% 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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+%\usepackage{epsfig}
+
+%\newcommand{\note}[1]{{\em Note: #1}}
+\newcommand{\note}[1]{{{\bf Note:}\sl #1}}
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+\newcommand{\Arg}[1]{\mbox{${\tt arg}_{#1}$}}
+\newcommand{\bottom}{bottom} % foo, can't remember the symbol name
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+\newcommand{\secref}[1]{Section~\ref{sec:#1}}
+\newcommand{\figref}[1]{Figure~\ref{fig:#1}}
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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
\newpage
\part{Introduction}
-\section{Overview}
+\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~\ref{sect:unboxed-tuples} which allow the programmer to
+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.
-\end{itemize}
+\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}
-\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{Glossary}
+\Subsection{Glossary}{glossary}
\ToDo{This terminology is not used consistently within the document.
-If you find soemthing which disagrees with this terminology, fix the
+If you find something which disagrees with this terminology, fix the
usage.}
-\begin{itemize}
-
-\item A {\em word} is (at least) 32 bits and can hold either a signed
-or an unsigned int.
-
-\item A {\em pointer} is (at least) 32 bits and big enough to hold a
-function pointer or a data pointer.
-
-\item A {\em boxed} type is one whose elements are heap allocated.
+In the type system, we have boxed and unboxed types.
-\item An {\em unboxed} type is one whose elements are {\em not} heap allocated.
+\begin{itemize}
-\item A {\em pointed} type is one that contains $\bot$. Variables with
+\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
-{\em entered} or {\em updated} and it requires that they be boxed.
+\emph{entered} or \emph{updated} and it requires that they be boxed.
-\item An {\em unpointed} type is one that does not contains $\bot$.
+\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 {\em entered} or {\em updated}. Unpointed objects
+that they cannot be \emph{entered} or \emph{updated}. Unpointed objects
may be boxed (like @Array#@) or unboxed (like @Int#@).
-\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.
+\end{itemize}
+
+In the implementation, we have different kinds of objects:
-\item A {\em thunk} is a (representation of) a value of a {\em pointed}
-type which is {\em not} in HNF.
+\begin{itemize}
+
+\item \emph{boxed} objects are heap objects used by the evaluators
+
+\item \emph{unboxed} objects are not heap allocated
+
+\item \emph{stack} objects are allocated on the stack
-\item A {\em value} is an object in HNF. It can be pointed or unpointed.
+\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.
+
+\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.
\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
-\subsection{Invariants}
+At the hardware level, we have \emph{word}s and \emph{pointer}s.
+
+\begin{itemize}
+
+\item A \emph{word} is (at least) 32 bits and can hold either a signed
+or an unsigned int.
+
+\item A \emph{pointer} is (at least) 32 bits and big enough to hold a
+function pointer or a data pointer.
+
+\end{itemize}
+
+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.}
+
+\subsection{Subtle Dependencies}
-There are a few system invariants which need to be mentioned ---
-though this is probably the wrong place for them.
+Some decisions have very subtle consequences which should be written
+down in case we want to change our minds.
\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
+
+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}).
+
+\item
+
+When we return to the scheduler, the top object on the stack is a closure.
+The scheduler restarts the thread by entering the closure.
+
+\secref{hugs-return-convention} discusses how Hugs returns an
+unboxed value to GHC and how GHC returns an unboxed value to Hugs.
+
+\item
+
+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.
+
+\item
+
+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
+
+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.
\end{itemize}
-\section{Source Language}
+\Section{Source Language}{source-language}
+
+\Subsection{Explicit Allocation}{explicit-allocation}
+
+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.
+
+For example, we now write
+@
+> cons = \ x xs -> let r = (:) x xs in r
+@
+instead of
+@
+> cons = \ x xs -> (:) x xs
+@
+
+\note{For historical reasons, GHC doesn't use this syntax --- but it should.}
-\subsection{Unboxed tuples}\label{sect:unboxed-tuples}
+\Subsection{Unboxed tuples}{unboxed-tuples}
Functions can take multiple arguments as easily as they can take one
argument: there's no cost for adding another argument. But functions
styles quite expensive. For example, consider a simple state transformer
monad:
@
-> type S a = State -> (a,State)
+> 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)
to the same restrictions as other unpointed types.
Operationally, unboxed tuples are never built on the heap. When
-unboxed tuples are returned, they are returned in multiple registers
+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.
-Note that unboxed tuples can only have one constructor and that
+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.
+\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.
-%-----------------------------------------------------------------------------
-\part{Evaluation Model}
-\section{Compiled Execution}
+\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 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{sect:switching-worlds}.
+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.
-\subsection{Calling conventions}
+\end{itemize}
-\subsubsection{The call/return registers}
+\Subsection{STG Syntax}{stg-syntax}
-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.
+\ToDo{Insert STG syntax with appropriate changes.}
-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.
-\subsubsection{Entering closures}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\part{System Overview}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-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.
+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.
-\subsubsection{Function call}
+The major components of the system are:
+\begin{itemize}
-The function-call mechanism is obviously crucial. There are five different
-cases to consider:
-\begin{enumerate}
+\item
-\item {\em Known combinator (function with no free variables) and enough arguments.}
+The evaluators (\secref{sm-overview}) are responsible for
+evaluating heap objects. The system supports two evaluators: the
+machine code evaluator; and the bytecode evaluator.
-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}.
+\item
-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.
+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.
-\item {\em Known combinator and insufficient arguments.}
+\item
-A slow call can be made: push all arguments onto stack and jump to
-function's {\em slow entry point}.
+The storage manager (\secref{evaluators-overview}) is
+responsible for allocating blocks of contiguous memory and for garbage
+collection.
-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.
+\item
-%Todo: forward ref about tagging?
-%Todo: picture?
+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.
-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).
+\item
-\begin{itemize}
+The compilers (\secref{compilers-overview}) generate machine
+code and bytecode files which can be loaded by the loader.
-\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}.
+\end{itemize}
-\item If the argument satisfaction check succeeds, we jump to the fast
-entry point with the arguments in \Arg{1} \ldots \Arg{arity}.
+\ToDo{Insert diagram showing all components underneath the scheduler
+and communicating only with the scheduler}
-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.
-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.
+\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 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}
+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.
-\item {\em Known function closure (function with free variables) and enough arguments.}
+\Subsection{Evaluation Model}{evaluation-model}
-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}.
+Whilst the evaluators differ internally, they share a common
+evaluation model and many object representations.
-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.
+\Subsubsection{Heap objects}{heap-objects-overview}
+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 {\em Known function closure and insufficient arguments.}
+All heap objects look like this:
-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}.
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+\emph{Header} & \emph{Payload} \\ \hline
+\end{tabular}
+\end{center}
-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.
+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.
+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.)
-\item {\em Unknown function closure, thunk or constructor.}
+\begin{description}
-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)@.
+\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.
-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}.
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+@CONSTR@ & \emph{Fields} \\ \hline
+\end{tabular}
+\end{center}
-The {\em entry code} for a non-updateable closure is just the
-closure's slow entry point.
+\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.
