back on top of the stack and enters the function closure. (It
+has to do a stack overflow test first.)
+
+PAPs are also used to implement Hugs functions (where the arguments
+are free variables). PAPs generated by Hugs can be static so we need
+both @PAP@ and @PAP_STATIC@.
+
+\Subsubsection{@AP@ objects}{AP}
+
+@AP@ objects are used to represent thunks built by Hugs. The only
+distintion between an @AP@ and a @PAP@ is that an @AP@ is updateable.
+
+\begin{center}
+\begin{tabular}{|l|l|l|l|}
+\hline
+\emph{Fixed Header} & \emph{No of arg words} & \emph{Function closure} & \emph{Arg stack} \\
+\hline
+\end{tabular}
+\end{center}
+
+The entry code pushes an update frame, copies the arg stack chunk on
+top of the stack, and enters the function closure. (It has to do a
+stack overflow test first.)
+
+The ``arg stack'' is a block of (tagged) arguments representing the
+free variables of the thunk; ``no of arg words'' tells how many words
+it consists of. The function closure is (a pointer to) the closure
+for the thunk. The argument stack may be empty if the thunk has no
+free variables.
+
+\note{Since @AP@s are updateable, the @MIN_UPD_PAYLOAD@ constraint
+applies here too.}
+
+\Subsubsection{Indirections}{IND}
+
+Indirection closures just point to other closures. They are introduced
+when a thunk is updated to point to its value. The entry code for all
+indirections simply enters the closure it points to.
+
+There are several forms of indirection:
+\begin{description}
+\item[@IND@] is the vanilla, dynamically-allocated indirection.
+It is removed by the garbage collector. It has the following
+shape:
+\begin{center}
+\begin{tabular}{|l|l|l|}\hline
+\emph{Fixed header} & \emph{Mutable link field} & \emph{Target closure} \\ \hline
+\end{tabular}
+\end{center}
+It contains a \emph{mutable link field} that is used to string together
+indirections in each generation.
+
+
+\item[@IND_PERMANENT@]
+for lexical profiling, it is necessary to maintain cost centre
+information in an indirection, so ``permanent indirections'' are
+retained forever. Otherwise they are just like vanilla indirections.
+\note{If a permanent indirection points to another permanent
+indirection or a @CONST@ closure, it is possible to elide the indirection
+since it will have no effect on the profiler.}
+
+\note{Do we still need @IND@ in the profiling build, or do we just
+need @IND@ but its behaviour changes when profiling is on?}
+
+\item[@IND_STATIC@] is used for overwriting CAFs when they have been
+evaluated. Static indirections are not removed by the garbage
+collector; and are statically allocated outside the heap (and should
+stay there). Their static object link field is used just as for
+@FUN_STATIC@ closures.
+
+\begin{center}
+\begin{tabular}{|l|l|l|}
+\hline
+\emph{Fixed header} & \emph{Target closure} & \emph{Static object link} \\
+\hline
+\end{tabular}
+\end{center}
+
+\end{description}
+
+\Subsubsection{Black holes and blocking queues}{BH}
+
+Black hole closures are used to overwrite closures currently being
+evaluated. They inform the garbage collector that there are no live
+roots in the closure, thus removing a potential space leak.
+
+Black holes also become synchronization points in the threaded world.
+They contain a pointer to a list of blocked threads to be awakened
+when the black hole is updated (or @NULL@ if the list is empty).
+
+\begin{center}
+\begin{tabular}{|l|l|l|}
+\hline
+\emph{Fixed header} & \emph{Mutable link} & \emph{Blocked thread link} \\
+\hline
+\end{tabular}
+\end{center}
+
+The \emph{Blocked thread link} points to the TSO of the first thread
+waiting for the value of this thunk. All subsequent TSOs in the list
+are linked together using their @TSO_LINK@ field.
+
+When the blocking queue is non-@NULL@ and the @BH@ is in the old
+generation, the black hole must be added to the mutables list since
+the TSOs on the list may contain pointers into the new generation.
+There is no need to clutter up the mutables list with black holes with
+empty blocking queues.
+
+\note{In a single-threaded system, entering a black hole indicates an
+infinite loop. In a concurrent system, entering a black hole
+indicates an infinite loop only if the hole is being entered by the
+same thread that originally entered the closure.}
+
+
+\Subsubsection{FetchMes}{FETCHME}
+
+In the parallel systems, FetchMes are used to represent pointers into
+the global heap. When evaluated, the value they point to is read from
+the global heap.
+
+\ToDo{Describe layout}
+
+Because there may be offsets into these arrays, a primitive array
+cannot be handled as a FetchMe in the parallel system, but must be
+shipped in its entirety if its parent closure is shipped.
+
+
+
+\Subsection{Unpointed Objects}{unpointed-objects}
+
+A variable of unpointed type is always bound to a \emph{value}, never
+to a \emph{thunk}. For this reason, unpointed objects cannot be
+entered.
+
+\subsubsection{Immutable objects}
+\label{sec:ARR_WORDS1}
+\label{sec:ARR_PTRS}
+
+\begin{description}
+\item[@ARR_WORDS@] is a variable-sized object consisting solely of
+non-pointers. It is used for arrays of all sorts of things (bytes,
+words, floats, doubles... it doesn't matter).
+\begin{center}
+\begin{tabular}{|c|c|c|c|}
+\hline
+\emph{Fixed Hdr} & \emph{No of non-pointers} & \emph{Non-pointers\ldots} \\ \hline
+\end{tabular}
+\end{center}
+
+\item[@ARR_PTRS@] is an immutable, variable sized array of pointers.
+\begin{center}
+\begin{tabular}{|c|c|c|c|}
+\hline
+\emph{Fixed Hdr} & \emph{Mutable link} & \emph{No of pointers} & \emph{Pointers\ldots} \\ \hline
+\end{tabular}
+\end{center}
+The mutable link is present so that we can easily freeze and thaw an
+array (by changing the header and adding/removing the array to the
+mutables list).
+
+\end{description}
+
+\subsubsection{Mutable objects}
+\label{sec:mutables}
+\label{sec:ARR_WORDS2}
+\label{sec:MUTVAR}
+\label{sec:MUTARR_PTRS}
+\label{sec:MUTARR_PTRS_FROZEN}
+
+Some of these objects are \emph{mutable}; they represent objects which
+are explicitly mutated by Haskell code through the @ST@ monad.
+They're not used for thunks which are updated precisely once.
+Depending on the garbage collector, mutable closures may contain extra
+header information which allows a generational collector to implement
+the ``write barrier.''
+
+\begin{description}
+
+\item[@ARR_WORDS@] is also used to represent \emph{mutable} arrays of
+bytes, words, floats, doubles, etc. It's possible to use the same
+object type because even generational collectors don't need to
+distinguish them.
+
+\item[@MUTVAR@] is a mutable variable.
+\begin{center}
+\begin{tabular}{|c|c|c|}
+\hline
+\emph{Fixed Hdr} & \emph{Mutable link} & \emph{Pointer} \\ \hline
+\end{tabular}
+\end{center}
+
+\item[@MUTARR_PTRS@] is a mutable array of pointers.
+Such an array may be \emph{frozen}, becoming an @ARR_PTRS@, with a
+different info-table.
+\begin{center}
+\begin{tabular}{|c|c|c|c|}
+\hline
+\emph{Fixed Hdr} & \emph{Mutable link} & \emph{No of ptrs} & \emph{Pointers\ldots} \\ \hline
+\end{tabular}
+\end{center}
+
+\end{description}
+
+
+\Subsubsection{Foreign objects}{FOREIGN}
+
+Here's what a ForeignObj looks like:
+
+\begin{center}
+\begin{tabular}{|l|l|l|l|}
+\hline
+\emph{Fixed header} & \emph{Data} & \emph{Free Routine} & \emph{Foreign object link} \\
+\hline
+\end{tabular}
+\end{center}
+
+The @FreeRoutine@ is a reference to the finalisation routine to call
+when the @ForeignObj@ becomes garbage. If @freeForeignObject@ is
+called on a Foreign Object, the @FreeRoutine@ is set to zero and the
+garbage collector will not attempt to call @FreeRoutine@ when the
+object becomes garbage.
+
+The Foreign object link is a link to the next foreign object in the
+list. This list is traversed at the end of garbage collection: if an
+object is about to be deallocated (e.g.~it was not marked or
+evacuated), the free routine is called and the object is deleted from
+the list.
+
+\subsubsection{MVars and IVars}
+\label{sec:MVAR}
+\label{sec:IVAR}
+
+\ToDo{MVars and IVars}
+
+
+
+The remaining objects types are all administrative --- none of them
+may be entered.
+
+\subsection{Other weird objects}
+\label{sec:SPARK}
+\label{sec:BLOCKED_FETCH}
+
+\begin{description}
+\item[@BlockedFetch@ heap objects (`closures')] (parallel only)
+
+@BlockedFetch@s are inbound fetch messages blocked on local closures.
+They arise as entries in a local blocking queue when a fetch has been
+received for a local black hole. When awakened, we look at their
+contents to figure out where to send a resume.
+
+A @BlockedFetch@ closure has the form:
+\begin{center}
+\begin{tabular}{|l|l|l|l|l|l|}\hline
+\emph{Fixed header} & link & node & gtid & slot & weight \\ \hline
+\end{tabular}
+\end{center}
+
+\item[Spark Closures] (parallel only)
+
+Spark closures are used to link together all closures in the spark pool. When
+the current processor is idle, it may choose to speculatively evaluate some of
+the closures in the pool. It may also choose to delete sparks from the pool.
+\begin{center}
+\begin{tabular}{|l|l|l|l|l|l|}\hline
+\emph{Fixed header} & \emph{Spark pool link} & \emph{Sparked closure} \\ \hline
+\end{tabular}
+\end{center}
+
+\item[Slop Objects]\label{sec:slop-objects}
+
+Slop objects are used to overwrite the end of an updatee if it is
+larger than an indirection. Normal slop objects consist of an info
+pointer a size word and a number of slop words.
+
+\begin{center}
+\begin{tabular}{|l|l|l|l|l|l|}\hline
+\emph{Info Pointer} & \emph{Size} & \emph{Slop Words} \\ \hline
+\end{tabular}
+\end{center}
+
+This is too large for single word slop objects which consist of a
+single info table.
+
+Note that slop objects only contain an info pointer, not a standard
+fixed header. This doesn't cause problems because slop objects are
+always unreachable --- they can only be accessed by linearly scanning
+the heap.
+
+\end{description}
+
+\Subsection{Thread State Objects (TSOs)}{TSO}
+
+\ToDo{This is very out of date. We now embed a single stack object
+within the TSO. TSOs include an ID number which can be used to
+generate a hash value. The gransim, profiling and ticky info is
+surely bogus.}
+
+In the multi-threaded system, the state of a suspended thread is
+packed up into a Thread State Object (TSO) which contains all the
+information needed to restart the thread and for the garbage collector
+to find all reachable objects. When a thread is running, it may be
+``unpacked'' into machine registers and various other memory locations
+to provide faster access.
+
+Single-threaded systems don't really \emph{need\/} TSOs --- but they do
+need some way to tell the storage manager about live roots so it is
+convenient to use a single TSO to store the mutator state even in
+single-threaded systems.
+
+Rather than manage TSOs' alloc/dealloc, etc., in some \emph{ad hoc}
+way, we instead alloc/dealloc/etc them in the heap; then we can use
+all the standard garbage-collection/fetching/flushing/etc machinery on
+them. So that's why TSOs are ``heap objects,'' albeit very special
+ones.
+\begin{center}
+\begin{tabular}{|l|l|}
+ \hline \emph{Fixed header}
+\\ \hline @TSO_LINK@
+\\ \hline @TSO_STATE@
+\\ \hline \emph{Exception Handlers}
+\\ \hline \emph{Ticky Info}
+\\ \hline \emph{Profiling Info}
+\\ \hline \emph{Parallel Info}
+\\ \hline \emph{GranSim Info}
+\\ \hline
+\\
+ \emph{Stack}
+\\
+\\ \hline
+\end{tabular}
+\end{center}
+The contents of a TSO are:
+\begin{itemize}
+
+\item A pointer (@TSO_LINK@) used to maintain a list of threads with a similar
+ state (e.g.~all runnable, all sleeping, all blocked on the same black
+ hole, all blocked on the same MVar, etc.)
+
+\item A word (@TSO_STATE@) which records the current state of a thread: running, runnable, blocked, etc.
+
+\item Optional information for ``Ticky Ticky'' statistics: @TSO_STK_HWM@ is
+ the maximum number of words allocated to this thread.
+
+\item Optional information for profiling:
+ @TSO_CCC@ is the current cost centre.
+
+\item Optional information for parallel execution:
+
+% \begin{itemize}
+%
+% \item The types of threads (@TSO_TYPE@):
+% \begin{description}
+% \item[@T_MAIN@] Must be executed locally.
+% \item[@T_REQUIRED@] A required thread -- may be exported.
+% \item[@T_ADVISORY@] An advisory thread -- may be exported.
+% \item[@T_FAIL@] A failure thread -- may be exported.
+% \end{description}
+%
+% \item I've no idea what else
+%
+% \end{itemize}
+%
+% \item Optional information for GranSim execution:
+% \begin{itemize}
+% \item locked
+% \item sparkname
+% \item started at
+% \item exported
+% \item basic blocks
+% \item allocs
+% \item exectime
+% \item fetchtime
+% \item fetchcount
+% \item blocktime
+% \item blockcount
+% \item global sparks
+% \item local sparks
+% \item queue
+% \item priority
+% \item clock (gransim light only)
+% \end{itemize}
+%
+%
+% Here are the various queues for GrAnSim-type events.
+%
+% Q_RUNNING
+% Q_RUNNABLE
+% Q_BLOCKED
+% Q_FETCHING
+% Q_MIGRATING
+%
+
+\end{itemize}
+
+\subsection{Stack Objects}
+\label{sec:STACK_OBJECT}
+\label{sec:stacks}
+
+\ToDo{Merge this in with the section on TSOs}
+
+These are ``stack objects,'' which are used in the threaded world as
+the stack for each thread is allocated from the heap in smallish
+chunks. (The stack in the sequential world is allocated outside of
+the heap.)
+
+Each reduction thread has to have its own stack space. As there may
+be many such threads, and as any given one may need quite a big stack,
+a naive give-'em-a-big-stack-and-let-'em-run approach will cost a {\em
+lot} of memory.
+
+Our approach is to give a thread a small stack space, and then link
+on/off extra ``chunks'' as the need arises. Again, this is a
+storage-management problem, and, yet again, we choose to graft the
+whole business onto the existing heap-management machinery. So stack
+objects will live in the heap, be garbage collected, etc., etc..
+
+A stack object is laid out like this:
+
+\begin{center}
+\begin{tabular}{|l|}
+\hline
+\emph{Fixed header}
+\\ \hline
+\emph{Link to next stack object (0 for last)}
+\\ \hline
+\emph{N, the payload size in words}
+\\ \hline
+\emph{@Sp@ (byte offset from head of object)}
+\\ \hline
+\emph{@Su@ (byte offset from head of object)}
+\\ \hline
+\emph{Payload (N words)}
+\\ \hline
+\end{tabular}
+\end{center}
+
+The stack grows downwards, towards decreasing
+addresses. This makes it easier to print out the stack
+when debugging, and it means that a return address is
+at the lowest address of the chunk of stack it ``knows about''
+just like an info pointer on a closure.
+
+The garbage collector needs to be able to find all the
+pointers in a stack. How does it do this?
+\begin{itemize}
-\subsection{Heap and Stack Checks}
+\item Within the stack there are return addresses, pushed
+by @case@ expressions. Below a return address (i.e. at higher
+memory addresses, since the stack grows downwards) is a chunk
+of stack that the return address ``knows about'', namely the
+activation record of the currently running function.
-\note{I reckon these deserve a subsection of their own}
+\item Below each such activation record is a \emph{pending-argument
+section}, a chunk of
+zero or more words that are the arguments to which the result
+of the function should be applied. The return address does not
+statically
+``know'' how many pending arguments there are, or their types.
+(For example, the function might return a result of type $\alpha$.)
-Don't move heap pointer before success occurs.
-Talk about how stack check looks ahead into the branches of case expressions.
+\item Below each pending-argument section is another return address,
+and so on. Actually, there might be an update frame instead, but we
+can consider update frames as a special case of a return address with
+a well-defined activation record.
-\subsection{Handling interrupts/signals}
+\end{itemize}
-@
-May have to keep C stack pointer in register to placate OS?
-May have to revert black holes - ouch!
-@
+The game plan is this. The garbage collector
+walks the stack from the top, traversing pending-argument sections and
+activation records alternately. Next we discuss how it finds
+the pointers in each of these two stack regions.
-\section{Switching Worlds}
-Because this is a combined compiled/interpreted system, the
-interpreter will sometimes encounter compiled code, and vice-versa.
+\Subsubsection{Activation records}{activation-records}
-There are six cases we need to consider:
-\begin{enumerate}
-\item A GHC thread enters a Hugs-built thunk.
-\item A GHC thread calls a Hugs-compiled function.
-\item A GHC thread returns to a Hugs-compiled return address.
-\item A Hugs thread enters a GHC-built thunk.
-\item A Hugs thread calls a GHC-compiled function.
-\item A Hugs thread returns to a Hugs-compiled return address.
-\end{enumerate}
+An \emph{activation record} is a contiguous chunk of stack,
+with a return address as its first word, followed by as many
+data words as the return address ``knows about''. The return
+address is actually a fully-fledged info pointer. It points
+to an info table, replete with:
-\subsection{A GHC thread enters a Hugs-built thunk}
+\begin{itemize}
+\item entry code (i.e. the code to return to).
-A Hugs-built thunk looks like this:
+\item closure type is either @RET_SMALL/RET_VEC_SMALL@ or
+@RET_BIG/RET_VEC_BIG@, depending on whether the activation record has
+more than 32 data words (\note{64 for 8-byte-word architectures}) and
+on whether to use a direct or a vectored return.
-\begin{center}
-\begin{tabular}{|l|l|}
-\hline
-\emph{Hugs} & \emph{Hugs-specific information} \\
-\hline
-\end{tabular}
-\end{center}
+\item the layout info for @RET_SMALL@ is a bitmap telling the layout
+of the activation record, one bit per word. The least-significant bit
+describes the first data word of the record (adjacent to the fixed
+header) and so on. A ``@1@'' indicates a non-pointer, a ``@0@''
+indicates a pointer. We don't need to indicate exactly how many words
+there are, because when we get to all zeros we can treat the rest of
+the activation record as part of the next pending-argument region.
-\noindent where \emph{Hugs} is a pointer to a small
-statically-compiled piece of code that does the following:
+For @RET_BIG@ the layout field points to a block of bitmap words,
+starting with a word that tells how many words are in the block.
-\begin{itemize}
-\item Push the address of the thunk on the stack.
-\item Push @entertop@ on the stack.
-\item Save the current state of the thread in the TSO.
-\item Return to the scheduler, with the @whatNext@ field set to
-@RunHugs@.
+\item the info table contains a Static Reference Table pointer for the
+return address (\secref{srt}).
\end{itemize}
-\noindent where @entertop@ is a small statically-compiled piece of
-code that does the following:
+The activation record is a fully fledged closure too. As well as an
+info pointer, it has all the other attributes of a fixed header
+(\secref{fixed-header}) including a saved cost centre which
+is reloaded when the return address is entered.
-\begin{itemize}
-\item pop the return address from the stack.
-\item pop the next word off the stack into \Arg{1}.
-\item enter \Arg{1}.
-\end{itemize}
+In other words, all the attributes of closures are needed for
+activation records, so it's very convenient to make them look alike.
-The infotable for @entertop@ has some byte-codes attached that do
-essentially the same thing if the code is entered from Hugs.
-\subsection{A GHC thread calls a Hugs-compiled function}
+\Subsubsection{Pending arguments}{pending-args}
-How do we do this?
+So that the garbage collector can correctly identify pointers in
+pending-argument sections we explicitly tag all non-pointers. Every
+non-pointer in a pending-argument section is preceded (at the next
+lower memory word) by a one-word byte count that says how many bytes
+to skip over (excluding the tag word).
-\subsection{A GHC thread returns to a Hugs-compiled return address}
+The garbage collector traverses a pending argument section from the
+top (i.e. lowest memory address). It looks at each word in turn:
-\subsection{A Hugs thread enters a GHC-compiled thunk}
+\begin{itemize}
+\item If it is less than or equal to a small constant @MAX_STACK_TAG@
+then it treats it as a tag heralding zero or more words of
+non-pointers, so it just skips over them.
