% 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.
-
% \documentstyle[preprint]{acmconf}
\documentclass[11pt]{article}
\oddsidemargin 0.1 in % Note that \oddsidemargin = \evensidemargin
\marginparsep 0 in
\sloppy
-\usepackage{epsfig}
+%\usepackage{epsfig}
\newcommand{\note}[1]{{\em Note: #1}}
% DIMENSION OF TEXT:
explicitly state where results should be returned in registers (or on
the stack) instead of on the heap.
+\item
+
+Lazy black-holing has been replaced by eager black-holing. The
+problem with lazy black-holing is that it leaves slop in the heap
+which conflicts with the use of a mostly-copying collector.
+
\end{itemize}
\subsection{Wish list}
\begin{description}
\item[@SEQUENTIAL@] No concurrency or parallelism support.
This configuration might not support interrupt recovery.
+
+ \note{There's probably not much point in supporting this option. If
+ we've gone to the effort of supporting concurency, we don't gain
+ much by being able to turn it off.}
+
\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.
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.
-\item An {\em unpointed} type is one that does not contains $\bot$.
+\item An {\em 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
\subsection{Subtle Dependencies}
Some decisions have very subtle consequences which should be written
-down in case we want to change our minds.
+down in case we want to change our minds.
\begin{itemize}
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
+section~\ref{sect:slop-objects}).
This is hard to arrange if we do \emph{lazy} blackholing
(section~\ref{sect:lazy-black-holing}) so we currently plan to
blackhole an object when we push the update frame.
+
+
\item
Info tables for constructors contain enough information to decide which
\subsection{Unboxed tuples}\label{sect:unboxed-tuples}
-\Note{We're not planning to implement this right away. There doesn't
-seem to be any real difficulty adding it to the runtime system but
-it'll take a lot of work adding it to the compiler. Since unboxed
-tuples do not trigger allocation, the syntax could be modified to allow
-unboxed tuples in expressions.}
-
Functions can take multiple arguments as easily as they can take one
argument: there's no cost for adding another argument. But functions
can only return one result: the cost of adding a second ``result'' is
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.
+
+\item
+The compiler generates a closure of the form
+@
+> c = \ x y z -> C x y z
+@
+for every constructor (whether boxed or unboxed).
+
+This closure is normally used during desugaring to ensure that
+constructors are saturated and to apply any strictness annotations.
+They are also used when returning unboxed constructors to the machine
+code evaluator from the bytecode evaluator and when a heap check fails
+in a return continuation for an unboxed-tuple scrutinee.
+
+\end{itemize}
+
\subsection{STG Syntax}
\ToDo{Insert STG syntax with appropriate changes.}
-%-----------------------------------------------------------------------------
-\part{Evaluation Model}
-
-\section{Overview}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\part{System Overview}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
This part is concerned with defining the external interfaces of the
major components of the system; the next part is concerned with their
The major components of the system are:
\begin{itemize}
\item The scheduler
-\item The loader
\item The storage manager
-\item The machine code evaluator (compiled code)
-\item The bytecode evaluator (interpreted code)
+\item The evaluators
+\item The loader
\item The compilers
\end{itemize}
-\section{The Compilers}
-
-Need to describe 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.();;
-@
+\ToDo{Insert diagram showing all components underneath the scheduler
+and communicating only with the scheduler}
-\section{The Scheduler}
+\section{Scheduler}
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:
+blocked threads. All threads consist of a stack and a few words of
+status information. 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.
-\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}
+\subsection{The scheduler's main loop}
-A running system has a global state, consisting of
+The scheduler consists of a loop which chooses a runnable thread and
+invokes one of the evaluators which performs some reduction and
+returns.
-\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}
+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.
-Each thread is represented by a Thread State Object (TSO), which is
-described in detail in Section \ref{sect:TSO}.
+\subsection{Creating a thread}
-The following is pseudo-code for the inner loop of the scheduler
-itself.
+Threads are created:
-@
-while (threads_exist) {
- // handle global problems: GC, parallelism, etc
- if (need_gc) gc();
- if (external_message) service_message();
- // deal with other urgent stuff
+\begin{itemize}
- 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);
-}
-@
+\item
-\subsection{Invoking the garbage collector}
-\subsection{Putting the thread to sleep}
+When the scheduler is first invoked.
-\subsection{Calling C from Haskell}
+\item
-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.
+When a message is received from another processor (I think). (Parallel
+system only.)