-\end{enumerate}
+\item[Function closures] are used to represent functions. Their
+payload (if any) consists of the free variables of the function.
-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.
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+@FUN@ & \emph{Free Variables} \\ \hline
+\end{tabular}
+\end{center}
-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}
+Function closures are only generated by the machine code compiler.
+\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.
-\subsection{Case expressions and return conventions}
-\label{sect:return-conventions}
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+@THUNK@ & \emph{Free Variables} \\ \hline
+\end{tabular}
+\end{center}
-The {\em evaluation} of a thunk is always initiated by
-a @case@ expression. For example:
-@
- case x of (a,b) -> E
-@
+Thunks are only generated by the machine code evaluator.
-The code for a @case@ expression looks like this:
+\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.
-\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).
-\end{itemize}
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+@BCO@ & \emph{Constant Pool} & \emph{Bytecodes} \\ \hline
+\end{tabular}
+\end{center}
-Once evaluation of the scrutinee is complete, execution resumes at the
-return address, which points to the code for the expression @E@.
+\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.
-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@:
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+@PAP@ & \emph{Function Closure} & \emph{Arguments} \\ \hline
+\end{tabular}
+\end{center}
-\begin{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.
-\item
+\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.
-(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.
+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.
-\item If @x@ has a boxed type (e.g.~a data constructor or a function),
-a pointer to @x@ is returned in \Arg{1}.
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+@AP@ & \emph{Function Closure} & \emph{Arguments} \\ \hline
+\end{tabular}
+\end{center}
-\ToDo{Warn that components of E should be extracted as soon as
-possible to avoid a space leak.}
+@AP@s are only generated by the bytecode compiler.
-\item If @x@ is an unboxed type (e.g.~@Int#@ or @Float#@), @x@ is
-returned in \Arg{1}
+\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.
-\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.
+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.
-When passing an unboxed tuple to a function, the components are
-flattened out and passed in \Arg{1} \ldots \Arg{n} as usual.
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+@BH@ & \emph{Blocked threads} \\ \hline
+\end{tabular}
+\end{center}
-\end{itemize}
+\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?}
-\subsection{Vectored Returns}
+\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.)
-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.
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+@IND@ & \emph{Closure} \\ \hline
+\end{tabular}
+\end{center}
-\begin{itemize}
+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.
-\item
+\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.
-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}.
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+@TSO@ & \emph{Thread Id} & \emph{Status} & \emph{Stack} \\ \hline
+\end{tabular}
+\end{center}
-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}).
+\end{description}
-\item
+\Subsubsection{Stack objects}{stack-objects-overview}
-The constructor @Just@ returns by jumping to the second element of the
-return vector with a pointer to the closure in \Arg{1}.
+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@.
-In this way no test need be made to see which constructor returns;
-instead, execution resumes immediately in the appropriate branch of
-the @case@.
+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.
-\subsection{Direct Returns}
+There are three kinds of stack object: return addresses, update frames
+and seq frames. All stack objects look like this
-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
+\emph{Header} & \emph{Payload} \\ \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.
+As with heap objects, the header starts with a pointer to a pair
+consisting of an \emph{info table} and some \emph{entry code}.
-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}).
+\begin{description}
-Single-constructor data types also use direct returns, although in
-that case there is no need to return a tag in \Arg{2}.
+\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:
-\ToDo{Say whether we pop the return address before returning}
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+\emph{@RET_ADDR@} & \emph{Free Variables of the case alternatives} \\ \hline
+\end{tabular}
+\end{center}
-\ToDo{Stack stubbing?}
+The free variables are a mixture of pointers and non-pointers whose
+layout is described by the info table.
-\subsection{Updates}
-\label{sect:data-updates}
+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}
-The entry code for an updatable thunk (which must be of arity 0):
+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.
-\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[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.
-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
+\emph{@UPDATE@} & \emph{Next Update Frame} & \emph{Updatee} \\ \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}
+\item[Seq frames] are used to implement the polymorphic @seq@ primitive.
+They are a special kind of update frame.
-\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.
+\ToDo{Describe them properly}
-\item
-Start evaluating the body of the expression.
-\end{itemize}
+\end{description}
-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:
+\ToDo{We also need a stop frame which goes on the bottom of the stack
+when the thread terminates.}
-\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.)
-\item If the data type is a direct-return type rather than a
-vectored-return type, then the tag is in \Arg{2}.
+\Subsubsection{Case expressions}{case-expr-overview}
-\item The update frame is still on the stack.
-\end{itemize}
+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.
-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:
+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.
-@
- | ^ |
- | return vector |
- |---------------|
- | fixed-size |
- | info table |
- |---------------| <- update code pointer
- | update code |
- | v |
-@
-Each entry in the return vector (which is large enough to cover the
-largest vectored-return type) points to the update code.
+\Subsubsection{Function applications}{fun-app-overview}
-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}.
-\end{itemize}
+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.
-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.
-\subsection{Semi-tagging}
-\label{sect:semi-tagging}
+\Subsubsection{Let expressions}{let-expr-overview}
-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.
+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.
-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>
-@
-and here is the code for the expression @(case x of { [] -> E1; x:xs -> E2 }@:
-@
- \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>
+\Subsubsection{Primitive operations}{primop-overview}
-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.
+\ToDo{}
-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
-@
-ind: \Arg{1} = \Arg{1}->next;
- goto ind_loop;
-@
-where @ind_loop@ is the second line of code.
+Most primops are simple, some aren't.
-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{Heap and Stack Checks}
-\note{I reckon these deserve a subsection of their own}
-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 {\em not} move the heap
-pointer unless the heap check fails. This is different from what
-happens in the current STG implementation.
-Talk about how stack check looks ahead into the branches of case expressions.
+\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{Handling interrupts/signals}
+\Subsection{The scheduler's main loop}{scheduler-main-loop}
-@
-May have to keep C stack pointer in register to placate OS?
-May have to revert black holes - ouch!
-@
+The scheduler consists of a loop which chooses a runnable thread and
+invokes one of the evaluators which performs some reduction and
+returns.
-\section{Interpreted Execution}
+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.
-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.
+\Subsection{Creating a thread}{create-thread}
-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 staticly
-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.
+Threads are created:
-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.
+\begin{itemize}
-\subsection{Hugs Heap Objects}
-\label{sect:hugs-heap-objects}
+\item
-\subsubsection{Byte-Code Objects}
+When the scheduler is first invoked.
-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 Section \ref{sect:BCO}, in this section we will describe
-their semantics.
+\item
-Since byte-code lives on the heap, it can be garbage collected just
-like any other heap-resident data. Hugs maintains a table of
-currently live BCOs, which is treated as a table of live pointers by
-the garbage collector. When a module is unloaded, the pointers to its
-BCOs are removed from the table, and the code will be garbage
-collected some time later.
+When a message is received from another processor (I think). (Parallel
+system only.)
-A BCO represents a basic block of code - all entry points are 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.
+\item
-\subsubsection{Thunks and partial applications}
+When a C program calls some Haskell code.
-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.