-When Hugs is called on to enter a non-Hugs closure (these are
-recognisable by the lack of a \emph{Hugs} pointer at the front), the
-following sequence of instructions is executed:
+\item If it points to the code segment, it must be a return
+address, so we have come to the end of the pending-argument section.
-\begin{itemize}
-\item Push the address of the thunk on the stack.
-\item Push @entertop@ on the stack.
-\item Save the current state of the thread in the TSO.
-\item Return to the scheduler, with the @whatNext@ field set to
-@RunGHC@.
+\item Otherwise it must be a bona fide heap pointer.
\end{itemize}
-\subsection{A Hugs thread calls a GHC-compiled function}
-Hugs never calls GHC-functions directly, it only enters closures
-(which point to the slow entry point for the function). Hence in this
-case, we just push the arguments on the stack and proceed as for a
-thunk.
+\Subsection{The Stable Pointer Table}{STABLEPTR_TABLE}
-\subsection{A Hugs thread returns to a GHC-compiled return address}
+A stable pointer is a name for a Haskell object which can be passed to
+the external world. It is ``stable'' in the sense that the name does
+not change when the Haskell garbage collector runs---in contrast to
+the address of the object which may well change.
-\section{Heap objects}
-\label{sect:fixed-header}
+A stable pointer is represented by an index into the
+@StablePointerTable@. The Haskell garbage collector treats the
+@StablePointerTable@ as a source of roots for GC.
-\ToDo{Fix this picture}
+In order to provide efficient access to stable pointers and to be able
+to cope with any number of stable pointers (eg $0 \ldots 100000$), the
+table of stable pointers is an array stored on the heap and can grow
+when it overflows. (Since we cannot compact the table by moving
+stable pointers about, it seems unlikely that a half-empty table can
+be reduced in size---this could be fixed if necessary by using a
+hash table of some sort.)
+
+In general a stable pointer table closure looks like this:
-\begin{figure}
\begin{center}
-\input{closure}
+\begin{tabular}{|l|l|l|l|l|l|l|l|l|l|l|}
+\hline
+\emph{Fixed header} & \emph{No of pointers} & \emph{Free} & $SP_0$ & \ldots & $SP_{n-1}$
+\\\hline
+\end{tabular}
\end{center}
-\caption{A closure}
-\label{fig:closure}
-\end{figure}
-Every {\em heap object}, or {\em closure} is a contiguous block
-of memory, consisting of a fixed-format {\em header} followed
-by zero or more {\em data words}.
-The header consists of
-the following fields:
-\begin{itemize}
-\item A one-word {\em info pointer}, which points to
-the object's static {\em info table}.
-\item Zero or more {\em admin words} that support
-\begin{itemize}
-\item Profiling (notably a {\em cost centre} word).
- \note{We could possibly omit the cost centre word from some
- administrative objects.}
-\item Parallelism (e.g. GranSim keeps the object's global address here,
-though GUM keeps a separate hash table).
-\item Statistics (e.g. a word to track how many times a thunk is entered.).
+The fields are:
+\begin{description}
-We add a Ticky word to the fixed-header part of closures. This is
-used to record indicate if a closure has been updated but not yet
-entered. It is set when the closure is updated and cleared when
-subsequently entered.
+\item[@NPtrs@:] number of (stable) pointers.
-NB: It is {\em not} an ``entry count'', it is an
-``entries-after-update count.'' The commoning up of @CONST@,
-@CHARLIKE@ and @INTLIKE@ closures is turned off(?) if this is
-required. This has only been done for 2s collection.
+\item[@Free@:] the byte offset (from the first byte of the object) of the first free stable pointer.
+\item[$SP_i$:] A stable pointer slot. If this entry is in use, it is
+an ``unstable'' pointer to a closure. If this entry is not in use, it
+is a byte offset of the next free stable pointer slot.
+\end{description}
-\end{itemize}
-\end{itemize}
-Most of the RTS is completely insensitive to the number of admin words.
-The total size of the fixed header is @FIXED_HS@.
+When a stable pointer table is evacuated
+\begin{enumerate}
+\item the free list entries are all set to @NULL@ so that the evacuation
+ code knows they're not pointers;
-Many heap objects contain fields allowing them to be inserted onto lists
-during evaluation or during garbage collection. The lists required by
-the evaluator and storage manager are as follows.
+\item The stable pointer slots are scanned linearly: non-@NULL@ slots
+are evacuated and @NULL@-values are chained together to form a new free list.
+\end{enumerate}
+
+There's no need to link the stable pointer table onto the mutable
+list because we always treat it as a root.
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\Subsection{Garbage Collecting CAFs}{CAF}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+
+% begin{direct quote from current paper}
+A CAF (constant applicative form) is a top-level expression with no
+arguments. The expression may need a large, even unbounded, amount of
+storage when it is fully evaluated.
+
+CAFs are represented by closures in static memory that are updated
+with indirections to objects in the heap space once the expression is
+evaluated. Previous version of GHC maintained a list of all evaluated
+CAFs and traversed them during GC, the result being that the storage
+allocated by a CAF would reside in the heap until the program ended.
+% end{direct quote from current paper}
+% begin{elaboration on why CAFs are very very bad}
+Treating CAFs this way has two problems:
\begin{itemize}
-\item 2 lists of threads: runnable threads and sleeping threads.
+\item
+It can cause a very large space leak. For example, this program
+should run in constant space but, instead, will run out of memory.
+\begin{verbatim}
+> main :: IO ()
+> main = print nats
+>
+> nats :: [Int]
+> nats = [0..maxInt]
+\end{verbatim}
-\item The {\em static object list} is a list of all statically
-allocated objects which might contain pointers into the heap.
-(Section~\ref{sect:static-objects}.)
+\item
+Expressions with no arguments have very different space behaviour
+depending on whether or not they occur at the top level. For example,
+if we make \verb+nats+ a local definition, the space leak goes away
+and the resulting program runs in constant space, as expected.
+\begin{verbatim}
+> main :: IO ()
+> main = print nats
+> where
+> nats :: [Int]
+> nats = [0..maxInt]
+\end{verbatim}
-\item The {\em updated thunk list} is a list of all thunks in the old
-generation which have been updated with an indirection.
-(Section~\ref{sect:IND_OLDGEN}.)
+This huge change in the operational behaviour of the program
+is a problem for optimising compilers and for programmers.
+For example, GHC will normally flatten a set of let bindings using
+this transformation:
+\begin{verbatim}
+let x1 = let x2 = e2 in e1 ==> let x2 = e2 in let x1 = e1
+\end{verbatim}
+but it does not do so if this would raise \verb+x2+ to the top level
+since that may create a CAF. Many Haskell programmers avoid creating
+large CAFs by adding a dummy argument to a CAF or by moving a CAF away
+from the top level.
-\item The {\em mutables list} is a list of all other objects in the
-old generation which might contain pointers into the new generation.
-Most of the object on this list are ``mutable.''
-(Section~\ref{sect:mutables}.)
+\end{itemize}
+% end{elaboration on why CAFs are very very bad}
-\item The {\em Foreign Object list} is a list of all foreign objects
- which have not yet been deallocated. (Section~\ref{sect:FOREIGN}.)
+Solving the CAF problem requires different treatment in interactive
+systems such as Hugs than in batch-mode systems such as GHC
+\begin{itemize}
+\item
+In a batch-mode the program the runtime system is terminated
+after every execution of the runtime system. In such systems,
+the garbage collector can completely ``destroy'' a CAF when it
+is no longer live --- in much the same way as it ``destroys''
+normal closures when they are no longer live.
-\item The {\em Spark pool} is a doubly(?) linked list of Spark objects
-maintained by the parallel system. (Section~\ref{sect:SPARK}.)
+\item
+In an interactive system, many expressions are evaluated without
+restarting the runtime system between each evaluation. In such
+systems, the garbage collector cannot completely ``destroy'' a CAF
+when it is no longer live because, whilst it might not be required in
+the evaluation of the current expression, it might be required in the
+next evaluation.
+
+There are two possible behaviours we migth want:
+\begin{enumerate}
+\item
+When a CAF is no longer required for the current evaluation, the CAF
+should be reverted to its original form. This behaviour ensures that
+the operational behaviour of the interactive system is a reasonable
+predictor of the operational behaviour of the batch-mode system. This
+allows us to use Hugs for performance debugging (in particular, trying
+to understand and reduce the heap usage of a program) --- an area of
+increasing importance as Haskell is used more and more to solve ``real
+problems'' in ``real problem domains''.
-\item The {\em Blocked Fetch list} (or
-lists?). (Section~\ref{sect:BLOCKED_FETCH}.)
+\item
+Even if a CAF is no longer required for the current evaluation, we might
+choose to hang onto it by collecting it in the normal way. This keeps
+the space leak but might be useful in a teaching environment when
+trying to teach the difference between call by name evaluation (which
+doesn't share work) and lazy evaluation (which does share work).
-\item For each thread, there is a list of all update frames on the
-stack. (Section~\ref{sect:data-updates}.)
+\end{enumerate}
+It turns out that it is easy to support both styles of use, so the
+runtime system provides a switch which lets us turn this on and off
+during execution. \ToDo{What is this switch called?} It would also
+be easy to provide a function \verb+RevertCAF+ to let the interpreter
+revert any CAF it wanted between (but not during) executions, if we so
+desired. Running \verb+RevertCAF+ during execution would lose some sharing
+but is otherwise harmless.
\end{itemize}
-\ToDo{The links for these fields are usually inserted immediately
-after the fixed header except ...}
+% % begin{even more pointless observation?}
+% The simplest fix would be to remove the special treatment of
+% top level variables. This works but is very inefficient.
+% ToDo: say why.
+% (Note: delete this paragraph from final version.)
+% % end{even more pointless observation?}
+
+% begin{pointless observation?}
+An easy but inefficient fix to the CAF problem would be to make a
+complete copy of the heap before every evaluation and discard the copy
+after evaluation. This works but is inefficient.
+% end{pointless observation?}
-\subsection{Info Tables}
+An efficient way to achieve a similar effect is to revert all
+updatable thunks to their original form as they become unnecessary for
+the current evaluation. To do this, we modify the compiler to ensure
+that the only updatable thunks generated by the compiler are CAFs and
+we modify the garbage collector to revert entered CAFs to unentered
+CAFs as their value becomes unnecessary.
-An {\em info table} is a contiguous block of memory, {\em laid out
-backwards}. That is, the first field in the list that follows
-occupies the highest memory address, and the successive fields occupy
-successive decreasing memory addresses.
+\subsubsection{New Heap Objects}
+
+We add three new kinds of heap object: unentered CAF closures, entered
+CAF objects and CAF blackholes. We first describe how they are
+evaluated and then how they are garbage collected.
+\begin{itemize}
+\item
+Unentered CAF closures contain a pointer to closure representing the
+body of the CAF. The ``body closure'' is not updatable.
+
+Unentered CAF closures contain two unused fields to make them the same
+size as entered CAF closures --- which allows us to perform an inplace
+update. \ToDo{Do we have to add another kind of inplace update operation
+to the storage manager interface or do we consider this to be internal
+to the SM?}
\begin{center}
-\begin{tabular}{|c|}
- \hline Parallelism Info
-\\ \hline Profile Info
-\\ \hline Debug Info
-\\ \hline Tag/bytecode pointer
-\\ \hline Static reference table
-\\ \hline Storage manager layout info
-\\ \hline Closure type
-\\ \hline entry code \ldots
-\\ \hline
+\begin{tabular}{|l|l|l|l|}\hline
+\verb+CAF_unentered+ & \emph{body closure} & \emph{unused} & \emph{unused} \\ \hline
\end{tabular}
\end{center}
-An info table has the following contents (working backwards in memory
-addresses):
+When an unentered CAF is entered, we do the following:
\begin{itemize}
-\item The {\em entry code} for the closure.
-This code appears literally as the (large) last entry in the
-info table, immediately preceded by the rest of the info table.
-An {\em info pointer} always points to the first byte of the entry code.
-
-\item A one-word {\em closure type field}, @INFO_TYPE@, identifies what kind
-of closure the object is. The various types of closure are described
-in Section~\ref{sect:closures}.
-In some configurations, some useful properties of
-closures (is it a HNF? can it be sparked?)
-are represented as high-order bits so they can be tested quickly.
-
-\item A single pointer or word --- the {\em storage manager info field},
-@INFO_SM@, contains auxiliary information describing the closure's
-precise layout, for the benefit of the garbage collector and the code
-that stuffs graph into packets for transmission over the network.
-
-\item A one-pointer {\em Static Reference Table (SRT) pointer}, @INFO_SRT@, points to
-a table which enables the garbage collector to identify all accessible
-code and CAFs. They are fully described in Section~\ref{sect:srt}.
-
-\item A one-pointer {\em tag/bytecode-pointer} field, @INFO_TAG@ or @INFO_BC@.
-For data constructors this field contains the constructor tag, in the
-range $0..n-1$ where $n$ is the number of constructors.
-
-For other objects that can be entered this field points to the byte
-codes for the object. For the constructor case you can think of the
-tag as the name of a a suitable bytecode sequence but it can also be used to
-implement semi-tagging (section~\ref{sect:semi-tagging}).
+\item
+allocate a CAF black hole;
-One awkward question (which may not belong here) is ``how does the
-bytecode interpreter know whether to do a vectored return?'' The
-answer is it examines the @INFO_TYPE@ field of the return address:
-@RET_VEC_@$sz$ requires a vectored return and @RET_@$sz$ requires a
-direct return.
+\item
+push an update frame (to update the CAF black hole) onto the stack;
-\item {\em Profiling info\/}
+\item
+overwrite the CAF with an entered CAF object (see below) with the same
+body and whose value field points to the black hole;
-Closure category records are attached to the info table of the
-closure. They are declared with the info table. We put pointers to
-these ClCat things in info tables. We need these ClCat things because
-they are mutable, whereas info tables are immutable. Hashing will map
-similar categories to the same hash value allowing statistics to be
-grouped by closure category.
+\item
+add the CAF to a list of all entered CAFs (called ``the CAF list'');
+and
-Cost Centres and Closure Categories are hashed to provide indexes
-against which arbitrary information can be stored. These indexes are
-memoised in the appropriate cost centre or category record and
-subsequent hashes avoided by the index routine (it simply returns the
-memoised index).
+\item
+the closure representing the value of the CAF is entered.
-There are different features which can be hashed allowing information
-to be stored for different groupings. Cost centres have the cost
-centre recorded (using the pointer), module and group. Closure
-categories have the closure description and the type
-description. Records with the same feature will be hashed to the same
-index value.
+\end{itemize}
-The initialisation routines, @init_index_<feature>@, allocate a hash
-table in which the cost centre / category records are stored. The
-lower bound for the table size is taken from @max_<feature>_no@. They
-return the actual table size used (the next power of 2). Unused
-locations in the hash table are indicated by a 0 entry. Successive
-@init_index_<feature>@ calls just return the actual table size.
+When evaluation of the CAF body returns a value, the update frame
+causes the CAF black hole to be updated with the value in the normal
+way.
-Calls to @index_<feature>@ will insert the cost centre / category
-record in the @<feature>@ hash table, if not already inserted. The hash
-index is memoised in the record and returned.
+\ToDo{Add a picture}
-CURRENTLY ONLY ONE MEMOISATION SLOT IS AVILABLE IN EACH RECORD SO
-HASHING CAN ONLY BE DONE ON ONE FEATURE FOR EACH RECORD. This can be
-easily relaxed at the expense of extra memoisation space or continued
-rehashing.
+\item
+Entered CAF closures contain two pointers: a pointer to the CAF body
+(the same as for unentered CAF closures); a pointer to the CAF value
+(this is initialised with a CAF blackhole, as previously described);
+and a link to the next CAF in the CAF list
-The initialisation routines must be called before initialisation of
-the stacks and heap as they require to allocate storage. It is also
-expected that the caller may want to allocate additional storage in
-which to store profiling information based on the return table size
-value(s).
+\ToDo{How is the end of the list marked? Null pointer or sentinel value?}.
\begin{center}
-\begin{tabular}{|l|}
- \hline Hash Index
-\\ \hline Selected
-\\ \hline Kind
-\\ \hline Description String
-\\ \hline Type String
-\\ \hline
+\begin{tabular}{|l|l|l|l|}\hline
+\verb+CAF_entered+ & \emph{body closure} & \emph{value} & \emph{link} \\ \hline
\end{tabular}
\end{center}
+When an entered CAF is entered, it enters its value closure.
-\begin{description}
-\item[Hash Index] Memoised copy
-\item[Selected]
- Is this category selected (-1 == not memoised, selected? 0 or 1)
-\item[Kind]
-One of the following values (defined in CostCentre.lh):
+\item
+CAF blackholes are identical to normal blackholes except that they
+have a different infotable. The only reason for having CAF blackholes
+is to allow an optimisation of lazy blackholing where we stop scanning
+the stack when we see the first {\em normal blackhole} but not
+when we see a {\em CAF blackhole.}
+\ToDo{The optimisation we want to allow should be described elsewhere
+so that all we have to do here is describe the difference.}
+
+Instead of allocating a blackhole to update with the value of the CAF,
+it might seem simpler to update the CAF directly. This would require
+a new kind of update frame which would update the value field of the
+CAF with a pointer to the value and wouldn't catch blackholes caused
+by CAFs that depend on themselves so we chose not to do so.
-\begin{description}
-\item[@CON_K@]
-A constructor.
-\item[@FN_K@]
-A literal function.
-\item[@PAP_K@]
-A partial application.
-\item[@THK_K@]
-A thunk, or suspension.
-\item[@BH_K@]
-A black hole.
-\item[@ARR_K@]
-An array.
-\item[@ForeignObj_K@]
-A Foreign object (non-Haskell heap resident).
-\item[@SPT_K@]
-The Stable Pointer table. (There should only be one of these but it
-represents a form of weak space leak since it can't shrink to meet
-non-demand so it may be worth watching separately? ADR)
-\item[@INTERNAL_KIND@]
-Something internal to the runtime system.
-\end{description}
+\end{itemize}
+\subsubsection{Garbage Collection}
-\item[Description] Source derived string detailing closure description.
-\item[Type] Source derived string detailing closure type.
-\end{description}
+To avoid the space leak, each run of the garbage collector must revert
+the entered CAFs which are not required to complete the current
+evaluation (that is all the closures reachable from the set of
+runnable threads and the stable pointer table).
-\item {\em Parallelism info\/}
-\ToDo{}
+It does this by performing garbage collection in three phases:
+\begin{enumerate}
+\item
+During the first phase, we ``mark'' all closures reachable from the
+scheduler state.
-\item {\em Debugging info\/}
-\ToDo{}
+How we ``mark'' closures depends on the garbage collector. For
+example, in a 2-space collector, closures are ``marked'' by copying
+them into ``to-space'', overwriting them with a forwarding node and
+``marking'' all the closures reachable from the copy. The only
+requirements are that we can test whether a closure is marked and if a
+closure is marked then so are all closures reachable from it.
-\end{itemize}
+\ToDo{At present we say that the scheduler state includes any state
+that Hugs may have. This is not true anymore.}
+Performing this phase first provides us with a cheap test for
+execution closures: at this stage in execution, the execution closures
+are precisely the marked closures.
+\item
+During the second phase, we revert all unmarked CAFs on the CAF list
+and remove them from the CAF list.
-\section{Kinds of Heap Object}
-\label{sect:closures}
+Since the CAF list is exactly the set of all entered CAFs, this reverts
+all entered CAFs which are not execution closures.
-Heap objects can be classified in several ways, but one useful one is
-this:
-\begin{itemize}
-\item
-{\em Static closures} occupy fixed, statically-allocated memory
-locations, with globally known addresses.
+\item
+During the third phase, we mark all top level objects (including CAFs)
+by calling \verb+MarkHugsRoots+ which will call \verb+MarkRoot+ for
+each top level object known to Hugs.
+
+\end{enumerate}
+
+To implement the second style of interactive behaviour (where we
+deliberately keep the CAF-related space leak), we simply omit the
+second phase. Omitting the second phase causes the third phase to
+mark any unmarked CAF value closures.
-\item
-{\em Dynamic closures} are individually allocated in the heap.
+So far, we have been describing a pure Hugs system which contains no
+machine generated code. The main difference in a hybrid system is
+that GHC-generated code is statically allocated in memory instead of
+being dynamically allocated on the heap. We split both
+\verb+CAF_unentered+ and \verb+CAF_entered+ into two versions: a
+static and a dynamic version. The static and dynamic versions of each
+CAF differ only in whether they are moved during garbage collection.