-Safe calls are performed without returning to the scheduler and are
-discussed elsewhere (\ToDo{discuss elsewhere}).
+\item
-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.
+When a C program calls some Haskell code.
-\ToDo{Describe this in more detail.}
+\end{itemize}
-\subsection{Calling Haskell from C}
+\subsection{Restarting a thread}
-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.
+The evaluators can reduce almost all types of closure except that only
+the machine code evaluator can reduce GHC-compiled closures and only
+the bytecode evaluator can reduce Hugs-compiled closures.
+Consequently, the scheduler may use either evaluator to restart a
+thread unless the top closure is a @BCO@ or contains machine code.
-When the closure returns, the scheduler sends back a message which
-awakens the (C) thread.
+However, if the top of the stack contains a constructor, the scheduler
+should use the machine code evaluator to restart the thread. 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 will happen very rarely if at all.
-\ToDo{Do we need to worry about the garbage collector deallocating the
-thread if it gets blocked?}
+\subsection{Returning from a thread}
-\subsection{Switching Worlds}
-\label{sect:switching-worlds}
+The evaluators return to the scheduler when any of the following
+conditions arise:
-\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.}
+\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 evaluator needs to perform an ``unsafe'' C call.
+\item The thread becomes blocked.
+\item The thread is preempted.
+\item The thread terminates.
+\end{itemize}
-Because this is a combined compiled/interpreted system, the
-interpreter will sometimes encounter compiled code, and vice-versa.
+Except when the thread terminates, the thread always terminates with a
+closure on the top of the stack.
-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.
+\subsection{Preempting a thread}
-\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}
+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.
-There are four cases we need to consider:
+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.
-\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}
+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.
-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.
+\subsection{``Safe'' and ``unsafe'' C calls}
-\ToDo{Hugs-built constructors?}
+There are two ways of calling C:
-We now examine the various cases one by one and describe how the
-switch happens in each situation.
+\begin{description}
-\subsection{A GHC thread enters a Hugs-built closure}
-\label{sect:ghc-to-hugs-closure}
+\item[``Safe'' C calls]
+are used if the programer is certain that the C function will not
+do anything dangerous such as calling a Haskell function or an
+operating system call which blocks the thread for a long period of time.
+\footnote{Warning: this use of ``safe'' and ``unsafe'' is the exact
+opposite of the usage for functions like @unsafePerformIO@.}
+Safe C calls are faster but must be hand-checked by the programmer.
+
+Safe C calls are performed by pushing the arguments onto the C stack
+and jumping to the C function's entry point. On exit, the result of
+the function is in a register which is returned to the Haskell code as
+an unboxed value.
+
+\item[``Unsafe'' C calls] are used if the programmer suspects that the
+thread may do something dangerous like blocking or calling a Haskell
+function. Unsafe C calls are relatively slow but are less problematic.
+
+Unsafe 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.
-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.
+\end{description}
-The entry code for a BCO does the following:
+The bytecode evaluator will probably treat all C calls as being unsafe.
-\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}
+\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 a
+safe 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.}
-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.
-\subsection{A GHC thread returns to a Hugs-compiled return address}
-\label{sect:ghc-to-hugs-return}
+\section{The Evaluators}
-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:
+All the scheduler needs to know about evaluation is how to manipulate
+threads and how to find the closure on top of the stack. The
+evaluators need to agree on the representations of certain objects and
+on how to return from the scheduler.
+
+\subsection{Returning to the Scheduler}
+\label{sect:switching-worlds}
+The evaluators return to the scheduler under three circumstances:
\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}
-\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.
+\item
-\subsection{A Hugs thread enters a GHC-compiled closure}
-\label{sect:hugs-to-ghc-closure}
+When they enter 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.
-Hugs can recognise a GHC-built closure as not being one of the
-following types of object:
+\item
-\begin{itemize}
-\item A @BCO@,
-\item A @AP@,
-\item A @PAP@,
-\item An indirection, or
-\item A constructor.
-\end{itemize}
+When they return 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.
-When Hugs is called on to enter a GHC closure, it executes the
-following sequence of instructions:
+\item
+
+When a heap or stack check fails or when the preemption flag is set.
-\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}
+In all cases, they return to the scheduler with a closure on 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 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:
+\subsubsection{Leaving the bytecode evaluator}
+\label{sect:hugs-to-ghc-switch}
-\begin{itemize}
-\item save the state of the thread in the TSO.
-\item return to the scheduler, setting @whatNext@ to @EnterGHC@.