+\item
-Hugs represents thunks with an @HUGS_AP@ object. The @HUGS_AP@ object
-contains one or more pointers to other heap objects. 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 @HUGS_AP@ objects is described in more detail in Section
-\ref{sect:HUGS-AP}.
+By @forkIO@, @takeMVar@ and (maybe) other Concurrent Haskell primitives.
-Partial applications are represented by @HUGS_PAP@ objects, which are
-identical to @HUGS_AP@s except that they are non-updatable.
+\end{itemize}
-\ToDo{Hugs Constructors}.
-\subsection{Calling conventions}
-\label{sect:hugs-calling-conventions}
+\Subsection{Restarting a thread}{thread-restart}
-The calling convention for any byte-code function is straightforward:
+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 Push any arguments on the stack.
-\item Push a pointer to the BCO.
-\item Begin interpreting the byte code.
+\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}
-But it isn't enough for the bytecode interpreter to handle just
-byte-code functions specially. At a minimum, it must also be able to
-enter constructors too because of the way that GHC returns to
-Hugs-compiled return addresses (Section~\ref{sect:ghc-to-hugs-return}).
+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.
-In principle, all other closure types could be handled by switching to
-the compiled world (as described in
-Section~\ref{sect:hugs-to-ghc-closure}) and entering the closure
-there. This would work but it would obviously be very inefficient if
-we entered a @HUGS_AP@ by switching worlds, entering the @HUGS_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.
+\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.}
-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:
+\Subsection{Returning from a thread}{thread-return}
-\begin{description}
-\item[@HUGS_AP@]
-To enter a @HUGS_AP@ we push an update frame, push the values from the
-@HUGS_AP@ on the stack, and enter its associated object.
+The evaluators return to the scheduler when any of the following
+conditions arise:
-\item[@HUGS_PAP@]
-To enter a @HUGS_PAP@, we push its values on the stack and enter the
-first one.
+\begin{itemize}
+\item A heap check fails, and a garbage collection is required.
-\item[@PAP@]
-Same as @HUGS_PAP@.
+\item A stack check fails, and the scheduler must either enlarge the
+current thread's stack, or flag an out of memory condition.
-\item[Constructor]
-To enter a constructor, we simply return (see Section
-\ref{sect:hugs-return-convention}).
+\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[Indirection]
-To enter an indirection, we simply enter the object it points to
-after possibly adjusting the current cost centre.
+\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[Selector]
-To enter a selector, 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.
+\item The evaluator needs to perform a ``safe'' C call
+(\secref{c-calls}).
-\end{description}
+\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}).
-The most obvious omissions from the above list are @BCO@s (which we
-dealt with above) and GHC-built closures (which are covered in Section
-\ref{sect:hugs-to-ghc-closure}).
+\item The thread is preempted (the preemption mechanism is described
+in \secref{thread-preemption}).
+\item The thread terminates.
+\end{itemize}
-\subsection{Return convention}
-\label{sect:hugs-return-convention}
+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.
-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
-will be described in Section \ref{sect:ghc-to-hugs-return}). The
-stack layout is shown in Figure \ref{fig:hugs-return-stack}.
+\Subsubsection{Leaving the bytecode evaluator}{hugs-to-ghc-switch}
-\begin{figure}
-\begin{center}
-\input{hugs_ret.pstex_t}
-\end{center}
-\caption{Stack layout for a Hugs return address}
-\label{fig:hugs-return-stack}
-\end{figure}
+\paragraph{Entering a machine code closure}
-\begin{figure}
+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.
+
+\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}
-\input{hugs_ret2.pstex_t}
+@
+| stack |
++----------+
+| bco |--> BCO
++----------+
+| HUGS_RET |
++----------+
+@
+%\input{hugs_return1.pstex_t}
\end{center}
-\caption{Stack layout on enterings a Hugs return address}
-\label{fig:hugs-return2}
+\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}
+@
+| 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}
+
+\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}
+@
+| 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}
-When a Hugs byte-code sequence is returning, it first places the
-return value on the stack. It then examines the return address (now
-the second word on the stack):
+\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 If the return address is @HUGS_RET@, rearrange the stack so that
-it has the returned object followed by the pointer to the BCO at the
-top, then enter the BCO (Figure \ref{fig:hugs-return2}).
+\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 If the top of the stack is not @HUGS_RET@, we need to do a world
-switch as described in Section \ref{sect:hugs-to-ghc-return}.
+\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.
-\section{The Scheduler}
+\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.
-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:
+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 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.
+\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}
-A running system has a global state, consisting of
+
+\Subsection{SM support for foreign function calls}{sm-foreign-calls}
\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.
+
+\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}
-Each thread is represented by a Thread State Object (TSO), which is
-described in detail in Section \ref{sect:TSO}.
+\Subsection{Misc}{sm-misc}
-The following is pseudo-code for the inner loop of the scheduler
-itself.
+\begin{itemize}
-@
-while (threads_exist) {
- // handle global problems: GC, parallelism, etc
- if (need_gc) gc();
- if (external_message) service_message();
- // deal with other urgent stuff
+\item
- pick a runnable thread;
- do {
- switch (thread->whatNext) {
- case (EnterGHC pc): status=runGHC(pc); break;
- case (EnterHugs 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);
-}
+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.
+@
+_interface_ Main 1
+_exports_
+Main main ;
+_declarations_
+1 main _:_ IOBase.IO PrelBase.();;
@
-Optimisations to avoid excess trampolining from Hugs into itself.
-How do we invoke GC, ccalls, etc.
-General ccall (@ccall-GC@) and optimised ccall.
+\Subsection{Bytecode files}{bytecode-files}
-\section{Switching Worlds}
+(All that matters here is what the loader sees.)
-\label{sect:switching-worlds}
+\Subsection{Machine code files}{asm-files}
-Because this is a combined compiled/interpreted system, the
-interpreter will sometimes encounter compiled code, and vice-versa.
+(Again, all that matters is what the loader sees.)
-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 (Section \ref{sect:TSO}) is checked to find out how to execute the
-thread.
+\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 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.
+\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}
-There are four cases we need to consider:
+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.
-\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}
+References in import lists are of two types:
+\begin{description}
+\item[ References in machine code ]
-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.
+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.
-\ToDo{dynamic linking stuff}
-\ToDo{Hugs-built constructors?}
+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.
-We now examine the various cases one by one and describe how the
-switch happens in each situation.
+\item[ References in data structures (SRTs and static data constructors) ]
-\subsection{A GHC thread enters a Hugs-built closure}
-\label{sect:ghc-to-hugs-closure}
+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.
-There are three possibilities: GHC has entered the BCO directly (for a
-top-level function closure), it has entered a @HUGS_AP@, or it has
-entered a @HUGS_PAP@.
+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.
-The code for all three objects is the same:
+\end{description}
-\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}
+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.
-\subsection{A GHC thread returns to a Hugs-compiled return address}
-\label{sect:ghc-to-hugs-return}
-Hugs return addresses are laid out as in Figure
-\ref{fig: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:
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\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 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@.