+When reverting CAFs, we revert dynamic entered CAFs to dynamic
+unentered CAFs and static entered CAFs to static unentered CAFs.
-\item
-{\em Stack closures} are closures allocated within a thread's stack
-(which is itself a heap object). Unlike other closures, there are
-never any pointers to stack closures. Stack closures are discussed in
-Section~\ref{sect:stacks}.
-\end{itemize}
-A second useful classification is this:
-\begin{itemize}
-\item
-{\em Executive closures}, such as thunks and data constructors,
-participate directly in a program's execution. They can be subdivided into
-two kinds of objects according to their type:
-\begin{itemize}
-\item
-{\em Pointed objects}, represent values of a {\em pointed} type
-(<.pointed types launchbury.>) --i.e.~a type that includes $\bottom$ such as @Int@ or @Int# -> Int#@.
-\item {\em Unpointed objects}, represent values of a {\em unpointed} type --i.e.~a type that does not include $\bottom$ such as @Int#@ or @Array#@.
-\item {\em Activation frames}, represent ``continuations''. They are
-always stored on the stack and are never pointed to by heap objects or
-passed as arguments. \note{It's not clear if this will still be true
-once we support speculative evaluation.}
+\Section{The Bytecode Evaluator}{bytecode-evaluator}
-\end{itemize}
+This section describes how the Hugs interpreter interprets code in the
+same environment as compiled code executes. Both evaluation models
+use a common garbage collector, so they must agree on the form of
+objects in the heap.
-\item {\em Administrative closures}, such as stack objects and thread
-state objects, do not represent values in the original program.
-\end{itemize}
+Hugs interprets code by converting it to byte-code and applying a
+byte-code interpreter to it. Wherever possible, we try to ensure that
+the byte-code is all that is required to interpret a section of code.
+This means not dynamically generating info tables, and hence we can
+only have a small number of possible heap objects each with a statically
+compiled info table. Similarly for stack objects: in fact we only
+have one Hugs stack object, in which all information is tagged for the
+garbage collector.
-Only pointed objects can be entered. All pointed objects share a
-common header format: the ``pointed header''; while all unpointed
-objects share a common header format: the ``unpointed header''.
-\ToDo{Describe the difference and update the diagrams to mention
-an appropriate header type.}
+There is, however, one exception to this rule. Hugs must generate
+info tables for any constructors it is asked to compile, since the
+alternative is to force a context-switch each time compiled code
+enters a Hugs-built constructor, which would be prohibitively
+expensive.
-This section enumerates all the kinds of heap objects in the system.
-Each is identified by a distinct @INFO_TYPE@ tag in its info table.
+We achieve this simplicity by forgoing some of the optimisations used
+by compiled code:
+\begin{itemize}
+\item
-\ToDo{Check this table very carefully}
+Whereas compiled code has five different ways of entering a closure
+(\secref{entering-closures}), interpreted code has only one.
+The entry point for interpreted code behaves like slow entry points for
+compiled code.
-\begin{tabular}{|l|l|l|l|l|l|l|l|l|l|l|}
-\hline
+\item
-closure kind & HNF & UPD & NS & STA & THU & MUT & UPT & BH & IND & Section \\
+We use just one info table for \emph{all\/} direct returns.
+This introduces two problems:
+\begin{enumerate}
+\item How does the interpreter know what code to execute?
-\hline
-{\em Pointed} \\
-\hline
+Instead of pushing just a return address, we push a return BCO and a
+trivial return address which just enters the return BCO.
-@CONSTR@ & 1 & & 1 & & & & & & & \ref{sect:CONSTR} \\
-@CONSTR_STATIC@ & 1 & & 1 & 1 & & & & & & \ref{sect:CONSTR} \\
-@CONSTR_STATIC_NOCAF@ & 1 & & 1 & 1 & & & & & & \ref{sect:CONSTR} \\
-
-@FUN@ & 1 & & ? & & & & & & & \ref{sect:FUN} \\
-@FUN_STATIC@ & 1 & & ? & 1 & & & & & & \ref{sect:FUN} \\
-
-@THUNK@ & 1 & 1 & & & 1 & & & & & \ref{sect:THUNK} \\
-@THUNK_STATIC@ & 1 & 1 & & 1 & 1 & & & & & \ref{sect:THUNK} \\
-@THUNK_SELECTOR@ & 1 & 1 & 1 & & 1 & & & & & \ref{sect:THUNK_SEL} \\
-
-@PAP@ & 1 & & ? & & & & & & & \ref{sect:PAP} \\
-
-@IND@ & & & 1 & & ? & & & & 1 & \ref{sect:IND} \\
-@IND_OLDGEN@ & 1 & & 1 & & ? & & & & 1 & \ref{sect:IND} \\
-@IND_PERM@ & & & 1 & & ? & & & & 1 & \ref{sect:IND} \\
-@IND_OLDGEN_PERM@ & 1 & & 1 & & ? & & & & 1 & \ref{sect:IND} \\
-@IND_STATIC@ & ? & & 1 & 1 & ? & & & & 1 & \ref{sect:IND} \\
+(In a purely interpreted system, we could avoid pushing the trivial
+return address.)
-\hline
-{\em Unpointed} \\
-\hline
+\item How can the garbage collector follow pointers within the
+activation record?
+We could push a third word ---a bitmask describing the location of any
+pointers within the record--- but, since we're already tagging unboxed
+function arguments on the stack, we use the same mechanism for unboxed
+values within the activation record.
-@ARR_WORDS@ & 1 & & 1 & & & 1 & 1 & & & \ref{sect:ARR_WORDS1},\ref{sect:ARR_WORDS2} \\
-@ARR_PTRS@ & 1 & & 1 & & & 1 & 1 & & & \ref{sect:ARR_PTRS} \\
-@MUTVAR@ & 1 & & 1 & & & 1 & 1 & & & \ref{sect:MUTVAR} \\
-@MUTARR_PTRS@ & 1 & & 1 & & & 1 & 1 & & & \ref{sect:MUTARR_PTRS} \\
-@MUTARR_PTRS_FROZEN@ & 1 & & 1 & & & 1 & 1 & & & \ref{sect:MUTARR_PTRS_FROZEN} \\
+\ToDo{Do we have to stub out dead variables in the activation frame?}
-@FOREIGN@ & 1 & & 1 & & & & 1 & & & \ref{sect:FOREIGN} \\
+\end{enumerate}
-@BH@ & ? & 0/1 & 1 & & ? & ? & & 1 & ? & \ref{sect:BH} \\
-@MVAR@ & & & & & & & & & & \ref{sect:MVAR} \\
-@IVAR@ & & & & & & & & & & \ref{sect:IVAR} \\
-@FETCHME@ & & & & & & & & & & \ref{sect:FETCHME} \\
-\hline
-\end{tabular}
+\item
-Activation frames do not live (directly) on the heap --- but they have
-a similar organisation. The classification bits are all zero in
-activation frames.
+We trivially support vectored returns by pushing a return vector whose
+entries are all the same.
-\begin{tabular}{|l|l|}\hline
-closure kind & Section \\ \hline
-@RET_SMALL@ & \ref{sect:activation-records} \\
-@RET_VEC_SMALL@ & \ref{sect:activation-records} \\
-@RET_BIG@ & \ref{sect:activation-records} \\
-@RET_VEC_BIG@ & \ref{sect:activation-records} \\
-@UPDATE_FRAME@ & \ref{sect:activation-records} \\
-\hline
-\end{tabular}
+\item
-There are also a number of administrative objects. The classification bits are
-all zero in administrative objects.
+We avoid the need to build SRTs by putting bytecode objects on the
+heap and restricting BCOs to a single basic block.
-\begin{tabular}{|l|l|}\hline
-closure kind & Section \\ \hline
-@TSO@ & \ref{sect:TSO} \\
-@STACK_OBJECT@ & \ref{sect:STACK_OBJECT} \\
-@STABLEPTR_TABLE@ & \ref{sect:STABLEPTR_TABLE} \\
-@SPARK_OBJECT@ & \ref{sect:SPARK} \\
-@BLOCKED_FETCH@ & \ref{sect:BLOCKED_FETCH} \\
-\hline
-\end{tabular}
+\end{itemize}
-\ToDo{I guess the parallel system has something like a stable ptr
-table. Is there any opportunity for sharing code/data structures
-here?}
+\Subsection{Hugs Info Tables}{hugs-info-tables}
+Hugs requires the following info tables and closures:
+\begin{description}
+\item [@HUGS_RET@].
-\subsection{Classification bits}
+Contains both a vectored return table and a direct entry point. All
+entry points are the same: they rearrange the stack to match the Hugs
+return convention (\secref{hugs-return-convention}) and return to the
+scheduler. When the scheduler restarts the thread, it will find a BCO
+on top of the stack and will enter the Hugs interpreter.
-The top bits of the @INFO_TYPE@ tag tells what sort of animal the
-closure is.
+\item [@UPD_RET@].
-\begin{tabular}{|l|l|l|} \hline
-Abbrev & Bit & Interpretation \\ \hline
-HNF & 0 & 1 $\Rightarrow$ Head normal form \\
-UPD & 4 & 1 $\Rightarrow$ May be updated (inconsistent with being a HNF) \\
-NS & 1 & 1 $\Rightarrow$ Don't spark me (Any HNF will have this set to 1)\\
-STA & 2 & 1 $\Rightarrow$ This is a static closure \\
-THU & 8 & 1 $\Rightarrow$ Is a thunk \\
-MUT & 3 & 1 $\Rightarrow$ Has mutable pointer fields \\
-UPT & 5 & 1 $\Rightarrow$ Has an unpointed type (eg a primitive array) \\
-BH & 6 & 1 $\Rightarrow$ Is a black hole \\
-IND & 7 & 1 $\Rightarrow$ Is an indirection \\
-\hline
-\end{tabular}
+This is just the standard info table for an update frame.
-Updatable structures (@_UP@) are thunks that may be shared. Primitive
-arrays (@_BM@ -- Big Mothers) are structures that are always held
-in-memory (basically extensions of a closure). Because there may be
-offsets into these arrays, a primitive array cannot be handled as a
-FetchMe in the parallel system, but must be shipped in its entirety if
-its parent closure is shipped.
+\item [Constructors].
-The other bits in the info-type field simply give a unique bit-pattern
-to identify the closure type.
+The entry code for a constructor jumps to a generic entry point in the
+runtime system which decides whether to do a vectored or unvectored
+return depending on the shape of the constructor/type. This implies that
+info tables must have enough info to make that decision.
-\iffalse
-@
-#define _NF 0x0001 /* Normal form */
-#define _NS 0x0002 /* Don't spark */
-#define _ST 0x0004 /* Is static */
-#define _MU 0x0008 /* Is mutable */
-#define _UP 0x0010 /* Is updatable (but not mutable) */
-#define _BM 0x0020 /* Is a "primitive" array */
-#define _BH 0x0040 /* Is a black hole */
-#define _IN 0x0080 /* Is an indirection */
-#define _TH 0x0100 /* Is a thunk */
+\item [@AP@ and @PAP@].
+\item [Indirections].
+\item [Selectors].
-SPEC
-SPEC_N SPEC | _NF | _NS
-SPEC_S SPEC | _TH
-SPEC_U SPEC | _UP | _TH
-
-GEN
-GEN_N GEN | _NF | _NS
-GEN_S GEN | _TH
-GEN_U GEN | _UP | _TH
-
-DYN _NF | _NS
-TUPLE _NF | _NS | _BM
-DATA _NF | _NS | _BM
-MUTUPLE _NF | _NS | _MU | _BM
-IMMUTUPLE _NF | _NS | _BM
-STATIC _NS | _ST
-CONST _NF | _NS
-CHARLIKE _NF | _NS
-INTLIKE _NF | _NS
+Hugs doesn't generate them itself but it ought to recognise them
-BH _NS | _BH
-BH_N BH
-BH_U BH | _UP
-
-BQ _NS | _MU | _BH
-IND _NS | _IN
-CAF _NF | _NS | _ST | _IN
+\item [Complex primops].
-FM
-FETCHME FM | _MU
-FMBQ FM | _MU | _BH
+Some of the primops are too complex for GHC to generate inline.
+Instead, these primops are hand-written and called as normal functions.
+Hugs only needs to know their names and types but doesn't care whether
+they are generated by GHC or by hand. Two things to watch:
-TSO _MU
+\begin{enumerate}
+\item
+Hugs must be able to enter these primops even if it is working on a
+standalone system that does not support genuine GHC generated code.
-STKO
-STKO_DYNAMIC STKO | _MU
-STKO_STATIC STKO | _ST
-
-SPEC_RBH _NS | _MU | _BH
-GEN_RBH _NS | _MU | _BH
-BF _NS | _MU | _BH
-INTERNAL
+\item The complex primops often involve unboxed tuple types (which
+Hugs does not support at the source level) so we cannot specify their
+types in a Haskell source file.
-@
-\fi
+\end{enumerate}
-Notes:
+\end{description}
-An indirection either points to HNF (post update); or is result of
-overwriting a FetchMe, in which case the thing fetched is either
-under evaluation (BH), or by now an HNF. Thus, indirections get NoSpark flag.
+\Subsection{Hugs Heap Objects}{hugs-heap-objects}
+\subsubsection{Byte-code objects}
+Compiled byte code lives on the global heap, in objects called
+Byte-Code Objects (or BCOs). The layout of BCOs is described in
+detail in \secref{BCO}, in this section we will describe
+their semantics.
-\subsection{Pointed Objects}
+Since byte-code lives on the heap, it can be garbage collected just
+like any other heap-resident data. Hugs arranges that any BCO's
+referred to by the Hugs symbol tables are treated as live objects by
+the garbage collector. When a module is unloaded, the pointers to its
+BCOs are removed from the symbol table, and the code will be garbage
+collected some time later.
-All pointed objects can be entered.
+A BCO represents a basic block of code --- the (only) entry points is
+at the beginning of a BCO, and it is impossible to jump into the
+middle of one. A BCO represents not only the code for a function, but
+also its closure; a BCO can be entered just like any other closure.
+Hugs performs lambda-lifting during compilation to byte-code, and each
+top-level combinator becomes a BCO in the heap.
-\subsubsection{Function closures}\label{sect:FUN}
-Function closures represent lambda abstractions. For example,
-consider the top-level declaration:
-@
- f = \x -> let g = \y -> x+y
- in g x
-@
-Both @f@ and @g@ are represented by function closures. The closure
-for @f@ is {\em static} while that for @g@ is {\em dynamic}.
+\subsubsection{Thunks and partial applications}
-The layout of a function closure is as follows:
-\begin{center}
-\begin{tabular}{|l|l|l|l|}\hline
-{\em Fixed header} & {\em Pointers} & {\em Non-pointers} \\ \hline
-\end{tabular}
-\end{center}
-The data words (pointers and non-pointers) are the free variables of
-the function closure.
-The number of pointers
-and number of non-pointers are stored in the @INFO_SM@ word, in the least significant
-and most significant half-word respectively.
+A thunk consists of a code pointer, and values for the free variables
+of that code. Since Hugs byte-code is lambda-lifted, free variables
+become arguments and are expected to be on the stack by the called
+function.
-There are several different sorts of function closure, distinguished
-by their @INFO_TYPE@ field:
-\begin{itemize}
-\item @FUN@: a vanilla, dynamically allocated on the heap.
+Hugs represents updateable thunks with @AP@ objects applying a closure
+to a list of arguments. (As for @PAP@s, unboxed arguments should be
+preceded by a tag.) When it is entered, it pushes an update frame
+followed by its payload on the stack, and enters the first word (which
+will be a pointer to a BCO). The layout of @AP@ objects is described
+in more detail in \secref{AP}.
-\item $@FUN_@p@_@np$: to speed up garbage collection a number of
-specialised forms of @FUN@ are provided, for particular $(p,np)$ pairs,
-where $p$ is the number of pointers and $np$ the number of non-pointers.
+Partial applications are represented by @PAP@ objects, which are
+non-updatable.
-\item @FUN_STATIC@. Top-level, static, function closures (such as
-@f@ above) have a different
-layout than dynamic ones:
-\begin{center}
-\begin{tabular}{|l|l|l|}\hline
-{\em Fixed header} & {\em Static object link} \\ \hline
-\end{tabular}
-\end{center}
-Static function closurs have no free variables. (However they may refer to other
-static closures; these references are recorded in the function closure's SRT.)
-They have one field that is not present in dynamic closures, the {\em static object
-link} field. This is used by the garbage collector in the same way that to-space
-is, to gather closures that have been determined to be live but that have not yet
-been scavenged.
-\note{Static function closures that have no static references, and hence
-a null SRT pointer, don't need the static object link field. Is it worth
-taking advantage of this? See @CONSTR_STATIC_NOCAF@.}
-\end{itemize}
+\ToDo{Hugs Constructors}.
-Each lambda abstraction, $f$, in the STG program has its own private info table.
-The following labels are relevant:
+\Subsection{Calling conventions}{hugs-calling-conventions}
+
+The calling convention for any byte-code function is straightforward:
\begin{itemize}
-\item $f$@_info@ is $f$'s info table.
-\item $f$@_entry@ is $f$'s slow entry point (i.e. the entry code of its
-info table; so it will label the same byte as $f$@_info@).
-\item $f@_fast_@k$ is $f$'s fast entry point. $k$ is the number of arguments
-$f$ takes; encoding this number in the fast-entry label occasionally catches some nasty
-code-generation errors.
+\item Push any arguments on the stack.
+\item Push a pointer to the BCO.
+\item Begin interpreting the byte code.
\end{itemize}
-\subsubsection{Data Constructors}\label{sect:CONSTR}
+In a system containing both GHC and Hugs, the bytecode interpreter
+only has to be able to enter BCOs: everything else can be handled by
+returning to the compiled world (as described in
+\secref{hugs-to-ghc-switch}) and entering the closure
+there.
+
+This would work but it would obviously be very inefficient if we
+entered a @AP@ by switching worlds, entering the @AP@, pushing the
+arguments and function onto the stack, and entering the function
+which, likely as not, will be a byte-code object which we will enter
+by \emph{returning} to the byte-code interpreter. To avoid such
+gratuitious world switching, we choose to recognise certain closure
+types as being ``standard'' --- and duplicate the entry code for the
+``standard closures'' in the bytecode interpreter.
+
+A closure is said to be ``standard'' if its entry code is entirely
+determined by its info table. \emph{Standard Closures} have the
+desirable property that the byte-code interpreter can enter the
+closure by simply ``interpreting'' the info table instead of switching
+to the compiled world. The standard closures include:
-Data-constructor closures represent values constructed with
-algebraic data type constructors.
-The general layout of data constructors is the same as that for function
-closures. That is
-\begin{center}
-\begin{tabular}{|l|l|l|l|}\hline
-{\em Fixed header} & {\em Pointers} & {\em Non-pointers} \\ \hline
-\end{tabular}
-\end{center}
+\begin{description}
+\item[Constructor] To enter a constructor, we simply return (see
+\secref{hugs-return-convention}).
-The SRT pointer in a data constructor's info table is never used --- the
-code for a constructor does not make any static references.
-\note{Use it for something else?? E.g. tag?}
+\item[Indirection]
+To enter an indirection, we simply enter the object it points to
+after possibly adjusting the current cost centre.
-There are several different sorts of constructor:
-\begin{itemize}
-\item @CONSTR@: a vanilla, dynamically allocated constructor.
-\item @CONSTR_@$p$@_@$np$: just like $@FUN_@p@_@np$.
-\item @CONSTR_INTLIKE@.
-A dynamically-allocated heap object that looks just like an @Int@. The
-garbage collector checks to see if it can common it up with one of a fixed
-set of static int-like closures, thus getting it out of the dynamic heap
-altogether.
+\item[@AP@]
-\item @CONSTR_CHARLIKE@: same deal, but for @Char@.
+To enter an @AP@, we push an update frame, push the
+arguments, push the function and enter the function.
+(Not forgetting a stack check at the start.)
-\item @CONSTR_STATIC@ is similar to @FUN_STATIC@, with the complication that
-the layout of the constructor must mimic that of a dynamic constructor,
-because a static constructor might be returned to some code that unpacks it.
-So its layout is like this:
-\begin{center}
-\begin{tabular}{|l|l|l|l|l|}\hline
-{\em Fixed header} & {\em Pointers} & {\em Non-pointers} & {\em Static object link}\\ \hline
-\end{tabular}
-\end{center}
-The static object link, at the end of the closure, serves the same purpose
-as that for @FUN_STATIC@. The pointers in the static constructor can point
-only to other static closures.
+\item[@PAP@]
-The static object link occurs last in the closure so that static
-constructors can store their data fields in exactly the same place as
-dynamic constructors.