+\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}
-\noindent When Hugs runs, it will enter the return value, which will
-return using the correct Hugs convention (Section
-\ref{sect:hugs-return-convention}) to the return address underneath it
-on the stack.
+\Section{The Scheduler}{scheduler-internals}
-\subsection{A Hugs thread enters a GHC-compiled closure}
-\label{sect:hugs-to-ghc-closure}
+\ToDo{Detailed description of scheduler}
-Hugs can recognise a GHC-built closure as not being one of the
-following types of object:
+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 A @BCO@,
-\item A @HUGS_AP@,
-\item A @HUGS_PAP@,
-\item An indirection, or
-\item A constructor.
-\end{itemize}
-When Hugs is called on to enter a GHC closure, it executes the
-following sequence of instructions:
+\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.
-\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}
-\subsection{A Hugs thread returns to a GHC-compiled return address}
-\label{sect:hugs-to-ghc-return}
+\ToDo{The links for these fields are usually inserted immediately
+after the fixed header except ...}
-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:
+
+\Section{The Storage Manager}{storage-manager-internals}
+
+\subsection{Misc Text looking for a home}
+
+A \emph{value} may be:
\begin{itemize}
-\item save the state of the thread in the TSO.
-\item return to the scheduler, setting @whatNext@ to @EnterGHC@.
+\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}.
-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.
-
-\part{Implementation}
-\section{Heap objects}
-\label{sect:fixed-header}
-\ToDo{Fix this picture}
+\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 {\em heap object} is a contiguous block
-of memory, consisting of a fixed-format {\em header} followed
-by zero or more {\em data words}.
-
-\ToDo{I absolutely do not believe that every heap object has a header
-like this - ADR. I believe that they all have an info pointer but I
-see no readon why stack objects and unpointed heap objects would have
-an entry count since this will always be zero.}
+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 {\em info pointer}, which points to
-the object's static {\em info table}.
-\item Zero or more {\em admin words} that support
+\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 {\em cost centre} word).
+\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,
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.
-
-NB: It is {\em not} an ``entry count'', it is an
-``entries-after-update count.'' The commoning up of @CONST@,
+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.
+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@.
-
-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 2 lists of threads: runnable threads and sleeping threads.
-
-\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 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}.)
-
-\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}.)
-
-\item The {\em Foreign Object list} is a list of all foreign objects
- which have not yet been deallocated. (Section~\ref{sect:FOREIGN}.)
-
-\item The {\em Spark pool} is a doubly(?) linked list of Spark objects
-maintained by the parallel system. (Section~\ref{sect:SPARK}.)
-
-\item The {\em Blocked Fetch list} (or
-lists?). (Section~\ref{sect:BLOCKED_FETCH}.)
-
-\item For each thread, there is a list of all update frames on the
-stack. (Section~\ref{sect:data-updates}.)
-
-
-\end{itemize}
-
-\ToDo{The links for these fields are usually inserted immediately
-after the fixed header except ...}
+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}
+\Subsection{Info Tables}{info-tables}
-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.
+An \emph{info table} is a contiguous block of memory, laid out as follows:
\begin{center}
-\begin{tabular}{|c|}
- \hline Parallelism Info
-\\ \hline Profile Info
-\\ \hline Debug Info
-\\ \hline Tag / Static reference table
-\\ \hline Storage manager layout info
-\\ \hline Closure type
+\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 {\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
+
+\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:
-\item A one-word {\em Tag/Static Reference Table} field, @INFO_SRT@.
-For data constructors, this field contains the constructor tag, in the
-range $0..n-1$ where $n$ is the number of constructors. For all other
-objects it contains a pointer to a table which enables the garbage
-collector to identify all accessible code and CAFs. They are fully
-described in Section~\ref{sect:srt}.
+\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 {\em Profiling info\/}
+\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
\item[Type] Source derived string detailing closure type.
\end{description}
-\item {\em Parallelism info\/}
+\fi % end of commented out stuff
+
+\item \emph{Parallelism info\/}
\ToDo{}
-\item {\em Debugging info\/}
+\item \emph{Debugging info\/}
\ToDo{}
\end{itemize}
%-----------------------------------------------------------------------------
-\subsection{Kinds of Heap Object}
-\label{sect:closures}
+\Subsection{Kinds of Heap Object}{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
+\emph{Static closures} occupy fixed, statically-allocated memory
locations, with globally known addresses.
\item
-{\em Dynamic closures} are individually allocated in the heap.
+\emph{Dynamic closures} are individually allocated in the heap.
\item
-{\em Stack closures} are closures allocated within a thread's stack
+\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
-Section~\ref{sect:stacks}.
+\secref{stacks}.
\end{itemize}
A second useful classification is this:
\begin{itemize}
-\item
-{\em 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
-{\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 \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 {\em Activation frames}, represent ``continuations''. They are
+\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 {\em Administrative objects}, such as stack objects and thread
+\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.}
+\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 @INFO_TYPE@ tag in its info table.
-
-\ToDo{Check this table very carefully}
+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 kind & HNF & UPD & NS & STA & THU & MUT & UPT & BH & IND & Section \\
-
-\hline
-{\em Pointed} \\
-\hline
-
-@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 & & & & & \ref{sect:THUNK} \\
-@THUNK_STATIC@ & & 1 & & 1 & 1 & & & & & \ref{sect:THUNK} \\
-@THUNK_SELECTOR@ & & 1 & 1 & & 1 & & & & & \ref{sect:THUNK_SEL} \\
-
-@BCO@ & 1 & & 1 & & & & & & & \ref{sect:BCO} \\
-@BCO_CAF@ & & 1 & & & 1 & & & & & \ref{sect:BCO} \\
-
-@HUGS_AP@ & & 1 & & & 1 & & & & & \ref{sect:HUGS-AP} \\
-@HUGS_PAP@ & 1 & & 1 & & & & & & & \ref{sect:HUGS-AP} \\
-
-@PAP@ & 1 & & 1 & & & & & & & \ref{sect:PAP} \\
-
-@IND@ & ? & & ? & & ? & & & & 1 & \ref{sect:IND} \\
-@IND_OLDGEN@ & ? & & ? & & ? & & & & 1 & \ref{sect:IND} \\
-@IND_PERM@ & ? & & ? & & ? & & & & 1 & \ref{sect:IND} \\
-@IND_OLDGEN_PERM@ & ? & & ? & & ? & & & & 1 & \ref{sect:IND} \\
-@IND_STATIC@ & ? & & ? & 1 & ? & & & & 1 & \ref{sect:IND} \\
-
-\hline
-{\em Unpointed} \\
-\hline
-
-
-@ARR_WORDS@ & 1 & & 1 & & & & 1 & & & \ref{sect:ARR_WORDS1},\ref{sect:ARR_WORDS2} \\
-@ARR_PTRS@ & 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} \\
-
-@FOREIGN@ & 1 & & 1 & & & & 1 & & & \ref{sect:FOREIGN} \\
-
-@BH@ & & 1 & 1 & & ? & ? & & 1 & ? & \ref{sect:BH} \\
-@MVAR@ & 1 & & 1 & & & & & & & \ref{sect:MVAR} \\
-@IVAR@ & 1 & & 1 & & & & & & & \ref{sect:IVAR} \\
-@FETCHME@ & 1 & & 1 & & & & & & & \ref{sect:FETCHME} \\
+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. The classification bits are all zero in
-activation frames.