+To enter a @PAP@, we push the arguments, push the function and enter
+the function. (Not forgetting a stack check at the start.)
-\item @CONSTR_STATIC_NOCAF@. A statically allocated data constructor
-that guarantees not to point (directly or indirectly) to any CAF
-(section~\ref{sect:CAF}). This means it does not need a static object
-link field. Since we expect that there might be quite a lot of static
-constructors this optimisation makes sense. Furthermore, the @NOCAF@
-tag allows the compiler to indicate that no CAFs can be reached
-anywhere {\em even indirectly}.
+\item[Selector]
+To enter a selector (\secref{THUNK_SEL}), we test whether the
+selectee is a value. If so, we simply select the appropriate
+component; if not, it's simplest to treat it as a GHC-built closure
+--- though we could interpret it if we wanted.
-\end{itemize}
+\end{description}
-For each data constructor $Con$, two
-info tables are generated:
-\begin{itemize}
-\item $Con$@_info@ labels $Con$'s dynamic info table,
-shared by all dynamic instances of the constructor.
-\item $Con$@_static@ labels $Con$'s static info table,
-shared by all static instances of the constructor.
-\end{itemize}
+The most obvious omissions from the above list are @BCO@s (which we
+dealt with above) and GHC-built closures (which are covered in
+\secref{hugs-to-ghc-switch}).
-\subsubsection{Thunks}
-\label{sect:THUNK}
-\label{sect:THUNK_SEL}
-A thunk represents an expression that is not obviously in head normal
-form. For example, consider the following top-level definitions:
+\Subsection{Return convention}{hugs-return-convention}
+
+When Hugs pushes a return address, it pushes both a pointer to the BCO
+to return to, and a pointer to a static code fragment @HUGS_RET@ (this
+is described in \secref{ghc-to-hugs-switch}). The
+stack layout is shown in \figref{hugs-return-stack}.
+
+\begin{figure}[ht]
+\begin{center}
@
- range = between 1 10
- f = \x -> let ys = take x range
- in sum ys
+| stack |
++----------+
+| bco |--> BCO
++----------+
+| HUGS_RET |
++----------+
@
-Here the right-hand sides of @range@ and @ys@ are both thunks; the former
-is static while the latter is dynamic.
+%\input{hugs_ret.pstex_t}
+\end{center}
+\caption{Stack layout for a Hugs return address}
+\label{fig:hugs-return-stack}
+\end{figure}
-The layout of a thunk is the same as that for a function closure,
-except that it may have some words of ``slop'' at the end to make sure
-that it has
-at least @MIN_UPD_PAYLOAD@ words in addition to its
-fixed header.
+\begin{figure}[ht]
\begin{center}
-\begin{tabular}{|l|l|l|l|l|}\hline
-{\em Fixed header} & {\em Pointers} & {\em Non-pointers} & {\em Slop} \\ \hline
-\end{tabular}
+@
+| stack |
++----------+
+| con |--> CON
++----------+
+@
+%\input{hugs_ret2.pstex_t}
\end{center}
-The @INFO_SM@ word contains the same information as for function
-closures; that is, number of pointers and number of non-pointers (excluding slop).
-
-A thunk differs from a function closure in that it can be updated.
-
-There are several forms of thunk:
-\begin{itemize}
-\item @THUNK@: a vanilla, dynamically allocated thunk.
-The garbage collection code for thunks whose
-pointer + non-pointer words is less than @MIN_UPD_PAYLOAD@ differs from
-that for function closures and data constructors, because the GC code
-has to account for the slop.
-\item $@THUNK_@p@_@np$. Similar comments apply.
-\item @THUNK_STATIC@. A static thunk is also known as
-a {\em constant applicative form}, or {\em CAF}.
+\caption{Stack layout on enterings a Hugs return address}
+\label{fig:hugs-return2}
+\end{figure}
+\begin{figure}[ht]
\begin{center}
-\begin{tabular}{|l|l|l|l|l|}\hline
-{\em Fixed header} & {\em Pointers} & {\em Non-pointers} & {\em Slop} & {\em Static object link}\\ \hline
-\end{tabular}
+@
+| stack |
++----------+
+| 3# |
++----------+
+| I# |
++----------+
+@
+%\input{hugs_ret2.pstex_t}
\end{center}
+\caption{Stack layout on entering a Hugs return address with an unboxed value}
+\label{fig:hugs-return-int}
+\end{figure}
-\item @THUNK_SELECTOR@ is a (dynamically allocated) thunk
-whose entry code performs a simple selection operation from
-a data constructor drawn from a single-constructor type. For example,
-the thunk
+\begin{figure}[ht]
+\begin{center}
@
- x = case y of (a,b) -> a
+| stack |
++----------+
+| ghc_ret |
++----------+
+| con |--> CON
++----------+
@
-is a selector thunk. A selector thunk is laid out like this:
-\begin{center}
-\begin{tabular}{|l|l|l|l|}\hline
-{\em Fixed header} & {\em Selectee pointer} \\ \hline
-\end{tabular}
+%\input{hugs_ret3.pstex_t}
\end{center}
-The @INFO_SM@ word contains the byte offset of the desired word in
-the selectee. Note that this is different from all other thunks.
-
-The garbage collector ``peeks'' at the selectee's
-tag (in its info table). If it is evaluated, then it goes ahead and do
-the selection, and then behaves just as if the selector thunk was an
-indirection to the selected field.
-If it is not
-evaluated, it treats the selector thunk like any other thunk of that
-shape. [Implementation notes.
-Copying: only the evacuate routine needs to be special.
-Compacting: only the PRStart (marking) routine needs to be special.]
-\end{itemize}
-
-
-The only label associated with a thunk is its info table:
-\begin{description}
-\item[$f$@_info@] is $f$'s info table.
-\end{description}
-
-
-\subsubsection{Partial applications (PAPs)}\label{sect:PAP}
+\caption{Stack layout on enterings a GHC return address}
+\label{fig:hugs-return3}
+\end{figure}
-A partial application (PAP) represents a function applied to too few arguments.
-It is only built as a result of updating after an argument-satisfaction
-check failure. A PAP has the following shape:
+\begin{figure}[ht]
\begin{center}
-\begin{tabular}{|l|l|l|l|}\hline
-{\em Fixed header} & {\em No of arg words} & {\em Function closure} & {\em Arg stack} \\ \hline
-\end{tabular}
+@
+| stack |
++----------+
+| ghc_ret |
++----------+
+| 3# |
++----------+
+| I# |
++----------+
+| restart |--> id_Int#_closure
++----------+
+@
+%\input{hugs_ret2.pstex_t}
\end{center}
-The ``arg stack'' is a copy of of the chunk of stack above the update
-frame; ``no of arg words'' tells how many words it consists of. The
-function closure is (a pointer to) the closure for the function whose
-argument-satisfaction check failed.
+\caption{Stack layout on enterings a GHC return address with an unboxed value}
+\label{fig:hugs-return-int}
+\end{figure}
-There is just one standard form of PAP with @INFO_TYPE@ = @PAP@.
-There is just one info table too, called @PAP_info@.
-Its entry code simply copies the arg stack chunk back on top of the
-stack and enters the function closure. (It has to do a stack overflow test first.)
+When a Hugs byte-code sequence enters a closure, it examines the
+return address on top of the stack.
-There are no static PAPs.
+\begin{itemize}
-\subsubsection{Indirections}
-\label{sect:IND}
-\label{sect:IND_OLDGEN}
+\item If the return address is @HUGS_RET@, pop the @HUGS_RET@ and the
+bco for the continuation off the stack, push a pointer to the constructor onto
+the stack and enter the BCO with the current object pointer set to the BCO
+(\figref{hugs-return2}).
-Indirection closures just point to other closures. They are introduced
-when a thunk is updated to point to its value.
-The entry code for all indirections simply enters the closure it points to.
+\item If the top of the stack is not @HUGS_RET@, we need to do a world
+switch as described in \secref{hugs-to-ghc-switch}.
-There are several forms of indirection:
-\begin{description}
-\item[@IND@] is the vanilla, dynamically-allocated indirection.
-It is removed by the garbage collector. It has the following
-shape:
-\begin{center}
-\begin{tabular}{|l|l|l|}\hline
-{\em Fixed header} & {\em Target closure} \\ \hline
-\end{tabular}
-\end{center}
+\end{itemize}
-\item[@IND_OLDGEN@] is the indirection used to update an old-generation
-thunk. Its shape is like this:
-\begin{center}
-\begin{tabular}{|l|l|l|}\hline
-{\em Fixed header} & {\em Mutable link field} & {\em Target closure} \\ \hline
-\end{tabular}
-\end{center}
-It contains a {\em mutable link field} that is used to string together
-all old-generation indirections that might have a pointer into
-the new generation.
+\ToDo{This duplicates what we say about switching worlds
+(\secref{switching-worlds}) - kill one or t'other.}
-\item[@IND_PERMANENT@ and @IND_OLDGEN_PERMANENT@.]
-for lexical profiling, it is necessary to maintain cost centre
-information in an indirection, so ``permanent indirections'' are
-retained forever. Otherwise they are just like vanilla indirections.
-\note{If a permanent indirection points to another permanent
-indirection or a @CONST@ closure, it is possible to elide the indirection
-since it will have no effect on the profiler.}
-\note{Do we still need @IND@ and @IND_OLDGEN@
-in the profiling build, or can we just make
-do with one pair whose behaviour changes when profiling is built?}
+\ToDo{This was in the evaluation model part but it really belongs in
+this part which is about the internal details of each of the major
+sections.}
-\item[@IND_STATIC@] is used for overwriting CAFs when they have been
-evaluated. Static indirections are not removed by the garbage
-collector; and are statically allocated outside the heap (and should
-stay there). Their static object link field is used just as for
-@FUN_STATIC@ closures.
+\Subsection{Addressing Modes}{hugs-addressing-modes}
-\begin{center}
-\begin{tabular}{|l|l|l|}
-\hline
-{\em Fixed header} & {\em Target closure} & {\em Static object link} \\
-\hline
-\end{tabular}
-\end{center}
+To avoid potential alignment problems and simplify garbage collection,
+all literal constants are stored in two tables (one boxed, the other
+unboxed) within each BCO and are referred to by offsets into the tables.
+Slots in the constant tables are word aligned.
+
+\ToDo{How big can the offsets be? Is the offset specified in the
+address field or in the instruction?}
-\end{description}
+Literals can have the following types: char, int, nat, float, double,
+and pointer to boxed object. There is no real difference between
+char, int, nat and float since they all occupy 32 bits --- but it
+costs almost nothing to distinguish them and may improve portability
+and simplify debugging.
-\subsubsection{Activation Records}
+\Subsection{Compilation}{hugs-compilation}
-Activation records are parts of the stack described by return address
-info tables (closures with @INFO_TYPE@ values of @RET_SMALL@,
-@RET_VEC_SMALL@, @RET_BIG@ and @RET_VEC_BIG@). They are described in
-section~\ref{sect:activation-records}.
+\def\is{\mbox{\it is}}
+\def\ts{\mbox{\it ts}}
+\def\as{\mbox{\it as}}
+\def\bs{\mbox{\it bs}}
+\def\cs{\mbox{\it cs}}
+\def\rs{\mbox{\it rs}}
+\def\us{\mbox{\it us}}
+\def\vs{\mbox{\it vs}}
+\def\ws{\mbox{\it ws}}
+\def\xs{\mbox{\it xs}}
-\subsubsection{Black holes, MVars and IVars}
-\label{sect:BH}
-\label{sect:MVAR}
-\label{sect:IVAR}
+\def\e{\mbox{\it e}}
+\def\alts{\mbox{\it alts}}
+\def\fail{\mbox{\it fail}}
+\def\panic{\mbox{\it panic}}
+\def\ua{\mbox{\it ua}}
+\def\obj{\mbox{\it obj}}
+\def\bco{\mbox{\it bco}}
+\def\tag{\mbox{\it tag}}
+\def\entry{\mbox{\it entry}}
+\def\su{\mbox{\it su}}
-Black hole closures are used to overwrite closures currently being
-evaluated. They inform the garbage collector that there are no live
-roots in the closure, thus removing a potential space leak.
+\def\Ind#1{{\mbox{\it Ind}\ {#1}}}
+\def\update#1{{\mbox{\it update}\ {#1}}}
-Black holes also become synchronization points in the threaded world.
-They contain a pointer to a list of blocked threads to be awakened
-when the black hole is updated (or @NULL@ if the list is empty).
-\begin{center}
-\begin{tabular}{|l|l|l|}
-\hline
-{\em Fixed header} & {\em Mutable link} & {\em Blocked thread link} \\
-\hline
-\end{tabular}
-\end{center}
-The {\em Blocked thread link} points to the TSO of the first thread
-waiting for the value of this thunk. All subsequent TSOs in the list
-are linked together using their @TSO_LINK@ field.
+\def\next{$\Longrightarrow$}
+\def\append{\mathrel{+\mkern-6mu+}}
+\def\reverse{\mbox{\it reverse}}
+\def\size#1{{\vert {#1} \vert}}
+\def\arity#1{{\mbox{\it arity}{#1}}}
-When the blocking queue is non-@NULL@, the black hole must be added to
-the mutables list since the TSOs on the list may contain pointers into
-the new generation. There is no need to clutter up the mutables list
-with black holes with empty blocking queues.
+\def\AP{\mbox{\it AP}}
+\def\PAP{\mbox{\it PAP}}
+\def\GHCRET{\mbox{\it GHCRET}}
+\def\GHCOBJ{\mbox{\it GHCOBJ}}
-\ToDo{MVars}
+To make sense of the instructions, we need a sense of how they will be
+used. Here is a small compiler for the STG language.
+@
+> cg (f{a1, ... am}) = do
+> pushAtom am; ... pushAtom a1
+> pushVar f
+> SLIDE (m+1) |env|
+> ENTER
+> cg (let {x1=rhs1; ... xm=rhsm} in e) = do
+> ALLOC x1 |rhs1|, ... ALLOC xm |rhsm|
+> build x1 rhs1, ... build xm rhsm
+> cg e
+> cg (case e of alts) = do
+> PUSHALTS (cgAlts alts)
+> cg e
+
+> cgAlts { alt1; ... altm } = cgAlt alt1 $ ... $ cgAlt altm pmFail
+>
+> cgAlt (x@C{xs} -> e) fail = do
+> TEST C fail
+> HEAPCHECK (heapUse e)
+> UNPACK xs
+> cg e
+
+> build x (C{a1, ... am}) = do
+> pushUntaggedAtom am; ... pushUntaggedAtom a1
+> PACK x C
+> -- A useful optimisation
+> build x ({v1, ... vm} \ {}. f{a1, ... am}) = do
+> pushVar am; ... pushVar a1
+> pushVar f
+> MKAP x m
+> build x ({v1, ... vm} \ {}. e) = do
+> pushVar vm; ... pushVar v1
+> PUSHBCO (cgRhs ({v1, ... vm} \ {}. e))
+> MKAP x m
+> build x ({v1, ... vm} \ {x1, ... xm}. e) = do
+> pushVar vm; ... pushVar v1
+> PUSHBCO (cgRhs ({v1, ... vm} \ {x1, ... xm}. e))
+> MKPAP x m
+
+> cgRhs (vs \ xs. e) = do
+> ARGCHECK (xs ++ vs) -- can be omitted if xs == {}
+> STACKCHECK min(stackUse e,heapOverflowSlop)
+> HEAPCHECK (heapUse e)
+> cg e
+
+> pushAtom x = pushVar x
+> pushAtom i# = PUSHINT i#
+
+> pushVar x = if isGlobalVar x then PUSHGLOBAL x else PUSHLOCAL x
+
+> pushUntaggedAtom x = pushVar x
+> pushUntaggedAtom i# = PUSHUNTAGGEDINT i#
+
+> pushVar x = if isGlobalVar x then PUSHGLOBAL x else PUSHLOCAL x
+@
-\subsubsection{FetchMes}\label{sect:FETCHME}
+\ToDo{Is there an easy way to add semi-tagging? Would it be that different?}
-In the parallel systems, FetchMes are used to represent pointers into
-the global heap. When evaluated, the value they point to is read from
-the global heap.
+\ToDo{Optimise thunks of the form @f{x1,...xm}@ so that we build an AP directly}
-\ToDo{Describe layout}
+\Subsection{Instructions}{hugs-instructions}
+We specify the semantics of instructions using transition rules of
+the form:
-\subsection{Unpointed Objects}
+\begin{tabular}{|llrrrrr|}
+\hline
+ & $\is$ & $s$ & $\su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is'$ & $s'$ & $\su'$ & $h'$ & $hp'$ & $\sigma$ \\
+\hline
+\end{tabular}
-A variable of unpointed type is always bound to a {\em value}, never to a {\em thunk}.
-For this reason, unpointed objects cannot be entered.
+where $\is$ is an instruction stream, $s$ is the stack, $\su$ is the
+update frame pointer and $h$ is the heap.
-A {\em value} may be:
-\begin{itemize}
-\item {\em Boxed}, i.e.~represented indirectly by a pointer to a heap object (e.g.~foreign objects, arrays); or
-\item {\em Unboxed}, i.e.~represented directly by a bit-pattern in one or more registers (e.g.~@Int#@ and @Float#@).
-\end{itemize}
-All {\em pointed} values are {\em boxed}.
-\subsubsection{Immutable Objects}
-\label{sect:ARR_WORDS1}
-\label{sect:ARR_PTRS}
+\Subsection{Stack manipulation}{hugs-stack-manipulation}
\begin{description}
-\item[@ARR_WORDS@] is a variable-sized object consisting solely of
-non-pointers. It is used for arrays of all
-sorts of things (bytes, words, floats, doubles... it doesn't matter).
-\begin{center}
-\begin{tabular}{|c|c|c|c|}
+
+\item[ Push a global variable ].
+
+\begin{tabular}{|llrrrrr|}
+\hline
+ & PUSHGLOBAL $o$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $\sigma!o:s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
\hline
-{\em Fixed Hdr} & {\em No of non-pointers} & {\em Non-pointers\ldots} \\ \hline
\end{tabular}
-\end{center}
-\item[@ARR_PTRS@] is an immutable, variable sized array of pointers.
-\begin{center}
-\begin{tabular}{|c|c|c|c|}
+\item[ Push a local variable ].
+
+\begin{tabular}{|llrrrrr|}
+\hline
+ & PUSHLOCAL $o$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $s!o : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
\hline
-{\em Fixed Hdr} & {\em Mutable link} & {\em No of pointers} & {\em Pointers\ldots} \\ \hline
\end{tabular}
-\end{center}
-The mutable link is present so that we can easily freeze and thaw an
-array (by changing the header and adding/removing the array to the
-mutables list).
-\end{description}
+\item[ Push an unboxed int ].
-\subsubsection{Mutable Objects}
-\label{sect:mutables}
-\label{sect:ARR_WORDS2}
-\label{sect:MUTVAR}
-\label{sect:MUTARR_PTRS}
-\label{sect:MUTARR_PTRS_FROZEN}
+\begin{tabular}{|llrrrrr|}
+\hline
+ & PUSHINT $o$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $I\# : \sigma!o : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\hline
+\end{tabular}
-Some of these objects are {\em mutable}; they represent objects which
-are explicitly mutated by Haskell code through the @ST@ monad.
-They're not used for thunks which are updated precisely once.
-Depending on the garbage collector, mutable closures may contain extra
-header information which allows a generational collector to implement
-the ``write barrier.''
+The $I\#$ is a tag included for the benefit of the garbage collector.
+Similar rules exist for floats, doubles, chars, etc.
-\begin{description}
+\item[ Push an unboxed int ].
-\item[@ARR_WORDS@] is also used to represent {\em mutable} arrays of
-bytes, words, floats, doubles, etc. It's possible to use the same
-object type because even generational collectors don't need to
-distinguish them.
+\begin{tabular}{|llrrrrr|}
+\hline
+ & PUSHUNTAGGEDINT $o$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $\sigma!o : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\hline
+\end{tabular}
-\item[@MUTVAR@] is a mutable variable.
-\begin{center}
-\begin{tabular}{|c|c|c|}
+Similar rules exist for floats, doubles, chars, etc.
+
+\item[ Delete environment from stack --- ready for tail call ].
+
+\begin{tabular}{|llrrrrr|}
+\hline
+ & SLIDE $m$ $n$ : $\is$ & $\as \append \bs \append \cs$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $\as \append \cs$ & $su$ & $h$ & $hp$ & $\sigma$ \\
\hline
-{\em Fixed Hdr} & {\em Mutable link} & {\em Pointer} \\ \hline
\end{tabular}
-\end{center}
+\\
+where $\size{\as} = m$ and $\size{\bs} = n$.
-\item[@MUTARR_PTRS@] is a mutable array of pointers.
-Such an array may be {\em frozen}, becoming an @SM_MUTARR_PTRS_FROZEN@, with a
-different info-table.
-\begin{center}
-\begin{tabular}{|c|c|c|c|}
+
+\item[ Push a return address ].
+
+\begin{tabular}{|llrrrrr|}
+\hline
+ & PUSHALTS $o$:$\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $@HUGS_RET@:\sigma!o:s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
\hline
-{\em Fixed Hdr} & {\em Mutable link} & {\em No of ptrs} & {\em Pointers\ldots} \\ \hline
\end{tabular}
-\end{center}
-\item[@MUTARR_PTRS_FROZEN@] is a frozen @MUTARR_PTRS@ closure.
-The garbage collector converts @MUTARR_PTRS_FROZEN@ to @ARR_PTRS@ as it removes them from
-the mutables list.
+\item[ Push a BCO ].
+
+\begin{tabular}{|llrrrrr|}
+\hline
+ & PUSHBCO $o$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $\sigma!o : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\hline
+\end{tabular}
\end{description}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\Subsection{Heap manipulation}{hugs-heap-manipulation}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-\subsubsection{Foreign Objects}\label{sect:FOREIGN}
+\begin{description}
-Here's what a ForeignObj looks like:
+\item[ Allocate a heap object ].
-\begin{center}
-\begin{tabular}{|l|l|l|l|}
-\hline
-{\em Fixed header} & {\em Data} & {\em Free Routine} & {\em Foreign object link} \\
+\begin{tabular}{|llrrrrr|}
+\hline
+ & ALLOC $m$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $hp:s$ & $su$ & $h$ & $hp+m$ & $\sigma$ \\
\hline
\end{tabular}
-\end{center}
-
-The @FreeRoutine@ is a reference to the finalisation routine to call
-when the @ForeignObj@ becomes garbage. If @freeForeignObject@ is
-called on a Foreign Object, the @FreeRoutine@ is set to zero and the
-garbage collector will not attempt to call @FreeRoutine@ when the
-object becomes garbage.
-The Foreign object link is a link to the next foreign object in the
-list. This list is traversed at the end of garbage collection: if an
-object is about to be deallocated (e.g.~it was not marked or
-evacuated), the free routine is called and the object is deleted from
-the list.
+\item[ Build a constructor ].
+\begin{tabular}{|llrrrrr|}
+\hline
+ & PACK $o$ $o'$ : $\is$ & $\ws \append s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $s$ & $su$ & $h[s!o \mapsto Pack C\{\ws\}]$ & $hp$ & $\sigma$ \\
+\hline
+\end{tabular}
+\\
+where $C = \sigma!o'$ and $\size{\ws} = \arity{C}$.
-The remaining objects types are all administrative --- none of them may be entered.
+\item[ Build an AP or PAP ].
-\subsection{Thread State Objects (TSOs)}\label{sect:TSO}
+\begin{tabular}{|llrrrrr|}
+\hline
+ & MKAP $o$ $m$:$\is$ & $f : \ws \append s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $s$ & $su$ & $h[s!o \mapsto \AP(f,\ws)]$ & $hp$ & $\sigma$ \\
+\hline
+\end{tabular}
+\\
+where $\size{\ws} = m$.
-In the multi-threaded system, the state of a suspended thread is
-packed up into a Thread State Object (TSO) which contains all the
-information needed to restart the thread and for the garbage collector
-to find all reachable objects. When a thread is running, it may be
-``unpacked'' into machine registers and various other memory locations
-to provide faster access.
+\begin{tabular}{|llrrrrr|}
+\hline
+ & MKPAP $o$ $m$:$\is$ & $f : \ws \append s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $s$ & $su$ & $h[s!o \mapsto \PAP(f,\ws)]$ & $hp$ & $\sigma$ \\
+\hline
+\end{tabular}
+\\
+where $\size{\ws} = m$.
-Single-threaded systems don't really {\em need\/} TSOs --- but they do
-need some way to tell the storage manager about live roots so it is
-convenient to use a single TSO to store the mutator state even in
-single-threaded systems.
+\item[ Unpacking a constructor ].
-Rather than manage TSOs' alloc/dealloc, etc., in some {\em ad hoc}
-way, we instead alloc/dealloc/etc them in the heap; then we can use
-all the standard garbage-collection/fetching/flushing/etc machinery on
-them. So that's why TSOs are ``heap objects,'' albeit very special
-ones.
-\begin{center}
-\begin{tabular}{|l|l|}
- \hline {\em Fixed header}
-\\ \hline @TSO_LINK@
-\\ \hline @TSO_WHATNEXT@
-\\ \hline @TSO_WHATNEXT_INFO@
-\\ \hline @TSO_STACK@
-\\ \hline {\em Exception Handlers}
-\\ \hline {\em Ticky Info}
-\\ \hline {\em Profiling Info}
-\\ \hline {\em Parallel Info}
-\\ \hline {\em GranSim Info}
-\\ \hline
+\begin{tabular}{|llrrrrr|}
+\hline
+ & UNPACK : $is$ & $a : s$ & $su$ & $h[a \mapsto C\ \ws]$ & $hp$ & $\sigma$ \\
+\next & $is'$ & $(\reverse\ \ws) \append a : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\hline
\end{tabular}
-\end{center}
-The contents of a TSO are:
-\begin{itemize}
-\item A pointer (@TSO_LINK@) used to maintain a list of threads with a similar
- state (e.g.~all runable, all sleeping, all blocked on the same black
- hole, all blocked on the same MVar, etc.)
+The $\reverse\ \ws$ looks expensive but, since the stack grows down
+and the heap grows up, that's actually the cheap way of copying from
+heap to stack. Looking at the compilation rules, you'll see that we
+always push the args in reverse order.
+
+\end{description}
-\item A word (@TSO_WHATNEXT@) which is in suspended threads to record
- how to awaken it. This typically requires a program counter which is stored
- in the pointer @TSO_WHATNEXT_INFO@
-\item A pointer (@TSO_STACK@) to the top stack block.
+\Subsection{Entering a closure}{hugs-entering}
-\item Optional information for ``Ticky Ticky'' statistics: @TSO_STK_HWM@ is
- the maximum number of words allocated to this thread.
+\begin{description}
-\item Optional information for profiling:
- @TSO_CCC@ is the current cost centre.
+\item[ Enter a BCO ].
-\item Optional information for parallel execution:
-\begin{itemize}
+\begin{tabular}{|llrrrrr|}
+\hline
+ & [ENTER] & $a : s$ & $su$ & $h[a \mapsto BCO\{\is\} ]$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $a : s$ & $su$ & $h$ & $hp$ & $a$ \\
+\hline
+\end{tabular}
-\item The types of threads (@TSO_TYPE@):
-\begin{description}
-\item[@T_MAIN@] Must be executed locally.
-\item[@T_REQUIRED@] A required thread -- may be exported.
-\item[@T_ADVISORY@] An advisory thread -- may be exported.
-\item[@T_FAIL@] A failure thread -- may be exported.
-\end{description}
+\item[ Enter a PAP closure ].
+
+\begin{tabular}{|llrrrrr|}
+\hline
+ & [ENTER] & $a : s$ & $su$ & $h[a \mapsto \PAP(f,\ws)]$ & $hp$ & $\sigma$ \\
+\next & [ENTER] & $f : \ws \append s$ & $su$ & $h$ & $hp$ & $???$ \\
+\hline
+\end{tabular}
-\item I've no idea what else
+\item[ Entering an AP closure ].
-\end{itemize}
+\begin{tabular}{|llrrrrr|}
+\hline
+ & [ENTER] & $a : s$ & $su$ & $h[a \mapsto \AP(f,ws)]$ & $hp$ & $\sigma$ \\
+\next & [ENTER] & $f : \ws \append @UPD_RET@:\su:a:s$ & $su'$ & $h$ & $hp$ & $???$ \\
+\hline
+\end{tabular}
-\item Optional information for GranSim execution:
+Optimisations:
\begin{itemize}
-\item locked
-\item sparkname
-\item started at
-\item exported
-\item basic blocks
-\item allocs
-\item exectime
-\item fetchtime
-\item fetchcount
-\item blocktime
-\item blockcount
-\item global sparks
-\item local sparks
-\item queue
-\item priority
-\item clock (gransim light only)
+\item Instead of blindly pushing an update frame for $a$, we can first test whether there's already
+ an update frame there. If so, overwrite the existing updatee with an indirection to $a$ and
+ overwrite the updatee field with $a$. (Overwriting $a$ with an indirection to the updatee also
+ works.) This results in update chains of maximum length 2.
\end{itemize}
-Here are the various queues for GrAnSim-type events.
-@
-Q_RUNNING
-Q_RUNNABLE
-Q_BLOCKED
-Q_FETCHING
-Q_MIGRATING
-@
-
-\end{itemize}
+\item[ Returning a constructor ].
-\subsection{Other weird objects}
-\label{sect:SPARK}
-\label{sect:BLOCKED_FETCH}
+\begin{tabular}{|llrrrrr|}
+\hline
+ & [ENTER] & $a : @HUGS_RET@ : \alts : s$ & $su$ & $h[a \mapsto C\{\ws\}]$ & $hp$ & $\sigma$ \\
+\next & $\alts.\entry$ & $a:s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\hline
+\end{tabular}
-\begin{description}
-\item[@BlockedFetch@ heap objects (`closures')] (parallel only)
-@BlockedFetch@s are inbound fetch messages blocked on local closures.
-They arise as entries in a local blocking queue when a fetch has been
-received for a local black hole. When awakened, we look at their
-contents to figure out where to send a resume.
+\item[ Entering an indirection node ].
-A @BlockedFetch@ closure has the form:
-\begin{center}
-\begin{tabular}{|l|l|l|l|l|l|}\hline
-{\em Fixed header} & link & node & gtid & slot & weight \\ \hline
+\begin{tabular}{|llrrrrr|}
+\hline
+ & [ENTER] & $a : s$ & $su$ & $h[a \mapsto \Ind{a'}]$ & $hp$ & $\sigma$ \\
+\next & [ENTER] & $a' : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\hline
\end{tabular}
-\end{center}
-\item[Spark Closures] (parallel only)
+\item[Entering GHC closure].
-Spark closures are used to link together all closures in the spark pool. When
-the current processor is idle, it may choose to speculatively evaluate some of
-the closures in the pool. It may also choose to delete sparks from the pool.
-\begin{center}
-\begin{tabular}{|l|l|l|l|l|l|}\hline
-{\em Fixed header} & {\em Spark pool link} & {\em Sparked closure} \\ \hline
+\begin{tabular}{|llrrrrr|}
+\hline
+ & [ENTER] & $a : s$ & $su$ & $h[a \mapsto \GHCOBJ]$ & $hp$ & $\sigma$ \\
+\next & [ENTERGHC] & $a : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\hline
\end{tabular}
-\end{center}
+\item[Returning a constructor to GHC].
+
+\begin{tabular}{|llrrrrr|}
+\hline
+ & [ENTER] & $a : \GHCRET : s$ & $su$ & $h[a \mapsto C \ws]$ & $hp$ & $\sigma$ \\
+\next & [ENTERGHC] & $a : \GHCRET : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\hline
+\end{tabular}
\end{description}
-\subsection{Stack Objects}
-\label{sect:STACK_OBJECT}
-\label{sect:stacks}
+\Subsection{Updates}{hugs-updates}
-These are ``stack objects,'' which are used in the threaded world as
-the stack for each thread is allocated from the heap in smallish
-chunks. (The stack in the sequential world is allocated outside of
-the heap.)
+\begin{description}
-Each reduction thread has to have its own stack space. As there may
-be many such threads, and as any given one may need quite a big stack,
-a naive give-'em-a-big-stack-and-let-'em-run approach will cost a {\em
-lot} of memory.
+\item[ Updating with a constructor].
-Our approach is to give a thread a small stack space, and then link
-on/off extra ``chunks'' as the need arises. Again, this is a
-storage-management problem, and, yet again, we choose to graft the
-whole business onto the existing heap-management machinery. So stack
-objects will live in the heap, be garbage collected, etc., etc..
+\begin{tabular}{|llrrrrr|}
+\hline
+ & [ENTER] & $a : @UPD_RET@ : ua : s$ & $su$ & $h[a \mapsto C\{\ws\}]$ & $hp$ & $\sigma$ \\
+\next & [ENTER] & $a \append s$ & $su$ & $h[au \mapsto \Ind{a}$ & $hp$ & $\sigma$ \\
+\hline
+\end{tabular}
-A stack object is laid out like this:
+\item[ Argument checks].
-\begin{center}
-\begin{tabular}{|l|}
+\begin{tabular}{|llrrrrr|}
+\hline
+ & ARGCHECK $m$:$\is$ & $a : \as \append s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $a : \as \append s$ & $su$ & $h'$ & $hp$ & $\sigma$ \\
\hline
-{\em Fixed header}
-\\ \hline
-{\em Link to next stack object (0 for last)}
-\\ \hline
-{\em N, the payload size in words}
-\\ \hline
-{\em @Sp@ (byte offset from head of object)}
-\\ \hline
-{\em @Su@ (byte offset from head of object)}
-\\ \hline
-{\em Payload (N words)}
-\\ \hline
\end{tabular}
-\end{center}
+\\
+where $m \ge (su - sp)$
-\ToDo{Are stack objects on the mutable list?}
+\begin{tabular}{|llrrrrr|}
+\hline
+ & ARGCHECK $m$:$\is$ & $a : \as \append @UPD_RET@:su:ua:s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $a : \as \append s$ & $su$ & $h'$ & $hp$ & $\sigma$ \\
+\hline
+\end{tabular}
+\\
+where $m < (su - sp)$ and
+ $h' = h[ua \mapsto \Ind{a'}, a' \mapsto \PAP(a,\reverse\ \as) ]$
-The stack grows downwards, towards decreasing
-addresses. This makes it easier to print out the stack
-when debugging, and it means that a return address is
-at the lowest address of the chunk of stack it ``knows about''
-just like an info pointer on a closure.
+Again, we reverse the list of values as we transfer them from the
+stack to the heap --- reflecting the fact that the stack and heap grow
+in different directions.
-The garbage collector needs to be able to find all the
-pointers in a stack. How does it do this?
+\end{description}
-\begin{itemize}
+\Subsection{Branches}{hugs-branches}
-\item Within the stack there are return addresses, pushed
-by @case@ expressions. Below a return address (i.e. at higher
-memory addresses, since the stack grows downwards) is a chunk
-of stack that the return address ``knows about'', namely the
-activation record of the currently running function.
+\begin{description}
-\item Below each such activation record is a {\em pending-argument
-section}, a chunk of
-zero or more words that are the arguments to which the result
-of the function should be applied. The return address does not
-statically
-``know'' how many pending arguments there are, or their types.
-(For example, the function might return a result of type $\alpha$.)
+\item[ Testing a constructor ].
-\item Below each pending-argument section is another return address,
-and so on. Actually, there might be an update frame instead, but we
-can consider update frames as a special case of a return address with
-a well-defined activation record.
+\begin{tabular}{|llrrrrr|}
+\hline
+ & TEST $tag$ $is'$ : $is$ & $a : s$ & $su$ & $h[a \mapsto C\ \ws]$ & $hp$ & $\sigma$ \\
+\next & $is$ & $a : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\hline
+\end{tabular}
+\\
+where $C.\tag = tag$
+
+\begin{tabular}{|llrrrrr|}
+\hline
+ & TEST $tag$ $is'$ : $is$ & $a : s$ & $su$ & $h[a \mapsto C\ \ws]$ & $hp$ & $\sigma$ \\
+\next & $is'$ & $a : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\hline
+\end{tabular}
+\\
+where $C.\tag \neq tag$
+
+\end{description}
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\Subsection{Heap and stack checks}{hugs-heap-stack-checks}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+
+\begin{tabular}{|llrrrrr|}
+\hline
+ & STACKCHECK $stk$:$\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\hline
+\end{tabular}
+\\
+if $s$ has $stk$ free slots.
+
+\begin{tabular}{|llrrrrr|}
+\hline
+ & HEAPCHECK $hp$:$\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\hline
+\end{tabular}
+\\
+if $h$ has $hp$ free slots.
+
+If either check fails, we push the current bco ($\sigma$) onto the
+stack and return to the scheduler. When the scheduler has fixed the
+problem, it pops the top object off the stack and reenters it.
-\ToDo{Doesn't it {\em have} to be an update frame? After all, the arg
-satisfaction check is @Su - Sp >= ...@.}
+Optimisations:
+\begin{itemize}
+\item The bytecode CHECK1000 conservatively checks for 1000 words of heap space and 1000 words of stack space.
+ We use it to reduce code space and instruction decoding time.
+\item The bytecode HEAPCHECK1000 conservatively checks for 1000 words of heap space.
+ It is used in case alternatives.
\end{itemize}
-The game plan is this. The garbage collector
-walks the stack from the top, traversing pending-argument sections and
-activation records alternately. Next we discuss how it finds
-the pointers in each of these two stack regions.
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\Subsection{Primops}{hugs-primops}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-\subsubsection{Activation records}\label{sect:activation-records}
+\ToDo{primops take m words and return n words. The expect boxed arguments on the stack.}
-An {\em activation record} is a contiguous chunk of stack,
-with a return address as its first word, followed by as many
-data words as the return address ``knows about''. The return
-address is actually a fully-fledged info pointer. It points
-to an info table, replete with:
-\begin{itemize}
-\item entry code (i.e. the code to return to).
-\item @INFO_TYPE@ is either @RET_SMALL/RET_VEC_SMALL@ or @RET_BIG/RET_VEC_BIG@, depending
-on whether the activation record has more than 32 data words (\note{64 for 8-byte-word architectures}) and on whether
-to use a direct or a vectored return.
-\item @INFO_SM@ for @RET_SMALL@ is a bitmap telling the layout
-of the activation record, one bit per word. The least-significant bit
-describes the first data word of the record (adjacent to the fixed
-header) and so on. A ``@1@'' indicates a non-pointer, a ``@0@''
-indicates
-a pointer. We don't need to indicate exactly how many words there
-are,
-because when we get to all zeros we can treat the rest of the
-activation record as part of the next pending-argument region.
+\Section{The Machine Code Evaluator}{asm-evaluator}
-For @RET_BIG@ the @INFO_SM@ field points to a block of bitmap
-words, starting with a word that tells how many words are in
-the block.
+This section describes the framework in which compiled code evaluates
+expressions. Only at certain points will compiled code need to be
+able to talk to the interpreted world; these are discussed in
+\secref{switching-worlds}.
-\item @INFO_SRT@ is the Static Reference Table for the return
-address (Section~\ref{sect:srt}).
-\end{itemize}
+\Subsection{Calling conventions}{ghc-calling-conventions}
-The activation record is a fully fledged closure too.
-As well as an info pointer, it has all the other attributes of
-a fixed header (Section~\ref{sect:fixed-header}) including a saved cost
-centre which is reloaded when the return address is entered.
+\Subsubsection{The call/return registers}{ghc-regs}
-In other words, all the attributes of closures are needed for
-activation records, so it's very convenient to make them look alike.
+One of the problems in designing a virtual machine is that we want it
+abstract away from tedious machine details but still reveal enough of
+the underlying hardware that we can make sensible decisions about code
+generation. A major problem area is the use of registers in
+call/return conventions. On a machine with lots of registers, it's
+cheaper to pass arguments and results in registers than to pass them
+on the stack. On a machine with very few registers, it's cheaper to
+pass arguments and results on the stack than to use ``virtual
+registers'' in memory. We therefore use a hybrid system: the first
+$n$ arguments or results are passed in registers; and the remaining
+arguments or results are passed on the stack. For register-poor
+architectures, it is important that we allow $n=0$.
+We'll label the arguments and results \Arg{1} \ldots \Arg{m} --- with
+the understanding that \Arg{1} \ldots \Arg{n} are in registers and
+\Arg{n+1} \ldots \Arg{m} are on top of the stack.
-\subsubsection{Pending arguments}
+Note that the mapping of arguments \Arg{1} \ldots \Arg{n} to machine
+registers depends on the \emph{kinds} of the arguments. For example,
+if the first argument is a Float, we might pass it in a different
+register from if it is an Int. In fact, we might find that a given
+architecture lets us pass varying numbers of arguments according to
+their types. For example, if a CPU has 2 Int registers and 2 Float
+registers then we could pass between 2 and 4 arguments in machine
+registers --- depending on whether they all have the same kind or they
+have different kinds.