+a similar organisation.
\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} \\
+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. The classification bits are
-all zero in administrative objects.
+There are also a number of administrative objects.
\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} \\
+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}
here?}
-\subsection{Classification bits}
+\Subsection{Predicates}{closure-predicates}
-The top bits of the @INFO_TYPE@ tag tells what sort of animal the
-closure is.
+\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.}
-\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}
+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.
-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.
+\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.
-The other bits in the info-type field simply give a unique bit-pattern
-to identify the closure type.
\iffalse
@
#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 _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 */
@
\fi
-Notes:
-
-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 Objects}
+\subsection{Closures (aka Pointed Objects)}
-\subsubsection{Byte-Code Objects}
-\label{sect:BCO}
+An object can be entered iff it is a closure.
-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.
-
-There are two kinds of BCO: a standard @BCO@ which has an arity of one
-or more, and a @BCO_CAF@ which takes no arguments and can be updated.
-When a @BCO_CAF@ is updated, the code is thrown away!
-
-The semantics of BCOs are described in Section
-\ref{sect: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 (Section
-\ref{sect:ghc-to-hugs-closure}).
-\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{@HUGS_AP@ objects}
-\label{sect:HUGS-AP}
-
-There are two kinds of @HUGS_AP@ objects: a standard @HUGS_AP@, used
-to represent thunks buit by Hugs, and a @HUGS_PAP@, used for partial
-applications. The only difference between the two is that a
-@HUGS_PAP@ is non-updatable.
-
-\begin{center}
-\begin{tabular}{|l|l|l|l|}
-\hline
-\emph{Fixed Header} & \emph{BCO} & \emph{Layout} & \emph{Free Variables} \\
-\hline
-\end{tabular}
-\end{center}
-
-\noindent where:
-
-\begin{itemize}
-
-\item The entry code is a statically-compiled code fragment/info table
-that returns to the scheduler to invoke Hugs (Sections
-\ref{sect:ghc-to-hugs-closure}, \ref{sect:ghc-to-hugs-return}).
-
-\item \emph{BCO} is a pointer to the BCO for the thunk.
-
-\item \emph{Layout} contains the number of pointers and the size of
-the \emph{Free Variables} field.
-
-\item \emph{Free Variables} contains the free variables of the
-thunk/partial application/return address, pointers first.
-
-\end{itemize}
-
-\subsection{Pointed Objects}
-
-All pointed objects can be entered.
-
-\subsubsection{Function closures}\label{sect:FUN}
+\Subsubsection{Function closures}{FUN}
Function closures represent lambda abstractions. For example,
consider the top-level declaration:
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}.
+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
-{\em Fixed header} & {\em Pointers} & {\em Non-pointers} \\ \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
and most significant half-word respectively.
There are several different sorts of function closure, distinguished
-by their @INFO_TYPE@ field:
+by their closure type field:
+
\begin{itemize}
-\item @FUN@: a vanilla, dynamically allocated on the heap.
+
+\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.
+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:
-\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
+\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 {\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
+
+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@.}
+
+\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:
+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.
+
+\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}\label{sect:CONSTR}
+\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
-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
+\emph{Fixed header} & \emph{Pointers} & \emph{Non-pointers} \\ \hline
\end{tabular}
\end{center}
-The SRT pointer in a data constructor's info table is used for the
-constructor tag, since a constructor never has any static references.
-
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_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:
+\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
+\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.
\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
+(\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 {\em even indirectly}.
-
+anywhere \emph{even indirectly}.
\end{itemize}
-For each data constructor $Con$, two
-info tables are generated:
+For each data constructor $Con$, two info tables are generated:
+
\begin{itemize}
-\item $Con$@_info@ labels $Con$'s dynamic info table,
+\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{sect:THUNK}
-\label{sect:THUNK_SEL}
+\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:
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,
-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.
+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
-{\em Fixed header} & {\em Pointers} & {\em Non-pointers} & {\em Slop} \\ \hline
+\emph{Fixed header} & \emph{Pointers} & \emph{Non-pointers} \\ \hline
\end{tabular}
\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).
+
+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@: 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}.
+
+\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
-{\em Fixed header} & {\em Pointers} & {\em Non-pointers} & {\em Slop} & {\em Static object link}\\ \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
+\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
-{\em Fixed header} & {\em Selectee pointer} \\ \hline
+\emph{Fixed header} & \emph{Selectee pointer} \\ \hline
\end{tabular}
\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 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}
\end{description}
-\subsubsection{Partial applications (PAPs)}\label{sect:PAP}
+\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
-{\em Fixed header} & {\em No of arg words} & {\em Function closure} & {\em Arg stack} \\ \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
function closure is (a pointer to) the closure for the function whose
argument-satisfaction check failed.
-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.)
+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.
-There are no static PAPs.
+\note{Since @AP@s are updateable, the @MIN_UPD_PAYLOAD@ constraint
+applies here too.}
-\subsubsection{Indirections}
-\label{sect:IND}
-\label{sect:IND_OLDGEN}
+\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.
+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}
shape:
\begin{center}
\begin{tabular}{|l|l|l|}\hline
-{\em Fixed header} & {\em Target closure} \\ \hline
-\end{tabular}
-\end{center}
-
-\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
+\emph{Fixed header} & \emph{Mutable link field} & \emph{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.
+It contains a \emph{mutable link field} that is used to string together
+indirections in each generation.
-\item[@IND_PERMANENT@ and @IND_OLDGEN_PERMANENT@.]
+\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@ and @IND_OLDGEN@
-in the profiling build, or can we just make
-do with one pair whose behaviour changes when profiling is built?}
+
+\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
\begin{center}
\begin{tabular}{|l|l|l|}
\hline
-{\em Fixed header} & {\em Target closure} & {\em Static object link} \\
+\emph{Fixed header} & \emph{Target closure} & \emph{Static object link} \\
\hline
\end{tabular}
\end{center}
\end{description}
-\subsubsection{Activation Records}
-
-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}.
-
-
-\subsubsection{Black holes, MVars and IVars}
-\label{sect:BH}
-\label{sect:MVAR}
-\label{sect:IVAR}
+\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
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} \\
+\emph{Fixed header} & \emph{Mutable link} & \emph{Blocked thread link} \\
\hline
\end{tabular}
\end{center}
-The {\em Blocked thread link} points to the TSO of the first thread
+
+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@, 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.
+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.
-\ToDo{MVars}
+\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}\label{sect:FETCHME}
+\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
\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}
-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.
-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}.
+\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{sect:ARR_WORDS1}
-\label{sect:ARR_PTRS}
+\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).