-So that the garbage collector can correctly identify pointers
-in pending-argument sections we explicitly tag all non-pointers.
-Every non-pointer in a pending-argument section is preceded
-(at the next lower memory word) by a one-word byte count that
-says how many bytes to skip over (excluding the tag word).
+\Subsubsection{Entering closures}{entering-closures}
-The garbage collector traverses a pending argument section from
-the top (i.e. lowest memory address). It looks at each word in turn:
+To evaluate a closure we jump to the entry code for the closure
+passing a pointer to the closure in \Arg{1} so that the entry code can
+access its environment.
-\begin{itemize}
-\item If it is less than or equal to a small constant @MAX_STACK_TAG@
-then
-it treats it as a tag heralding zero or more words of non-pointers,
-so it just skips over them.
+\Subsubsection{Function call}{ghc-fun-call}
-\item If it points to the code segment, it must be a return
-address, so we have come to the end of the pending-argument section.
+The function-call mechanism is obviously crucial. There are five different
+cases to consider:
+\begin{enumerate}
-\item Otherwise it must be a bona fide heap pointer.
-\end{itemize}
+\item \emph{Known combinator (function with no free variables) and
+enough arguments.}
+A fast call can be made: push excess arguments onto stack and jump to
+function's \emph{fast entry point} passing arguments in \Arg{1} \ldots
+\Arg{m}.
-\subsection{The Stable Pointer Table}\label{sect:STABLEPTR_TABLE}
+The \emph{fast entry point} is only called with exactly the right
+number of arguments (in \Arg{1} \ldots \Arg{m}) so it can instantly
+start doing useful work without first testing whether it has enough
+registers or having to pop them off the stack first.
-A stable pointer is a name for a Haskell object which can be passed to
-the external world. It is ``stable'' in the sense that the name does
-not change when the Haskell garbage collector runs---in contrast to
-the address of the object which may well change.
+\item \emph{Known combinator and insufficient arguments.}
-A stable pointer is represented by an index into the
-@StablePointerTable@. The Haskell garbage collector treats the
-@StablePointerTable@ as a source of roots for GC.
+A slow call can be made: push all arguments onto stack and jump to
+function's \emph{slow entry point}.
-In order to provide efficient access to stable pointers and to be able
-to cope with any number of stable pointers (eg $0 \ldots 100000$), the
-table of stable pointers is an array stored on the heap and can grow
-when it overflows. (Since we cannot compact the table by moving
-stable pointers about, it seems unlikely that a half-empty table can
-be reduced in size---this could be fixed if necessary by using a
-hash table of some sort.)
+Any unpointed arguments which are pushed on the stack must be tagged.
+This means pushing an extra word on the stack below the unpointed
+words, containing the number of unpointed words above it.
-In general a stable pointer table closure looks like this:
+%Todo: forward ref about tagging?
+%Todo: picture?
-\begin{center}
-\begin{tabular}{|l|l|l|l|l|l|l|l|l|l|l|}
-\hline
-{\em Fixed header} & {\em No of pointers} & {\em Free} & $SP_0$ & \ldots & $SP_{n-1}$
-\\\hline
-\end{tabular}
-\end{center}
+The \emph{slow entry point} might be called with insufficient arguments
+and so it must test whether there are enough arguments on the stack.
+This \emph{argument satisfaction check} consists of checking that
+@Su-Sp@ is big enough to hold all the arguments (including any tags).
-The fields are:
-\begin{description}
+\begin{itemize}
-\item[@NPtrs@:] number of (stable) pointers.
+\item If the argument satisfaction check fails, it is because there is
+one or more update frames on the stack before the rest of the
+arguments that the function needs. In this case, we construct a PAP
+(partial application, \secref{PAP}) containing the arguments
+which are on the stack. The PAP construction code will return to the
+update frame with the address of the PAP in \Arg{1}.
-\item[@Free@:] the byte offset (from the first byte of the object) of the first free stable pointer.
+\item If the argument satisfaction check succeeds, we jump to the fast
+entry point with the arguments in \Arg{1} \ldots \Arg{arity}.
-\item[$SP_i$:] A stable pointer slot. If this entry is in use, it is
-an ``unstable'' pointer to a closure. If this entry is not in use, it
-is a byte offset of the next free stable pointer slot.
+If the fast entry point expects to receive some of \Arg{i} on the
+stack, we can reduce the amount of movement required by making the
+stack layout for the fast entry point look like the stack layout for
+the slow entry point. Since the slow entry point is entered with the
+first argument on the top of the stack and with tags in front of any
+unpointed arguments, this means that if \Arg{i} is unpointed, there
+should be space below it for a tag and that the highest numbered
+argument should be passed on the top of the stack.
-\end{description}
+We usually arrange that the fast entry point is placed immediately
+after the slow entry point --- so we can just ``fall through'' to the
+fast entry point without performing a jump.
-When a stable pointer table is evacuated
-\begin{enumerate}
-\item the free list entries are all set to @NULL@ so that the evacuation
- code knows they're not pointers;
+\end{itemize}
-\item The stable pointer slots are scanned linearly: non-@NULL@ slots
-are evacuated and @NULL@-values are chained together to form a new free list.
-\end{enumerate}
-There's no need to link the stable pointer table onto the mutable
-list because we always treat it as a root.
+\item \emph{Known function closure (function with free variables) and
+enough arguments.}
+
+A fast call can be made: push excess arguments onto stack and jump to
+function's \emph{fast entry point} passing a pointer to closure in
+\Arg{1} and arguments in \Arg{2} \ldots \Arg{m+1}.
+
+Like the fast entry point for a combinator, the fast entry point for a
+closure is only called with appropriate values in \Arg{1} \ldots
+\Arg{m+1} so we can start work straight away. The pointer to the
+closure is used to access the free variables of the closure.
+\item \emph{Known function closure and insufficient arguments.}
-\section{The Storage Manager}
+A slow call can be made: push all arguments onto stack and jump to the
+closure's slow entry point passing a pointer to the closure in \Arg{1}.
-The generational collector remembers the depth of the last generation
-collected and the value of the heap pointer at the end of the last GC.
-If the mutator has not moved the heap pointer, that means that the
-amount of space recovered is insufficient to satisfy even one request
-and it is time to collect an older generation or report a heap overflow.
+Again, the slow entry point performs an argument satisfaction check
+and either builds a PAP or pops the arguments off the stack into
+\Arg{2} \ldots \Arg{m+1} and jumps to the fast entry point.
-A deeper collection is also triggered when a minor collection fails to
-recover at least @...@ bytes of space.
-When can a GC happen?
+\item \emph{Unknown function closure, thunk or constructor.}
+Sometimes, the function being called is not statically identifiable.
+Consider, for example, the @compose@ function:
@
-- During updates (ie during returns)
-- When a heap check fails
-- When a stack check fails (concurrent system only)
-- When a context switch happens (concurrent system only)
-
-When do heap checks occur?
-- Immediately after entering a thunk
-- Immediately after entering a case alternative
-
-When do stack checks occur?
-- We calculate the worst-case stack usage of an entire
- thunk so there's no need to put a check inside each alternative.
-- Immediately after entering a thunk
- We can't make a similar worst-case calculation for heap usage
- because the heap isn't used in a stacklike manner so any
- evaluation inside a case might steal some of the heap we've
- checked for.
-
-Concurrency
-- Threads can be blocked
-- Threads can be put to sleep
- - Heap may move while we sleep
- - Black holing may happen while we sleep (ie during GC)
+ compose f g x = f (g x)
@
+Since @f@ and @g@ are passed as arguments to @compose@, the latter has
+to make a heap call. In a heap call the arguments are pushed onto the
+stack, and the closure bound to the function is entered. In the
+example, a thunk for @(g x)@ will be allocated, (a pointer to it)
+pushed on the stack, and the closure bound to @f@ will be
+entered. That is, we will jump to @f@s entry point passing @f@ in
+\Arg{1}. If \Arg{1} is passed on the stack, it is pushed on top of
+the thunk for @(g x)@.
-\subsection{The SM state}
+The \emph{entry code} for an updateable thunk (which must have arity 0)
+pushes an update frame on the stack and starts executing the body of
+the closure --- using \Arg{1} to access any free variables. This is
+described in more detail in \secref{data-updates}.
-Contains @Hp@, @HpLim@, @StablePtrTable@ plus version-specific info.
+The \emph{entry code} for a non-updateable closure is just the
+closure's slow entry point.
-\begin{itemize}
+\end{enumerate}
-\item Static Object list
-\item Foreign Object list
-\item Stable Pointer Table
+In addition to the above considerations, if there are \emph{too many}
+arguments then the extra arguments are simply pushed on the stack with
+appropriate tags.
-\end{itemize}
+To summarise, a closure's standard (slow) entry point performs the
+following:
-In addition, the generational collector requires:
+\begin{description}
+\item[Argument satisfaction check.] (function closure only)
+\item[Stack overflow check.]
+\item[Heap overflow check.]
+\item[Copy free variables out of closure.] %Todo: why?
+\item[Eager black holing.] (updateable thunk only) %Todo: forward ref.
+\item[Push update frame.]
+\item[Evaluate body of closure.]
+\end{description}
-\begin{itemize}
-\item Old Generation Indirection list
-\item Mutables list --- list of mutable objects in the old generation.
-\item @OldLim@ --- the boundary between the generations
-\item Old Foreign Object list --- foreign objects in the old generation
+\Subsection{Case expressions and return conventions}{return-conventions}
-\end{itemize}
+The \emph{evaluation} of a thunk is always initiated by
+a @case@ expression. For example:
+@
+ case x of (a,b) -> E
+@
-It is passed a table of {\em roots\/} containing
+The code for a @case@ expression looks like this:
\begin{itemize}
-
-\item All runnable TSOs
-
+\item Push the free variables of the branches on the stack (fv(@E@) in
+this case).
+\item Push a \emph{return address} on the stack.
+\item Evaluate the scrutinee (@x@ in this case).
\end{itemize}
+Once evaluation of the scrutinee is complete, execution resumes at the
+return address, which points to the code for the expression @E@.
-In the parallel system, there must be some extra magic associated with
-global GC.
+When execution resumes at the return point, there must be some {\em
+return convention} that defines where the components of the pair, @a@
+and @b@, can be found. The return convention varies according to the
+type of the scrutinee @x@:
-\subsection{The SM interface}
+\begin{itemize}
-@initSM@ finalizes any runtime parameters of the storage manager.
+\item
-@exitSM@ does any cleaning up required by the storage manager before
-the program is executed. Its main purpose is to print any summary
-statistics.
+(A space for) the return address is left on the top of the stack.
+Leaving the return address on the stack ensures that the top of the
+stack contains a valid activation record
+(\secref{activation-records}) --- should a garbage
+collection be required.
-@initHeap@ allocates the heap. It initialises the @hp@ and @hplim@
-fields of @sm@ to represent an empty heap for the compiled-in garbage
-collector. It also initialises @CAFlist@ to be the empty list. If we
-are using Appel's collector it also initialises the @OldLim@ field.
-It also initialises the stable pointer table and the @ForeignObjList@
-(and @OldForeignObjList@) fields.
+\item If @x@ has a boxed type (e.g.~a data constructor or a function),
+a pointer to @x@ is returned in \Arg{1}.
-@collectHeap@ invokes the garbage collector. @collectHeap@ requires
-all the fields of @sm@ to be initialised appropriately (from the
-STG-machine registers). The following are identified as heap roots:
-\begin{itemize}
-\item The updated CAFs recorded in @CAFlist@.
-\item Any pointers found on the stack.
-\item All runnable and sleeping TSOs.
-\item The stable pointer table.
-\end{itemize}
+\ToDo{Warn that components of E should be extracted as soon as
+possible to avoid a space leak.}
-There are two possible results from a garbage collection:
-\begin{description}
-\item[@GC_FAIL@]
-The garbage collector is unable to free up any more space.
+\item If @x@ is an unboxed type (e.g.~@Int#@ or @Float#@), @x@ is
+returned in \Arg{1}
-\item[@GC_SUCCESS@]
-The garbage collector managed to free up more space.
+\item If @x@ is an unboxed tuple constructor, the components of @x@
+are returned in \Arg{1} \ldots \Arg{n} but no object is constructed in
+the heap.
-\begin{itemize}
-\item @hp@ and @hplim@ will indicate the new space available for
-allocation.
+When passing an unboxed tuple to a function, the components are
+flattened out and passed in \Arg{1} \ldots \Arg{n} as usual.
-\item The elements of @CAFlist@ and the stable pointers will be
-updated to point to the new locations of the closures they reference.
+\end{itemize}
-\item Any members of @ForeignObjList@ which became garbage should have
-been reported (by calling their finalising routines; and the
-@(Old)ForeignObjList@ updated to contain only those Foreign objects
-which are still live.
+\Subsection{Vectored Returns}{vectored-returns}
-\end{itemize}
+Many algebraic data types have more than one constructor. For
+example, the @Maybe@ type is defined like this:
+@
+ data Maybe a = Nothing | Just a
+@
+How does the return convention encode which of the two constructors is
+being returned? A @case@ expression scrutinising a value of @Maybe@
+type would look like this:
+@
+ case E of
+ Nothing -> ...
+ Just a -> ...
+@
+Rather than pushing a return address before evaluating the scrutinee,
+@E@, the @case@ expression pushes (a pointer to) a \emph{return
+vector}, a static table consisting of two code pointers: one for the
+@Just@ alternative, and one for the @Nothing@ alternative.
-\end{description}
+\begin{itemize}
+\item
-%************************************************************************
-%* *
-\subsection{``What really happens in a garbage collection?''}
-%* *
-%************************************************************************
+The constructor @Nothing@ returns by jumping to the first item in the
+return vector with a pointer to a (statically built) Nothing closure
+in \Arg{1}.
-This is a brief tutorial on ``what really happens'' going to/from the
-storage manager in a garbage collection.
+It might seem that we could avoid loading \Arg{1} in this case since the
+first item in the return vector will know that @Nothing@ was returned
+(and can easily access the Nothing closure in the (unlikely) event
+that it needs it. The only reason we load \Arg{1} is in case we have to
+perform an update (\secref{data-updates}).
-\begin{description}
-%------------------------------------------------------------------------
-\item[The heap check:]
+\item
-[OLD-ISH: WDP]
+The constructor @Just@ returns by jumping to the second element of the
+return vector with a pointer to the closure in \Arg{1}.
-If you gaze into the C output of GHC, you see many macros calls like:
-\begin{verbatim}
-HEAP_CHK_2PtrsLive((_FHS+2));
-\end{verbatim}
+\end{itemize}
-This expands into the C (roughly speaking...):
-@
-Hp = Hp + (_FHS+2); /* optimistically move heap pointer forward */
+In this way no test need be made to see which constructor returns;
+instead, execution resumes immediately in the appropriate branch of
+the @case@.
-GC_WHILE_OR_IF (HEAP_OVERFLOW_OP(Hp, HpLim) OR_INTERVAL_EXPIRED) {
- STGCALL2_GC(PerformGC, <liveness-bits>, (_FHS+2));
-}
-@
+\Subsection{Direct Returns}{direct-returns}
-In the parallel world, where we will need to re-try the heap check,
-@GC_WHILE_OR_IF@ will be a ``while''; in the sequential world, it will
-be an ``if''.
+When a datatype has a large number of constructors, it may be
+inappropriate to use vectored returns. The vector tables may be
+large and sparse, and it may be better to identify the constructor
+using a test-and-branch sequence on the tag. For this reason, we
+provide an alternative return convention, called a \emph{direct
+return}.
-The ``heap lookahead'' checks, which are similar and used for
-multi-precision @Integer@ ops, have some further complications. See
-the commentary there (@StgMacros.lh@).
+In a direct return, the return address pushed on the stack really is a
+code pointer. The returning code loads a pointer to the closure being
+returned in \Arg{1} as usual, and also loads the tag into \Arg{2}.
+The code at the return address will test the tag and jump to the
+appropriate code for the case branch. If \Arg{2} isn't mapped to a
+real machine register on this architecture, then we don't load it on a
+return, instead using the tag directly from the info table.
-%------------------------------------------------------------------------
-\item[Into @callWrapper_GC@...:]
+The choice of whether to use a vectored return or a direct return is
+made on a type-by-type basis --- up to a certain maximum number of
+constructors imposed by the update mechanism
+(\secref{data-updates}).
-When we failed the heap check (above), we were inside the
-GCC-registerised ``threaded world.'' @callWrapper_GC@ is all about
-getting in and out of the threaded world. On SPARCs, with register
-windows, the name of the game is not shifting windows until we have
-what we want out of the old one. In tricky cases like this, it's best
-written in assembly language.
+Single-constructor data types also use direct returns, although in
+that case there is no need to return a tag in \Arg{2}.
-Performing a GC (potentially) means giving up the thread of control.
-So we must fill in the thread-state-object (TSO) [and its associated
-stk object] with enough information for later resumption:
-\begin{enumerate}
-\item
-Save the return address in the TSO's PC field.
-\item
-Save the machine registers used in the STG threaded world in their
-corresponding TSO fields. We also save the pointer-liveness
-information in the TSO.
-\item
-The registers that are not thread-specific, notably @Hp@ and
-@HpLim@, are saved in the @StorageMgrInfo@ structure.
-\item
-Call the routine it was asked to call; in this example, call
-@PerformGC@ with arguments @<liveness>@ and @_FHS+2@ (some constant)...
+\ToDo{for a nullary constructor we needn't return a pointer to the
+constructor in \Arg{1}.}
-\end{enumerate}
+\Subsection{Updates}{data-updates}
-%------------------------------------------------------------------------
-\item[Into the heap overflow wrapper, @PerformGC@ [parallel]:]
+The entry code for an updatable thunk (which must be of arity 0):
-Most information has already been saved in the TSO.
+\begin{itemize}
+\item copies the free variables out of the thunk into registers or
+ onto the stack.
+\item pushes an \emph{update frame} onto the stack.
-\begin{enumerate}
-\item
-The first argument (@<liveness>@, in our example) say what registers
-are live, i.e., are ``roots'' the storage manager needs to know.
-\begin{verbatim}
-StorageMgrInfo.rootno = 2;
-StorageMgrInfo.roots[0] = (P_) Ret1_SAVE;
-StorageMgrInfo.roots[1] = (P_) Ret2_SAVE;
-\end{verbatim}
+An update frame is a small activation record consisting of
+\begin{center}
+\begin{tabular}{|l|l|l|}
+\hline
+\emph{Fixed header} & \emph{Update Frame link} & \emph{Updatee} \\
+\hline
+\end{tabular}
+\end{center}
-\item
-We move the heap-pointer back [we had optimistically
-advanced it, in the initial heap check]
+\note{In the semantics part of the STG paper (section 5.6), an update
+frame consists of everything down to the last update frame on the
+stack. This would make sense too --- and would fit in nicely with
+what we're going to do when we add support for speculative
+evaluation.}
+\ToDo{I think update frames contain cost centres sometimes}
\item
-We load up the @smInfo@ data from the STG registers' @*_SAVE@ locations.
+If we are doing ``eager blackholing,'' we then overwrite the thunk
+with a black hole. Otherwise, we leave it to the garbage collector to
+black hole the thunk.
-\item
-We mark on the scheduler's big ``blackboard'' that a GC is
-required.
+\item
+Start evaluating the body of the expression.
-\item
-We reschedule, i.e., this thread gives up control. (The scheduler
-will presumably initiate a garbage-collection, but it may have to do
-any number of other things---flushing, for example---before ``normal
-execution'' resumes; and it most certainly may not be this thread that
-resumes at that point!)
-\end{enumerate}
+\end{itemize}
-IT IS AT THIS POINT THAT THE WORLD IS COMPLETELY TIDY.
+When the expression finishes evaluation, it will enter the update
+frame on the top of the stack. Since the returner doesn't know
+whether it is entering a normal return address/vector or an update
+frame, we follow exactly the same conventions as return addresses and
+return vectors. That is, on entering the update frame:
-%------------------------------------------------------------------------
-\item[Out of @callWrapper_GC@ [parallel]:]
+\begin{itemize}
+\item The value of the thunk is in \Arg{1}. (Recall that only thunks
+are updateable and that thunks return just one value.)
-When this thread is finally resumed after GC (and who knows what
-else), it will restart by the normal enter-TSO/enter-stack-object
-sequence, which has the effect of re-loading the registers, etc.,
-(i.e., restoring the state).
+\item If the data type is a direct-return type rather than a
+vectored-return type, then the tag is in \Arg{2}.
-Because the address we saved in the TSO's PC field was that at the end
-of the heap check, and because the check is a while-loop in the
-parallel system, we will now loop back around, and make sure there is
-enough space before continuing.
-\end{description}
+\item The update frame is still on the stack.
+\end{itemize}
+We can safely share a single statically-compiled update function
+between all types. However, the code must be able to handle both
+vectored and direct-return datatypes. This is done by arranging that
+the update code looks like this:
+@
+ | ^ |
+ | return vector |
+ |---------------|
+ | fixed-size |
+ | info table |
+ |---------------| <- update code pointer
+ | update code |
+ | v |
+@
-\subsection{Static Reference Tables (SRTs)}
-\label{sect:srt}
-\label{sect:CAF}
-\label{sect:static-objects}
+Each entry in the return vector (which is large enough to cover the
+largest vectored-return type) points to the update code.
-In the above, we assumed that objects always contained pointers to all
-their free variables. In fact, this isn't quite true: GHC omits
-pointers to top-level objects and allocates their closures in static
-memory. This optimisation reduces the number of free variables in
-heap objects - reducing memory usage and the effort needed to put them
-into heap objects. However, this optimisation comes at a cost: we
-need to complicate the garbage collector with machinery for tracing
-these static references.
+The update code:
+\begin{itemize}
+\item overwrites the \emph{updatee} with an indirection to \Arg{1};
+\item loads @Su@ from the Update Frame link;
+\item removes the update frame from the stack; and
+\item enters \Arg{1}.
+\end{itemize}
-Early versions of GHC used a very simple algorithm: it treated all
-static objects as roots. This is safe in the sense that no object is
-ever deallocated if there's a chance that it might be required later
-but can lead to some terrible space leaks. For example, this program
-ought to be able to run in constant space but, because @xs@ is never
-deallocated, it runs in linear space.
+We enter \Arg{1} again, having probably just come from there, because
+it knows whether to perform a direct or vectored return. This could
+be optimised by compiling special update code for each slot in the
+return vector, which performs the correct return.
-@
-main = print xs
-xs = [1..]
-@
+\Subsection{Semi-tagging}{semi-tagging}
-The correct behaviour is for the garbage collector to keep a static
-object alive iff it might be required later in execution. That is, if
-it is reachable from any live heap objects {\em or\/} from any return
-addresses found on the stack or from the current program counter.
-Since it is obviously infeasible for the garbage collector to scan
-machine code looking for static references, the code generator must
-generate a table of all static references in any piece of code (and we
-must place a pointer to this table next to any piece of code we
-generate).
+When a @case@ expression evaluates a variable that might be bound
+to a thunk it is often the case that the scrutinee is already evaluated.
+In this case we have paid the penalty of (a) pushing the return address (or
+return vector address) on the stack, (b) jumping through the info pointer
+of the scrutinee, and (c) returning by an indirect jump through the
+return address on the stack.
-Here's what the SRT has to contain:
+If we knew that the scrutinee was already evaluated we could generate
+(better) code which simply jumps to the appropriate branch of the
+@case@ with a pointer to the scrutinee in \Arg{1}. (For direct
+returns to multiconstructor datatypes, we might also load the tag into
+\Arg{2}).
+An obvious idea, therefore, is to test dynamically whether the heap
+closure is a value (using the tag in the info table). If not, we
+enter the closure as usual; if so, we jump straight to the appropriate
+alternative. Here, for example, is pseudo-code for the expression
+@(case x of { (a,_,c) -> E }@:
@
-...
+ \Arg{1} = <pointer to x>;
+ tag = \Arg{1}->entry->tag;
+ if (isWHNF(tag)) {
+ Sp--; \\ insert space for return address
+ goto ret;
+ }
+ push(ret);
+ goto \Arg{1}->entry;
+
+ <info table for return address goes here>
+ret: a = \Arg{1}->data1; \\ suck out a and c to avoid space leak
+ c = \Arg{1}->data3;
+ <code for E2>
@
-
-Here's how we represent it:
-
+and here is the code for the expression @(case x of { [] -> E1; x:xs -> E2 }@:
@
-...
-must be able to handle 0 references well
+ \Arg{1} = <pointer to x>;
+ tag = \Arg{1}->entry->tag;
+ if (isWHNF(tag)) {
+ Sp--; \\ insert space for return address
+ goto retvec[tag];
+ }
+ push(retinfo);
+ goto \Arg{1}->entry;
+
+ .addr ret2
+ .addr ret1
+retvec: \\ reversed return vector
+ <return info table for case goes here>
+retinfo:
+ panic("Direct return into vectored case");
+
+ret1: <code for E1>
+
+ret2: x = \Arg{1}->head;
+ xs = \Arg{1}->tail;
+ <code for E2>
@
+There is an obvious cost in compiled code size (but none in the size
+of the bytecodes). There is also a cost in execution time if we enter
+more thunks than data constructors.
+Both the direct and vectored returns are easily modified to chase chains
+of indirections too. In the vectored case, this is most easily done by
+making sure that @IND = TAG_1 - 1@, and adding an extra field to every
+return vector. In the above example, the indirection code would be
@
-Other trickery:
-o The CAF list
-o The scavenge list
-o Generational GC trickery
+ind: \Arg{1} = \Arg{1}->next;
+ goto ind_loop;
@
+where @ind_loop@ is the second line of code.
-\subsection{Space leaks and black holes}
-\label{sect:black-hole}
-
-\iffalse
-
-\ToDo{Insert text stolen from update paper}
+Note that we have to leave space for a return address since the return
+address expects to find one. If the body of the expression requires a
+heap check, we will actually have to write the return address before
+entering the garbage collector.
-\else
-A program exhibits a {\em space leak} if it retains storage that is
-sure not to be used again. Space leaks are becoming increasingly
-common in imperative programs that @malloc@ storage and fail
-subsequently to @free@ it. They are, however, also common in
-garbage-collected systems, especially where lazy evaluation is
-used.[.wadler leak, runciman heap profiling jfp.]
+\Subsection{Heap and Stack Checks}{heap-and-stack-checks}
-Quite a bit of experience has now accumulated suggesting that
-implementors must be very conscientious about avoiding gratuitous
-space leaks --- that is, ones which are an accidental artefact of some
-implementation technique.[.appel book.] The update mechanism is
-a case in point, as <.jones jfp leak.> points out. Consider a thunk for
-the expression
-@
- let xs = [1..1000] in last xs
-@
-where @last@ is a function that returns the last element of its
-argument list. When the thunk is entered it will call @last@, which
-will consume @xs@ until it finds the last element. Since the list
-@[1..1000]@ is produced lazily one might reasonably expect the
-expression to evaluate in constant space. But {\em until the moment
-of update, the thunk itself still retains a pointer to the beginning
-of the list @xs@}. So, until the update takes place the whole list
-will be retained!
-
-Of course, this is completely gratuitous. The pointer to @xs@ in the
-thunk will never be used again. In <.peyton stock hardware.> the solution to
-this problem that we advocated was to overwrite a thunk's info with a
-fixed ``black hole'' info pointer, {\em at the moment of entry}. The
-storage management information attached to a black-hole info pointer
-tells the garbage collector that the closure contains no pointers,
-thereby plugging the space leak.
-
-\subsubsection{Lazy black-holing}
-
-Black-holing is a well-known idea. The trouble is that it is
-gallingly expensive. To avoid the occasional space leak, for every
-single thunk entry we have to load a full-word literal constant into a
-register (often two instructions) and then store that register into a
-memory location.
-
-Fortunately, this cost can easily be avoided. The
-idea is simple: {\em instead of black-holing every thunk on entry,
-wait until the garbage collector is called, and then black-hole all
-(and only) the thunks whose evaluation is in progress at that moment}.
-There is no benefit in black-holing a thunk that is updated before
-garbage collection strikes! In effect, the idea is to perform the
-black-holing operation lazily, only when it is needed. This
-dramatically cuts down the number of black-holing operations, as our
-results show {\em forward ref}.
-
-How can we find all the thunks whose evaluation is in progress? They
-are precisely the ones for which update frames are on the stack. So
-all we need do is find all the update frames (via the @Su@ chain) and
-black-hole their thunks right at the start of garbage collection.
-Notice that it is not enough to refrain from treating update frames as
-roots: firstly because the thunks to which they point may need to be
-moved in a copying collector, but more importantly because the thunk
-might be accessible via some other route.
-
-\subsubsection{Detecting loops}
-
-Black-holing has a second minor advantage: evaluation of a thunk whose
-value depends on itself will cause a black hole closure to be entered,
-which can cause a suitable error message to be displayed. For example,
-consider the definition
-@
- x = 1+x
-@
-The code to evaluate @x@'s right hand side will evaluate @x@. In the
-absence of black-holing, the result will be a stack overflow, as the
-evaluator repeatedly pushes a return address and enters @x@. If
-thunks are black-holed on entry, then this infinite loop can be caught
-almost instantly.
-
-With our new method of lazy black-holing, a self-referential program
-might cause either stack overflow or a black-hole error message,
-depending on exactly when garbage collection strikes. It is quite
-easy to conceal these differences, however. If stack overflow occurs,
-all we need do is examine the update frames on the stack to see if
-more than one refers to the same thunk. If so, there is a loop that
-would have been detected by eager black-holing.
-
-\subsubsection{Lazy locking}
-\label{sect:lock}
-
-In a parallel implementation, it is necessary somehow to ``lock'' a
-thunk that is under evaluation, so that other parallel evaluators
-cannot simultaneously evaluate it and thereby duplicate work.
-Instead, an evaluator that enters a locked thunk should be blocked,
-and made runnable again when the thunk is updated.
-
-This locking is readily arranged in the same way as black-holing, by
-overwriting the thunk's info pointer with a special ``locked'' info
-pointer, at the moment of entry. If another evaluator enters the
-thunk before it has been updated, it will land in the entry code for
-the ``locked'' info pointer, which blocks the evaluator and queues it
-on the locked thunk.
-
-The details are given by <.portable parallel trinder.>. However, the close similarity
-between locking and black holing suggests the following question: can
-locking be done lazily too? The answer is that it can, except that
-locking can be postponed only until the next {\em context switch},
-rather than the next {\em garbage collection}. We are assuming here
-that the parallel implementation does not use shared memory to allow
-two processors to access the same closure. If such access is
-permitted then every thunk entry requires a hardware lock, and becomes
-much too expensive.
-
-Is lazy locking worth while, given that it requires extra work every
-context switch? We believe it is, because contexts switches are
-relatively infrequent, and thousands of thunk-entries typically take
-place between each.
-
-{\em Measurements elsewhere. Omit this section? If so, fix cross refs to here.}
+The storage manager detects that it needs to garbage collect the old
+generation when the evaluator requests a garbage collection without
+having moved the heap pointer since the last garbage collection. It
+is therefore important that the GC routines \emph{not} move the heap
+pointer unless the heap check fails. This is different from what
+happens in the current STG implementation.
-\fi
+Assuming that the stack can never shrink, we perform a stack check
+when we enter a closure but not when we return to a return
+continuation. This doesn't work for heap checks because we cannot
+predict what will happen to the heap if we call a function.
+If we wish to allow the stack to shrink, we need to perform a stack
+check whenever we enter a return continuation. Most of these checks
+could be eliminated if the storage manager guaranteed that a stack
+would always have 1000 words (say) of space after it was shrunk. Then
+we can omit stack checks for less than 1000 words in return
+continuations.
-\subsection{Squeezing identical updates}
+When an argument satisfaction check fails, we need to push the closure
+(in R1) onto the stack - so we need to perform a stack check. The
+problem is that the argument satisfaction check occurs \emph{before}
+the stack check. The solution is that the caller of a slow entry
+point or closure will guarantee that there is at least one word free
+on the stack for the callee to use.
-\iffalse
+Similarily, if a heap or stack check fails, we need to push the arguments
+and closure onto the stack. If we just came from the slow entry point,
+there's certainly enough space and it is the responsibility of anyone
+using the fast entry point to guarantee that there is enough space.
-\ToDo{Insert text stolen from update paper}
+\ToDo{Be more precise about how much space is required - document it
+in the calling convention section.}
-\else
+\Subsection{Handling interrupts/signals}{signals}
-Consider the following Haskell definition of the standard
-function @partition@ that divides a list into two, those elements
-that satisfy a predicate @p@ and those that do not:
-@
- partition :: (a->Bool) -> [a] -> ([a],[a])
- partition p [] = ([],[])
- partition p (x:xs) = if p x then (x:ys, zs)
- else (ys, x:zs)
- where
- (ys,zs) = partition p xs
-@
-By the time this definition has been desugared, it looks like this:
-@
- partition p xs
- = case xs of
- [] -> ([],[])
- (x:xs) -> let
- t = partition p xs
- ys = fst t
- zs = snd t
- in
- if p x then (x:ys,zs)
- else (ys,x:zs)
-@
-Lazy evaluation demands that the recursive call is bound to an
-intermediate variable, @t@, from which @ys@ and @zs@ are lazily
-selected. (The functions @fst@ and @snd@ select the first and second
-elements of a pair, respectively.)
-
-Now, suppose that @partition@ is applied to a list @[x1,x2]@,
-all of whose
-elements satisfy @p@. We can get a good idea of what will happen
-at runtime by unrolling the recursion a few times in our heads.
-Unrolling once, and remembering that @(p x1)@ is @True@, we get this:
@
- partition p [x1,x2]
-=
- let t1 = partition [x2]
- ys1 = fst t1
- zs1 = snd t1
- in (x1:ys1, zs1)
-@
-Unrolling the rest of the way gives this:
-@
- partition p [x1,x2]
-=
- let t2 = ([],[])
- ys2 = fst t2
- zs2 = snd t2
- t1 = (x2:ys2,zs2)
- ys1 = fst t1
- zs1 = snd t1
- in (x1:ys1,zs1)
+May have to keep C stack pointer in register to placate OS?
+May have to revert black holes - ouch!
@
-Now consider what happens if @zs1@ is evaluated. It is bound to a
-thunk, which will push an update frame before evaluating the
-expression @snd t1@. This expression in turn forces evaluation of
-@zs2@, which pushes an update frame before evaluating @snd t2@.
-Indeed the stack of update frames will grow as deep as the list is
-long when @zs1@ is evaluated. This is rather galling, since all the
-thunks @zs1@, @zs2@, and so on, have the same value.
-
-\ToDo{Describe the state-transformer case in which we get a space leak from
-pending update frames.}
-
-The solution is simple. The garbage collector, which is going to traverse the
-update stack in any case, can easily identify two update frames that are directly
-on top of each other. The second of these will update its target with the same
-value as the first. Therefore, the garbage collector can perform the update
-right away, by overwriting one update target with an indirection to the second,
-and eliminate the corresponding update frame. In this way ever-growing stacks of
-update frames are reduced to a single representative at garbage collection time.
-If this is done at the start of garbage collection then, if it turns out that
-some of these update targets are garbage they will be collected right away.
-\fi
-\subsection{Space leaks and selectors}
-\iffalse
+\section{The Loader}
+\section{The Compilers}
-\ToDo{Insert text stolen from update paper}
+\iffalse
+\part{Old stuff - needs to be mined for useful info}
-\else
+\section{The Scheduler}
-In 1987, Wadler identified an important source of space leaks in
-lazy functional programs. Consider the Haskell function definition:
-@
- f p = (g1 a, g2 b) where (a,b) = p
-@
-The pattern-matching in the @where@ clause is known as
-{\em lazy pattern-matching}, because it is performed only if @a@
-or @b@ is actually evaluated. The desugarer translates lazy pattern matching
-to the use of selectors, @fst@ and @snd@ in this case:
-@
- f p = let a = fst p
- b = snd p
- in
- (b, a)
-@
-Now suppose that the second component of the pair @(f p)@, namely @a@,
-is evaluated and discarded, but the first is not although it remains
-reachable. The garbage collector will find that the thunk for @b@ refers
-to @p@ and hence to @a@. Thus, although @a@ cannot ever be used again, its
-space is retained. It turns out that this space leak can have a very bad effect
-indeed on a program's space behaviour (Section~\ref{sect:selector-results}).
-
-Wadler's paper also proposed a solution: if the garbage collector
-encounters a thunk of the form @snd p@, where @p@ is evaluated, then
-the garbage collector should perform the selection and overwrite the
-thunk with a pointer to the second component of the pair. In effect, the
-garbage collector thereby performs a bounded amount of as-yet-undemanded evaluation
-in the hope of improving space behaviour.
-We implement this idea directly, by making the garbage collector
-eagerly execute all selector thunks\footnote{A word of caution: it is rather easy
-to make a mistake in the implementation, especially if the garbage collector
-uses pointer reversal to traverse the reachable graph.},
-with results
-reported in Section~\ref{sect:THUNK_SEL}.
-
-One could easily imagine generalisations of this idea, with the garbage
-collector performing bounded amounts of space-saving work. One example is
-this:
-@
- f x [] = (x,x)
- f x (y:ys) = f (x+1) ys
-@
-Most lazy evaluators will build up a chain of thunks for the accumulating
-parameter, @x@, each of which increments @x@. It is not safe to evaluate
-any of these thunks eagerly, since @f@ is not strict in @x@, and we know nothing
-about the value of @x@ passed in the initial call to @f@.
-On the other hand, if the garbage collector found a thunk @(x+1)@ where
-@x@ happened to be evaluated, then it could ``execute'' it eagerly.
-If done carefully, the entire chain could be eliminated in a single
-garbage collection. We have not (yet) implemented this idea.
-A very similar idea, dubbed ``stingy evaluation'', is described
-by <.stingy.>.
-
-<.sparud lazy pattern matching.> describes another solution to the
-lazy-pattern-matching
-problem. His solution involves adding code to the two thunks for
-@a@ and @b@ so that if either is evaluated it arranges to update the
-other as well as itself. The garbage-collector solution is a little
-more general, since it applies whether or not the selectors were
-generated by lazy pattern matching, and in our setting it was easier
-to implement than Sparud's.
+The Scheduler is the heart of the run-time system. A running program
+consists of a single running thread, and a list of runnable and
+blocked threads. The running thread returns to the scheduler when any
+of the following conditions arises:
-\fi
+\begin{itemize}
+\item A heap check fails, and a garbage collection is required
+\item Compiled code needs to switch to interpreted code, and vice
+versa.
+\item The thread becomes blocked.
+\item The thread is preempted.
+\end{itemize}
+A running system has a global state, consisting of
-\subsection{Internal workings of the Compacting Collector}
+\begin{itemize}
+\item @Hp@, the current heap pointer, which points to the next
+available address in the Heap.
+\item @HpLim@, the heap limit pointer, which points to the end of the
+heap.
+\item The Thread Preemption Flag, which is set whenever the currently
+running thread should be preempted at the next opportunity.
+\item A list of runnable threads.
+\item A list of blocked threads.
+\end{itemize}
-\subsection{Internal workings of the Copying Collector}
+Each thread is represented by a Thread State Object (TSO), which is
+described in detail in \secref{TSO}.
-\subsection{Internal workings of the Generational Collector}
+The following is pseudo-code for the inner loop of the scheduler
+itself.
+@
+while (threads_exist) {
+ // handle global problems: GC, parallelism, etc
+ if (need_gc) gc();
+ if (external_message) service_message();
+ // deal with other urgent stuff
+ pick a runnable thread;
+ do {
+ // enter object on top of stack
+ // if the top object is a BCO, we must enter it
+ // otherwise appply any heuristic we wish.
+ if (thread->stack[thread->sp]->info.type == BCO) {
+ status = runHugs(thread,&smInfo);
+ } else {
+ status = runGHC(thread,&smInfo);
+ }
+ switch (status) { // handle local problems
+ case (StackOverflow): enlargeStack; break;
+ case (Error e) : error(thread,e); break;
+ case (ExitWith e) : exit(e); break;
+ case (Yield) : break;
+ }
+ } while (thread_runnable);
+}
+@
-\section{Profiling}
+\Subsection{Invoking the garbage collector}{ghc-invoking-gc}
-Registering costs centres looks awkward - can we tidy it up?
+\Subsection{Putting the thread to sleep}{ghc-thread-sleeps}
-\section{Parallelism}
+\Subsection{Calling C from Haskell}{ghc-ccall}
-Something about global GC, inter-process messages and fetchmes.
+We distinguish between "safe calls" where the programmer guarantees
+that the C function will not call a Haskell function or, in a
+multithreaded system, block for a long period of time and "unsafe
+calls" where the programmer cannot make that guarantee.
-\section{Debugging}
+Safe calls are performed without returning to the scheduler and are
+discussed elsewhere (\ToDo{discuss elsewhere}).