+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
-{\em Fixed Hdr} & {\em No of non-pointers} & {\em Non-pointers\ldots} \\ \hline
+\emph{Fixed Hdr} & \emph{No of non-pointers} & \emph{Non-pointers\ldots} \\ \hline
\end{tabular}
\end{center}
\begin{center}
\begin{tabular}{|c|c|c|c|}
\hline
-{\em Fixed Hdr} & {\em Mutable link} & {\em No of pointers} & {\em Pointers\ldots} \\ \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
\end{description}
-\subsubsection{Mutable Objects}
-\label{sect:mutables}
-\label{sect:ARR_WORDS2}
-\label{sect:MUTVAR}
-\label{sect:MUTARR_PTRS}
-\label{sect:MUTARR_PTRS_FROZEN}
+\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 {\em mutable}; they represent objects which
+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
\begin{description}
-\item[@ARR_WORDS@] is also used to represent {\em mutable} arrays of
+\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.
\begin{center}
\begin{tabular}{|c|c|c|}
\hline
-{\em Fixed Hdr} & {\em Mutable link} & {\em Pointer} \\ \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 {\em frozen}, becoming an @SM_MUTARR_PTRS_FROZEN@, with a
+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
-{\em Fixed Hdr} & {\em Mutable link} & {\em No of ptrs} & {\em Pointers\ldots} \\ \hline
+\emph{Fixed Hdr} & \emph{Mutable link} & \emph{No of ptrs} & \emph{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.
-
\end{description}
-\subsubsection{Foreign Objects}\label{sect:FOREIGN}
+\Subsubsection{Foreign objects}{FOREIGN}
Here's what a ForeignObj looks like:
\begin{center}
\begin{tabular}{|l|l|l|l|}
\hline
-{\em Fixed header} & {\em Data} & {\em Free Routine} & {\em Foreign object link} \\
+\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
+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
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.
-The remaining objects types are all administrative --- none of them may be entered.
+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.
-\subsection{Thread State Objects (TSOs)}\label{sect:TSO}
+\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
``unpacked'' into machine registers and various other memory locations
to provide faster access.
-Single-threaded systems don't really {\em need\/} TSOs --- but they do
+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 {\em ad hoc}
+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 {\em Fixed header}
+ \hline \emph{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
+\\ \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 runable, all sleeping, all blocked on the same black
+ 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_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.
+\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.
@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
+% \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}
-\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{Other weird objects}
-\label{sect:SPARK}
-\label{sect: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
-{\em 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
-{\em Fixed header} & {\em Spark pool link} & {\em Sparked closure} \\ \hline
-\end{tabular}
-\end{center}
-
-
-\end{description}
-
-
\subsection{Stack Objects}
-\label{sect:STACK_OBJECT}
-\label{sect:stacks}
+\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
\begin{center}
\begin{tabular}{|l|}
\hline
-{\em Fixed header}
+\emph{Fixed header}
\\ \hline
-{\em Link to next stack object (0 for last)}
+\emph{Link to next stack object (0 for last)}
\\ \hline
-{\em N, the payload size in words}
+\emph{N, the payload size in words}
\\ \hline
-{\em @Sp@ (byte offset from head of object)}
+\emph{@Sp@ (byte offset from head of object)}
\\ \hline
-{\em @Su@ (byte offset from head of object)}
+\emph{@Su@ (byte offset from head of object)}
\\ \hline
-{\em Payload (N words)}
+\emph{Payload (N words)}
\\ \hline
\end{tabular}
\end{center}
-\ToDo{Are stack objects on the mutable list?}
-
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
of stack that the return address ``knows about'', namely the
activation record of the currently running function.
-\item Below each such activation record is a {\em pending-argument
+\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
can consider update frames as a special case of a return address with
a well-defined activation record.
-\ToDo{Doesn't it {\em have} to be an update frame? After all, the arg
-satisfaction check is @Su - Sp >= ...@.}
-
\end{itemize}
The game plan is this. The garbage collector
the pointers in each of these two stack regions.
-\subsubsection{Activation records}\label{sect:activation-records}
+\Subsubsection{Activation records}{activation-records}
-An {\em activation record} is a contiguous chunk of stack,
+
+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
\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
+
+\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.
+
+\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.
+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.
-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.
+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.
-\item @INFO_SRT@ is the Static Reference Table for the return
-address (Section~\ref{sect:srt}).
+\item the info table contains a Static Reference Table pointer for the
+return address (\secref{srt}).
\end{itemize}
-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.
+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.
In other words, all the attributes of closures are needed for
activation records, so it's very convenient to make them look alike.
-\subsubsection{Pending arguments}
+\Subsubsection{Pending arguments}{pending-args}
-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).
+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).
-The garbage collector traverses a pending argument section from
-the top (i.e. lowest memory address). It looks at each word in turn:
+The garbage collector traverses a pending argument section from the
+top (i.e. lowest memory address). It looks at each word in turn:
\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.
+then it treats it as a tag heralding zero or more words of
+non-pointers, so it just skips over them.
\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.
\end{itemize}
-\subsection{The Stable Pointer Table}\label{sect:STABLEPTR_TABLE}
+\Subsection{The Stable Pointer Table}{STABLEPTR_TABLE}
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
\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 & $SP_{n-1}$
+\emph{Fixed header} & \emph{No of pointers} & \emph{Free} & $SP_0$ & \ldots & $SP_{n-1}$
\\\hline
\end{tabular}
\end{center}
-\section{The Storage Manager}
+\Section{The Bytecode Evaluator}{bytecode-evaluator}
-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.
+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.
-A deeper collection is also triggered when a minor collection fails to
-recover at least @...@ bytes of space.
+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.
-When can a GC happen?
+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.
-@
-- 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)
-@
+We achieve this simplicity by forgoing some of the optimisations used
+by compiled code:
+\begin{itemize}
+\item
-\subsection{The SM state}
+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.
-Contains @Hp@, @HpLim@, @StablePtrTable@ plus version-specific info.
+\item
-\begin{itemize}
+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?
-\item Static Object list
-\item Foreign Object list
-\item Stable Pointer Table
+Instead of pushing just a return address, we push a return BCO and a
+trivial return address which just enters the return BCO.
-\end{itemize}
+(In a purely interpreted system, we could avoid pushing the trivial
+return address.)
-In addition, the generational collector requires:
+\item How can the garbage collector follow pointers within the
+activation record?
-\begin{itemize}
+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.
-\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
+\ToDo{Do we have to stub out dead variables in the activation frame?}
-\end{itemize}
+\end{enumerate}
-It is passed a table of {\em roots\/} containing
+\item
-\begin{itemize}
+We trivially support vectored returns by pushing a return vector whose
+entries are all the same.
-\item All runnable TSOs
+\item
+
+We avoid the need to build SRTs by putting bytecode objects on the
+heap and restricting BCOs to a single basic block.
\end{itemize}
+\Subsection{Hugs Info Tables}{hugs-info-tables}
+
+Hugs requires the following info tables and closures:
+\begin{description}
+\item [@HUGS_RET@].
+
+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.
+
+\item [@UPD_RET@].
+
+This is just the standard info table for an update frame.
+
+\item [Constructors].
+
+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.
+
+\item [@AP@ and @PAP@].
+
+\item [Indirections].
+
+\item [Selectors].
+
+Hugs doesn't generate them itself but it ought to recognise them
+
+\item [Complex primops].
+
+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:
+
+\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.
+
+\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.
+
+\end{enumerate}
+
+\end{description}
+
+\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.
+
+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.
+
+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.
+
-In the parallel system, there must be some extra magic associated with
-global GC.
+\subsubsection{Thunks and partial applications}
+
+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.
-\subsection{The SM interface}
+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}.
-@initSM@ finalizes any runtime parameters of the storage manager.
+Partial applications are represented by @PAP@ objects, which are
+non-updatable.
-@exitSM@ does any cleaning up required by the storage manager before
-the program is executed. Its main purpose is to print any summary
-statistics.
+\ToDo{Hugs Constructors}.
-@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.
+\Subsection{Calling conventions}{hugs-calling-conventions}
-@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:
+The calling convention for any byte-code function is straightforward:
\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.
+\item Push any arguments on the stack.
+\item Push a pointer to the BCO.
+\item Begin interpreting the byte code.
\end{itemize}
-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.
+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:
-\item[@GC_SUCCESS@]
-The garbage collector managed to free up more space.
+\begin{description}
+\item[Constructor] To enter a constructor, we simply return (see
+\secref{hugs-return-convention}).
-\begin{itemize}
-\item @hp@ and @hplim@ will indicate the new space available for
-allocation.
+\item[Indirection]
+To enter an indirection, we simply enter the object it points to
+after possibly adjusting the current cost centre.
-\item The elements of @CAFlist@ and the stable pointers will be
-updated to point to the new locations of the closures they reference.
+\item[@AP@]
-\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.
+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.)
-\end{itemize}
+\item[@PAP@]
+
+To enter a @PAP@, we push the arguments, push the function and enter
+the function. (Not forgetting a stack check at the start.)
+
+\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{description}
+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}).
-%************************************************************************
-%* *
-\subsection{``What really happens in a garbage collection?''}
-%* *
-%************************************************************************
-This is a brief tutorial on ``what really happens'' going to/from the
-storage manager in a garbage collection.
+\Subsection{Return convention}{hugs-return-convention}
-\begin{description}
-%------------------------------------------------------------------------
-\item[The heap check:]
+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}
+@
+| stack |
++----------+
+| bco |--> BCO
++----------+
+| HUGS_RET |
++----------+
+@
+%\input{hugs_ret.pstex_t}
+\end{center}
+\caption{Stack layout for a Hugs return address}
+\label{fig:hugs-return-stack}
+\end{figure}
-[OLD-ISH: WDP]
+\begin{figure}[ht]
+\begin{center}
+@
+| stack |
++----------+
+| con |--> CON
++----------+
+@
+%\input{hugs_ret2.pstex_t}
+\end{center}
+\caption{Stack layout on enterings a Hugs return address}
+\label{fig:hugs-return2}
+\end{figure}
-If you gaze into the C output of GHC, you see many macros calls like:
-\begin{verbatim}
-HEAP_CHK_2PtrsLive((_FHS+2));
-\end{verbatim}
+\begin{figure}[ht]
+\begin{center}
+@
+| 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}
-This expands into the C (roughly speaking...):
+\begin{figure}[ht]
+\begin{center}
+@
+| stack |
++----------+
+| ghc_ret |
++----------+
+| con |--> CON
++----------+
@
-Hp = Hp + (_FHS+2); /* optimistically move heap pointer forward */
+%\input{hugs_ret3.pstex_t}
+\end{center}
+\caption{Stack layout on enterings a GHC return address}
+\label{fig:hugs-return3}
+\end{figure}
-GC_WHILE_OR_IF (HEAP_OVERFLOW_OP(Hp, HpLim) OR_INTERVAL_EXPIRED) {
- STGCALL2_GC(PerformGC, <liveness-bits>, (_FHS+2));
-}
+\begin{figure}[ht]
+\begin{center}
@
+| stack |
++----------+
+| ghc_ret |
++----------+
+| 3# |
++----------+
+| I# |
++----------+
+| restart |--> id_Int#_closure
++----------+
+@
+%\input{hugs_ret2.pstex_t}
+\end{center}
+\caption{Stack layout on enterings a GHC return address with an unboxed value}
+\label{fig:hugs-return-int}
+\end{figure}
-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 Hugs byte-code sequence enters a closure, it examines the
+return address on top of the stack.
-The ``heap lookahead'' checks, which are similar and used for
-multi-precision @Integer@ ops, have some further complications. See
-the commentary there (@StgMacros.lh@).
+\begin{itemize}
-%------------------------------------------------------------------------
-\item[Into @callWrapper_GC@...:]
+\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}).
-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.
+\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}.
-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)...
+\end{itemize}
-\end{enumerate}
+\ToDo{This duplicates what we say about switching worlds
+(\secref{switching-worlds}) - kill one or t'other.}
-%------------------------------------------------------------------------
-\item[Into the heap overflow wrapper, @PerformGC@ [parallel]:]
-Most information has already been saved in the TSO.
+\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.}
-\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}
+\Subsection{Addressing Modes}{hugs-addressing-modes}
-\item
-We move the heap-pointer back [we had optimistically
-advanced it, in the initial heap check]
+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.
-\item
-We load up the @smInfo@ data from the STG registers' @*_SAVE@ locations.
+\ToDo{How big can the offsets be? Is the offset specified in the
+address field or in the instruction?}
-\item
-We mark on the scheduler's big ``blackboard'' that a GC is
-required.
+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.
+
+\Subsection{Compilation}{hugs-compilation}
+
+
+\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}}
+
+\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}}
+
+\def\Ind#1{{\mbox{\it Ind}\ {#1}}}
+\def\update#1{{\mbox{\it update}\ {#1}}}
+
+\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}}}
+
+\def\AP{\mbox{\it AP}}
+\def\PAP{\mbox{\it PAP}}
+\def\GHCRET{\mbox{\it GHCRET}}
+\def\GHCOBJ{\mbox{\it GHCOBJ}}
+
+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
+@
+
+\ToDo{Is there an easy way to add semi-tagging? Would it be that different?}
+
+\ToDo{Optimise thunks of the form @f{x1,...xm}@ so that we build an AP directly}
+
+\Subsection{Instructions}{hugs-instructions}
+
+We specify the semantics of instructions using transition rules of
+the form:
+
+\begin{tabular}{|llrrrrr|}
+\hline
+ & $\is$ & $s$ & $\su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is'$ & $s'$ & $\su'$ & $h'$ & $hp'$ & $\sigma$ \\
+\hline
+\end{tabular}
+
+where $\is$ is an instruction stream, $s$ is the stack, $\su$ is the
+update frame pointer and $h$ is the heap.
+
+
+\Subsection{Stack manipulation}{hugs-stack-manipulation}
+
+\begin{description}
+
+\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
+\end{tabular}
+
+\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
+\end{tabular}
+
+\item[ Push an unboxed int ].
+
+\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}
+
+The $I\#$ is a tag included for the benefit of the garbage collector.
+Similar rules exist for floats, doubles, chars, etc.
+
+\item[ Push an unboxed int ].