-\section{Ticky Ticky profiling}
+Unsafe calls are performed by returning an array (outside the Haskell
+heap) of arguments and a C function pointer to the scheduler. The
+scheduler allocates a new thread from the operating system
+(multithreaded system only), spawns a call to the function and
+continues executing another thread. When the ccall completes, the
+thread informs the scheduler and the scheduler adds the thread to the
+runnable threads list.
-Measure what proportion of ...:
-\begin{itemize}
-\item
-... Enters are to data values, function values, thunks.
-\item
-... allocations are for data values, functions values, thunks.
-\item
-... updates are for data values, function values.
-\item
-... updates ``fit''
-\item
-... return-in-heap (dynamic)
-\item
-... vectored return (dynamic)
-\item
-... updates are wasted (never re-entered).
-\item
-... constructor returns get away without hitting an update.
-\end{itemize}
+\ToDo{Describe this in more detail.}
-%************************************************************************
-%* *
-\subsubsection[ticky-stk-heap-use]{Stack and heap usage}
-%* *
-%************************************************************************
-Things we are interested in here:
-\begin{itemize}
-\item
-How many times we do a heap check and move @Hp@; comparing this with
-the allocations gives an indication of how many things we get per trip
-to the well:
+\Subsection{Calling Haskell from C}{ghc-c-calls-haskell}
-If we do a ``heap lookahead,'' we haven't really allocated any
-heap, so we need to undo the effects of an @ALLOC_HEAP@:
+When C calls a Haskell closure, it sends a message to the scheduler
+thread. On receiving the message, the scheduler creates a new Haskell
+thread, pushes the arguments to the C function onto the thread's stack
+(with tags for unboxed arguments) pushes the Haskell closure and adds
+the thread to the runnable list so that it can be entered in the
+normal way.
-\item
-The stack high-water mark.
+When the closure returns, the scheduler sends back a message which
+awakens the (C) thread.
-\item
-Re-use of stack slots, and stubbing of stack slots:
+\ToDo{Do we need to worry about the garbage collector deallocating the
+thread if it gets blocked?}
-\end{itemize}
+\Subsection{Switching Worlds}{switching-worlds}
-%************************************************************************
-%* *
-\subsubsection[ticky-allocs]{Allocations}
-%* *
-%************************************************************************
+\ToDo{This has all changed: we always leave a closure on top of the
+stack if we mean to continue executing it. The scheduler examines the
+top of the stack and tries to guess which world we want to be in. If
+it finds a @BCO@, it certainly enters Hugs, if it finds a @GHC@
+closure, it certainly enters GHC and if it finds a standard closure,
+it is free to choose either one but it's probably best to enter GHC
+for everything except @BCO@s and perhaps @AP@s.}
-We count things every time we allocate something in the dynamic heap.
-For each, we count the number of words of (1)~``admin'' (header),
-(2)~good stuff (useful pointers and data), and (3)~``slop'' (extra
-space, in hopes it will allow an in-place update).
+Because this is a combined compiled/interpreted system, the
+interpreter will sometimes encounter compiled code, and vice-versa.
-The first five macros are inserted when the compiler generates code
-to allocate something; the categories correspond to the @ClosureClass@
-datatype (manifest functions, thunks, constructors, big tuples, and
-partial applications).
+All world-switches go via the scheduler, ensuring that the world is in
+a known state ready to enter either compiled code or the interpreter.
+When a thread is run from the scheduler, the @whatNext@ field in the
+TSO (\secref{TSO}) is checked to find out how to execute the
+thread.
-We may also allocate space when we do an update, and there isn't
-enough space. These macros suffice (for: updating with a partial
-application and a constructor):
+\begin{itemize}
+\item If @whatNext@ is set to @ReturnGHC@, we load up the required
+registers from the TSO and jump to the address at the top of the user
+stack.
+\item If @whatNext@ is set to @EnterGHC@, we load up the required
+registers from the TSO and enter the closure pointed to by the top
+word of the stack.
+\item If @whatNext@ is set to @EnterHugs@, we enter the top thing on
+the stack, using the interpreter.
+\end{itemize}
-In the threaded world, we allocate space for the spark pool, stack objects,
-and thread state objects.
+There are four cases we need to consider:
-The histogrammy bit is fairly straightforward; the @-2@ is: one for
-0-origin C arrays; the other one because we do {\em no} one-word
-allocations, so we would never inc that histogram slot; so we shift
-everything over by one.
+\begin{enumerate}
+\item A GHC thread enters a Hugs-built closure.
+\item A GHC thread returns to a Hugs-compiled return address.
+\item A Hugs thread enters a GHC-built closure.
+\item A Hugs thread returns to a Hugs-compiled return address.
+\end{enumerate}
-Some hard-to-account-for words are allocated by/for primitives,
-includes Integer support. @ALLOC_PRIM2@ tells us about these. We
-count everything as ``goods'', which is not strictly correct.
-(@ALLOC_PRIM@ is the same sort of stuff, but we know the
-admin/goods/slop breakdown.)
+GHC-compiled modules cannot call functions in a Hugs-compiled module
+directly, because the compiler has no information about arities in the
+external module. Therefore it must assume any top-level objects are
+CAFs, and enter their closures.
-%************************************************************************
-%* *
-\subsubsection[ticky-enters]{Enters}
-%* *
-%************************************************************************
+\ToDo{Hugs-built constructors?}
-We do more magical things with @ENT_FUN_DIRECT@. Besides simply knowing
-how many ``fast-entry-point'' enters there were, we'd like {\em simple}
-information about where those enters were, and the properties thereof.
-@
-struct ent_counter {
- unsigned registeredp:16, /* 0 == no, 1 == yes */
- arity:16, /* arity (static info) */
- Astk_args:16, /* # of args off A stack */
- Bstk_args:16; /* # of args off B stack */
- /* (rest of args are in registers) */
- StgChar *f_str; /* name of the thing */
- StgChar *f_arg_kinds; /* info about the args types */
- StgChar *wrap_str; /* name of its wrapper (if any) */
- StgChar *wrap_arg_kinds;/* info about the orig wrapper's arg types */
- I_ ctr; /* the actual counter */
- struct ent_counter *link; /* link to chain them all together */
-};
-@
+We now examine the various cases one by one and describe how the
+switch happens in each situation.
-%************************************************************************
-%* *
-\subsubsection[ticky-returns]{Returns}
-%* *
-%************************************************************************
+\subsection{A GHC thread enters a Hugs-built closure}
+\label{sec:ghc-to-hugs-switch}
-Whenever a ``return'' occurs, it is returning the constituent parts of
-a data constructor. The parts can be returned either in registers, or
-by allocating some heap to put it in (the @ALLOC_*@ macros account for
-the allocation). The constructor can either be an existing one
-(@*OLD*@) or we could have {\em just} figured out this stuff
-(@*NEW*@).
+There is three possibilities: GHC has entered a @PAP@, or it has
+entered a @AP@, or it has entered the BCO directly (for a top-level
+function closure). @AP@s and @PAP@s are ``standard closures'' and
+so do not require us to enter the bytecode interpreter.
-Here's some special magic that Simon wants [edited to match names
-actually used]:
+The entry code for a BCO does the following:
-@
-From: Simon L Peyton Jones <simonpj>
-To: partain, simonpj
-Subject: counting updates
-Date: Wed, 25 Mar 92 08:39:48 +0000
+\begin{itemize}
+\item Push the address of the object entered on the stack.
+\item Save the current state of the thread in its TSO.
+\item Return to the scheduler, setting @whatNext@ to @EnterHugs@.
+\end{itemize}
-I'd like to count how many times we update in place when actually Node
-points to the thing. Here's how:
+BCO's for thunks and functions have the same entry conventions as
+slow entry points: they expect to find their arguments on the stac
+with unboxed arguments preceded by appropriate tags.
-@RET_OLD_IN_REGS@ sets the variable @ReturnInRegsNodeValid@ to @True@;
-@RET_NEW_IN_REGS@ sets it to @False@.
+\subsection{A GHC thread returns to a Hugs-compiled return address}
+\label{sec:ghc-to-hugs-switch}
-@RET_SEMI_???@ sets it to??? ToDo [WDP]
+Hugs return addresses are laid out as in \figref{hugs-return-stack}.
+If GHC is returning, it will return to the address at the top of the
+stack, namely @HUGS_RET@. The code at @HUGS_RET@ performs the
+following:
-@UPD_CON_IN_PLACE@ tests the variable, and increments @UPD_IN_PLACE_COPY_ctr@
-if it is true.
+\begin{itemize}
+\item pushes \Arg{1} (the return value) on the stack.
+\item saves the thread state in the TSO
+\item returns to the scheduler with @whatNext@ set to @EnterHugs@.
+\end{itemize}
-Then we need to report it along with the update-in-place info.
-@
+\noindent When Hugs runs, it will enter the return value, which will
+return using the correct Hugs convention
+(\secref{hugs-return-convention}) to the return address underneath it
+on the stack.
+\subsection{A Hugs thread enters a GHC-compiled closure}
+\label{sec:hugs-to-ghc-switch}
-Of all the returns (sum of four categories above), how many were
-vectored? (The rest were obviously unvectored).
+Hugs can recognise a GHC-built closure as not being one of the
+following types of object:
-%************************************************************************
-%* *
-\subsubsection[ticky-update-frames]{Update frames}
-%* *
-%************************************************************************
+\begin{itemize}
+\item A @BCO@,
+\item A @AP@,
+\item A @PAP@,
+\item An indirection, or
+\item A constructor.
+\end{itemize}
-These macros count up the following update information.
+When Hugs is called on to enter a GHC closure, it executes the
+following sequence of instructions:
-\begin{tabular}{|l|l|} \hline
-Macro & Counts \\ \hline
- & \\
-@UPDF_STD_PUSHED@ & Update frame pushed \\
-@UPDF_CON_PUSHED@ & Constructor update frame pushed \\
-@UPDF_HOLE_PUSHED@ & An update frame to update a black hole \\
-@UPDF_OMITTED@ & A thunk decided not to push an update frame \\
- & (all subsets of @ENT_THK@) \\
-@UPDF_RCC_PUSHED@ & Cost Centre restore frame pushed \\
-@UPDF_RCC_OMITTED@ & Cost Centres not required -- not pushed \\\hline
-\end{tabular}
+\begin{itemize}
+\item Push the address of the closure on the stack.
+\item Save the current state of the thread in the TSO.
+\item Return to the scheduler, with the @whatNext@ field set to
+@EnterGHC@.
+\end{itemize}
-%************************************************************************
-%* *
-\subsubsection[ticky-updates]{Updates}
-%* *
-%************************************************************************
+\subsection{A Hugs thread returns to a GHC-compiled return address}
+\label{sec:hugs-to-ghc-switch}
-These macros record information when we do an update. We always
-update either with a data constructor (CON) or a partial application
-(PAP).
+When Hugs encounters a return address on the stack that is not
+@HUGS_RET@, it knows that a world-switch is required. At this point
+the stack contains a pointer to the return value, followed by the GHC
+return address. The following sequence is then performed:
-\begin{tabular}{|l|l|}\hline
-Macro & Where \\ \hline
- & \\
-@UPD_EXISTING@ & Updating with an indirection to something \\
- & already in the heap \\
-@UPD_SQUEEZED@ & Same as @UPD_EXISTING@ but because \\
- & of stack-squeezing \\
-@UPD_CON_W_NODE@ & Updating with a CON: by indirecting to Node \\
-@UPD_CON_IN_PLACE@ & Ditto, but in place \\
-@UPD_CON_IN_NEW@ & Ditto, but allocating the object \\
-@UPD_PAP_IN_PLACE@ & Same, but updating w/ a PAP \\
-@UPD_PAP_IN_NEW@ & \\\hline
-\end{tabular}
+\begin{itemize}
+\item save the state of the thread in the TSO.
+\item return to the scheduler, setting @whatNext@ to @EnterGHC@.
+\end{itemize}
-%************************************************************************
-%* *
-\subsubsection[ticky-selectors]{Doing selectors at GC time}
-%* *
-%************************************************************************
+The first thing that GHC will do is enter the object on the top of the
+stack, which is a pointer to the return value. This value will then
+return itself to the return address using the GHC return convention.
-@GC_SEL_ABANDONED@: we could've done the selection, but we gave up
-(e.g., to avoid overflowing the C stack); @GC_SEL_MINOR@: did a
-selection in a minor GC; @GC_SEL_MAJOR@: ditto, but major GC.
+\fi
-\section{History}
+\part{History}
We're nuking the following:
STATIC SMReps are now called CONST
\item
- @SM_MUTVAR@ is new
+ @MUTVAR@ is new
\item The profiling ``kind'' field is now encoded in the @INFO_TYPE@ field.
This identifies the general sort of the closure for profiling purposes.
\end{itemize}
-\section{Old tricks}
-
-@CAF@ indirections:
-
-These are statically defined closures which have been updated with a
-heap-allocated result. Initially these are exactly the same as a
-@STATIC@ closure but with special entry code. On entering the closure
-the entry code must:
-
-\begin{itemize}
-\item Allocate a black hole in the heap which will be updated with
- the result.
-\item Overwrite the static closure with a special @CAF@ indirection.
-
-\item Link the static indirection onto the list of updated @CAF@s.
-\end{itemize}
-
-The indirection and the link field require the initial @STATIC@
-closure to be of at least size @MIN_UPD_SIZE@ (excluding the fixed
-header).
-
-@CAF@s are treated as special garbage collection roots. These roots
-are explicitly collected by the garbage collector, since they may
-appear in code even if they are not linked with the main heap. They
-consequently represent potentially enormous space-leaks. A @CAF@
-closure retains a fixed location in statically allocated data space.
-When updated, the contents of the @CAF@ indirection are changed to
-reflect the new closure. @CAF@ indirections require special garbage
-collection code.
-
-\section{Old stuff about SRTs}
-
-Garbage collection of @CAF@s is tricky. We have to cope with explicit
-collection from the @CAFlist@ as well as potential references from the
-stack and heap which will cause the @CAF@ evacuation code to be
-called. They are treated like indirections which are shorted out.
-However they must also be updated to point to the new location of the
-new closure as the @CAF@ may still be used by references which
-reside in the code.
-
-{\bf Copying Collection}
-
-A first scheme might use evacuation code which evacuates the reference
-and updates the indirection. This is no good as subsequent evacuations
-will result in an already evacuated closure being evacuated. This will
-leave a forward reference in to-space!
-
-An alternative scheme evacuates the @CAFlist@ first. The closures
-referenced are evacuated and the @CAF@ indirection updated to point to
-the evacuated closure. The @CAF@ evacuation code simply returns the
-updated indirection pointer --- the pointer to the evacuated closure.
-Unfortunately the closure the @CAF@ references may be a static
-closure, in fact, it may be another @CAF@. This will cause the second
-@CAF@'s evacuation code to be called before the @CAF@ has been
-evacuated, returning an unevacuated pointer.
-
-Another scheme leaves updating the @CAF@ indirections to the end of
-the garbage collection. All the references are evacuated and
-scavenged as usual (including the @CAFlist@). Once collection is
-complete the @CAFlist@ is traversed updating the @CAF@ references with
-the result of evacuating the referenced closure again. This will
-immediately return as it must be a forward reference, a static
-closure, or a @CAF@ which will indirect by evacuating its reference.
-
-The crux of the problem is that the @CAF@ evacuation code needs to
-know if its reference has already been evacuated and updated. If not,
-then the reference can be evacuated, updated and returned safely
-(possibly evacuating another @CAF@). If it has, then the updated
-reference can be returned. This can be done using two @CAF@
-info-tables. At the start of a collection the @CAFlist@ is traversed
-and set to an internal {\em evacuate and update} info-table. During
-collection, evacution of such a @CAF@ also results in the info-table
-being reset back to the standard @CAF@ info-table. Thus subsequent
-evacuations will simply return the updated reference. On completion of
-the collection all @CAF@s will have {\em return reference} info-tables
-again.
-
-This is the scheme we adopt. A @CAF@ indirection has evacuation code
-which returns the evacuated and updated reference. During garbage
-collection, all the @CAF@s are overwritten with an internal @CAF@ info
-table which has evacuation code which performs this evacuate and
-update and restores the original @CAF@ code. At some point during the
-collection we must ensure that all the @CAF@s are indeed evacuated.
-
-The only potential problem with this scheme is a cyclic list of @CAF@s
-all directly referencing (possibly via indirections) another @CAF@!
-Evacuation of the first @CAF@ will fail in an infinite loop of @CAF@
-evacuations. This is solved by ensuring that the @CAF@ info-table is
-updated to a {\em return reference} info-table before performing the
-evacuate and update. If this {\em return reference} evacuation code is
-called before the actual evacuation is complete it must be because
-such a cycle of references exists. Returning the still unevacuated
-reference is OK --- all the @CAF@s will now reference the same
-@CAF@ which will reference itself! Construction of such a structure
-indicates the program must be in an infinite loop.
-
-{\bf Compacting Collector}
-
-When shorting out a @CAF@, its reference must be marked. A first
-attempt might explicitly mark the @CAF@s, updating the reference with
-the marked reference (possibly short circuting indirections). The
-actual @CAF@ marking code can indicate that they have already been
-marked (though this might not have actually been done yet) and return
-the indirection pointer so it is shorted out. Unfortunately the @CAF@
-reference might point to an indirection which will be subsequently
-shorted out. Rather than returning the @CAF@ reference we treat the
-@CAF@ as an indirection, calling the mark code of the reference, which
-will return the appropriately shorted reference.
-
-Problem: Cyclic list of @CAF@s all directly referencing (possibly via
-indirections) another @CAF@!
-
-Before compacting, the locations of the @CAF@ references are
-explicitly linked to the closures they reference (if they reference
-heap allocated closures) so that the compacting process will update
-them to the closure's new location. Unfortunately these locations'
-@CAF@ indirections are static. This causes premature termination
-since the test to find the info pointer at the end of the location
-list will match more than one value. This can be solved by using an
-auxiliary dynamic array (on the top of the A stack). One location for
-each @CAF@ indirection is linked to the closure that the @CAF@
-references. Once collection is complete this array is traversed and
-the corresponding @CAF@ is then updated with the updated pointer from
-the auxiliary array.
-
-
-It is possible to use an alternative marking scheme, using a similar
-idea to the copying solution. This scheme avoids the need to update
-the @CAF@ references explicitly. We introduce an auxillary {\em mark
-and update} @CAF@ info-table which is used to update all @CAF@s at the
-start of a collection. The new code marks the @CAF@ reference,
-updating it with the returned reference. The returned reference is
-itself returned so the @CAF@ is shorted out. The code also modifies the
-@CAF@ info-table to be a {\em return reference}. Subsequent attempts to
-mark the @CAF@ simply return the updated reference.
-
-A cyclic @CAF@ reference will result in an attempt to mark the @CAF@
-before the marking has been completed and the reference updated. We
-cannot start marking the @CAF@ as it is already being marked. Nor can
-we return the reference as it has not yet been updated. Neither can we
-treat the CAF as an indirection since the @CAF@ reference has been
-obscured by the pointer reversal stack. All we can do is return the
-@CAF@ itself. This will result in some @CAF@ references not being
-shorted out.
-
-This scheme has not been adopted but has been implemented. The code is
-commented out with @#if 0@.
-
-\subsection{The virtual register set}
-
-We refer to any (atomic) part of the virtual machine state as a ``register.''
-These ``registers'' may be shared between all threads in the system or may be
-specific to each thread.
-
-Global:
-@
- Hp
- HpLim
- Thread preemption flag
-@
-
-Thread specific:
-@
- TSO - pointer to the TSO for when we have to pack thread away
- Sp
- SpLim
- Su - used to calculate number of arguments on stack
- - this is a more convenient representation
- Call/return registers (aka General purpose registers)
- Cost centre (and other debug/profile info)
- Statistic gathering (not in production system)
- Exception handlers
- Heap overflow - possible global?
- Stack overflow - possibly global?
- Pattern match failure
- maybe a failWith handler?
- maybe an exitWith handler?
- ...
-@
-
-Some of these virtual ``registers'' are used very frequently and should
-be mapped onto machine registers if at all possible. Others are used
-very infrequently and can be kept in memory to free up registers for
-other uses.
-
-On register-poor architectures, we can play a few tricks to reduce the
-number of virtual registers which need to be accessed on a regular
-basis:
-
-@
-- HpLim trick
-- Grow stack and heap towards each other (single-threaded system only)
-- We might need to keep the C stack pointer in a register if that
- is what the OS expects when a signal occurs.
-- Preemption flag trick
-- If any of the frequently accessed registers cannot be mapped onto
- machine registers we should keep the TSO in a machine register to
- allow faster access to all the other non-machine registers.
-@
-
\end{document}
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