+
+\begin{tabular}{|llrrrrr|}
+\hline
+ & PUSHUNTAGGEDINT $o$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $\sigma!o : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\hline
+\end{tabular}
+
+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
+\end{tabular}
+\\
+where $\size{\as} = m$ and $\size{\bs} = n$.
-\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}
-IT IS AT THIS POINT THAT THE WORLD IS COMPLETELY TIDY.
+\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
+\end{tabular}
-%------------------------------------------------------------------------
-\item[Out of @callWrapper_GC@ [parallel]:]
+\item[ Push a BCO ].
-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).
+\begin{tabular}{|llrrrrr|}
+\hline
+ & PUSHBCO $o$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $\sigma!o : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\hline
+\end{tabular}
-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}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\Subsection{Heap manipulation}{hugs-heap-manipulation}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+
+\begin{description}
+
+\item[ Allocate a heap object ].
+
+\begin{tabular}{|llrrrrr|}
+\hline
+ & ALLOC $m$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $hp:s$ & $su$ & $h$ & $hp+m$ & $\sigma$ \\
+\hline
+\end{tabular}
+
+\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 $\size{\ws} = \arity{C}$.
+
+\item[ Build an AP or PAP ].
+
+\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 $\size{\ws} = m$.
+
+\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 $\size{\ws} = m$.
+\item[ Unpacking a constructor ].
+
+\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}
+
+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}
+
+
+\Subsection{Entering a closure}{hugs-entering}
+
+\begin{description}
+
+\item[ Enter a BCO ].
+
+\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[ 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[ Entering an AP closure ].
+
+\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}
+
+Optimisations:
+\begin{itemize}
+\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}
+
+
+\item[ Returning a constructor ].
+
+\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}
+
+
+\item[ Entering an indirection node ].
+
+\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}
+
+\item[Entering GHC closure].
+
+\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}
+
+\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{Updates}{hugs-updates}
+
+\begin{description}
+
+\item[ Updating with a constructor].
+
+\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}
+
+\item[ Argument checks].
+
+\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
+\end{tabular}
+\\
+where $m \ge (su - sp)$
+
+\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
+ $h' = h[ua \mapsto \Ind{a'}, a' \mapsto \PAP(a,\reverse\ \as) ]$
+
+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.
+
+\end{description}
+
+\Subsection{Branches}{hugs-branches}
+
+\begin{description}
+
+\item[ Testing a constructor ].
+
+\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.
+
+
+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}
+
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\Subsection{Primops}{hugs-primops}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+
+\ToDo{primops take m words and return n words. The expect boxed arguments on the stack.}
+
+
+\Section{The Machine Code Evaluator}{asm-evaluator}
+
+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}.
+
+\Subsection{Calling conventions}{ghc-calling-conventions}
+
+\Subsubsection{The call/return registers}{ghc-regs}
+
+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.
+
+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.
+
+\Subsubsection{Entering closures}{entering-closures}
+
+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.
+
+\Subsubsection{Function call}{ghc-fun-call}
+
+The function-call mechanism is obviously crucial. There are five different
+cases to consider:
+\begin{enumerate}
+
+\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}.
+
+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.
+
+\item \emph{Known combinator and insufficient arguments.}
+
+A slow call can be made: push all arguments onto stack and jump to
+function's \emph{slow entry point}.
+
+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.
+
+%Todo: forward ref about tagging?
+%Todo: picture?
+
+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).
+
+\begin{itemize}
+
+\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 If the argument satisfaction check succeeds, we jump to the fast
+entry point with the arguments in \Arg{1} \ldots \Arg{arity}.
+
+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.
+
+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.
+
+\end{itemize}
+
+
+\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.}
+
+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}.
+
+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 \emph{Unknown function closure, thunk or constructor.}
+
+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 \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}.
+
+The \emph{entry code} for a non-updateable closure is just the
+closure's slow entry point.
+
+\end{enumerate}
+
+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.
+
+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}
+
+
+\Subsection{Case expressions and return conventions}{return-conventions}
+
+The \emph{evaluation} of a thunk is always initiated by
+a @case@ expression. For example:
+@
+ case x of (a,b) -> E
+@
+
+The code for a @case@ expression looks like this:
+
+\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).
+\end{itemize}
+
+Once evaluation of the scrutinee is complete, execution resumes at the
+return address, which points to the code for the expression @E@.
+
+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@:
+
+\begin{itemize}
+
+\item
+
+(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.
+
+\item If @x@ has a boxed type (e.g.~a data constructor or a function),
+a pointer to @x@ is returned in \Arg{1}.
+
+\ToDo{Warn that components of E should be extracted as soon as
+possible to avoid a space leak.}
+
+\item If @x@ is an unboxed type (e.g.~@Int#@ or @Float#@), @x@ is
+returned in \Arg{1}
+
+\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.
+
+When passing an unboxed tuple to a function, the components are
+flattened out and passed in \Arg{1} \ldots \Arg{n} as usual.
+
+\end{itemize}
+
+\Subsection{Vectored Returns}{vectored-returns}
+
+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.
+
+\begin{itemize}
+
+\item
+
+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}.
+
+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}).
+
+\item
+
+The constructor @Just@ returns by jumping to the second element of the
+return vector with a pointer to the closure in \Arg{1}.
+
+\end{itemize}
+
+In this way no test need be made to see which constructor returns;
+instead, execution resumes immediately in the appropriate branch of
+the @case@.
+
+\Subsection{Direct Returns}{direct-returns}
+
+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}.
+
+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.
+
+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}).
+
+Single-constructor data types also use direct returns, although in
+that case there is no need to return a tag in \Arg{2}.
+
+\ToDo{for a nullary constructor we needn't return a pointer to the
+constructor in \Arg{1}.}
+
+\Subsection{Updates}{data-updates}
+
+The entry code for an updatable thunk (which must be of arity 0):
+
+\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.
+
+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}
+
+\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
+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
+Start evaluating the body of the expression.
+
+\end{itemize}
+
+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:
+
+\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.)
+
+\item If the data type is a direct-return type rather than a
+vectored-return type, then the tag is in \Arg{2}.
+
+\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 |
+@
+
+Each entry in the return vector (which is large enough to cover the
+largest vectored-return type) points to the update code.
+
+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}
+
+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.
-\subsection{Static Reference Tables (SRTs)}
-\label{sect:srt}
-\label{sect:CAF}
-\label{sect:static-objects}
+\Subsection{Semi-tagging}{semi-tagging}
-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.
+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.
-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.
+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 }@:
@
-main = print xs
-xs = [1..]
-@
-
-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).
-
-Here's what the SRT has to contain:
-
+ \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>
@
-...
+and here is the code for the expression @(case x of { [] -> E1; x:xs -> E2 }@:
@
+ \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>
-Here's how we represent it:
-
-@
-...
-must be able to handle 0 references well
+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}\label{sect: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.>.
-
-\ToDo{Simple generalisation: handle all the ``standard closures'' this way.}
-
-<.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
-\section{Dynamic Linking}
+ 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}
projects / ghc-hetmet.git / blobdiff