-\end{itemize}
+\paragraph{Entering a machine code closure}
-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.
+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.
-\section{The Loader}
+\paragraph{Returning a constructor}
-\ToDo{Is it ok to load code when threads are running?}
+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.
-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.
+\note{This is why the scheduler must enter the machine code evaluator
+if it finds a constructor on top of the stack.}
-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.
+\paragraph{Returning an unboxed value}
-A portable solution is to perform some of the linking ourselves. Each module
-should provide three global symbols:
-\begin{itemize}
-\item
-An initialisation routine. (Might also be used for finalisation.)
-\item
-A table of symbols it exports.
-Entries in this table consist of the symbol name and the address of the
-names value.
-\item
-A table of symbols it imports.
-Entries in this table consist of the symbol name and a list of references
-to that symbol.
-\end{itemize}
+\note{Hugs doesn't support unboxed values in source programs but they
+are used for a few complex primops.}
-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.
+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 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.
-References in import lists are of two types:
-\begin{description}
-\item[ References in machine code ]
+The runtime system (or GHC?) provides one of these closures for each
+unboxed type. Hugs cannot generate them itself since the entry code is
+really very tricky.
-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.
+\paragraph{Heap/Stack overflow and preemption}
-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.
+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.
-\item[ References in data structures (SRTs and static data constructors) ]
+\subsubsection{Leaving the machine code evaluator}
+\label{sect:ghc-to-hugs-switch}
-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. Using an
-indirection is possible but tricky.
+\paragraph{Entering a BCO}
-Note: We could avoid patching machine code if all references to
-eternal 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 entry code for a BCO pushes the BCO onto the stack and returns to
+the scheduler.
-\end{description}
+\paragraph{Returning a constructor}
-\section{Compiled Execution}
+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. Figure~\ref{fig:hugs-return-stack}
+shows the state of the stack just before evaluating the scrutinee.
-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}.
+\begin{figure}[ht]
+\begin{center}
+@
+| stack |
++----------+
+| bco |--> BCO
++----------+
+| HUGS_RET |
++----------+
+@
+%\input{hugs_return1.pstex_t}
+\end{center}
+\caption{Stack layout for evaluating a scrutinee}
+\label{fig:hugs-return-stack}
+\end{figure}
-\subsection{Calling conventions}
+This return address rearranges the stack so that the bco pointer is
+above the constructor on the stack (as shown in
+figure~\ref{fig:hugs-boxed-return}) and returns to the scheduler.
-\subsubsection{The call/return registers}
+\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}
-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 $n=0$.
+\paragraph{Returning an unboxed value}
-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.
+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 unboxed value (as shown in
+figure~\ref{fig:hugs-entering-unboxed-return}) and returns to the scheduler.
-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.
+\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}
-\subsubsection{Entering closures}
-\label{sect:entering-closures}
+\paragraph{Heap/Stack overflow and preemption}
-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.
+\ToDo{}
-\subsubsection{Function call}
+\subsection{Shared Representations}
-The function-call mechanism is obviously crucial. There are five different
-cases to consider:
-\begin{enumerate}
+We share @AP@s, @PAP@s, constructors, indirections, selectors(?) and
+update frames. These are described in section~\ref{sect:heap-objects}.
-\item {\em 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 {\em fast entry point} passing arguments in \Arg{1} \ldots
-\Arg{m}.
+\section{The Storage Manager}
-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 storage manager is responsible for managing the heap and all
+objects stored in it. Most objects are just copied in the normal way
+but a number receive special treatment by the storage manager:
-\item {\em Known combinator and insufficient arguments.}
+\begin{itemize}
+\item
-A slow call can be made: push all arguments onto stack and jump to
-function's {\em slow entry point}.
+Indirections are shorted out.
-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?
+Weak pointers and stable pointers are treated specially.
-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}
+Thread State Objects (TSOs) and the stacks within them are treated specially.
+In particular:
-\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}.
+\begin{itemize}
+\item
-\item If the argument satisfaction check succeeds, we jump to the fast
-entry point with the arguments in \Arg{1} \ldots \Arg{arity}.
+Update frames pointing to unreachable objects are squeezed out.
-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.
+\item
-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.
+Adjacent update frames (for different closures) are compressed to a
+single update frame pointing to a single black hole.
-\end{itemize}
+\item
+If the stack contains a large amount of free space, the storage
+manager may shrink the stack. If it shrinks the stack, it guarantees
+never to leave less than @MIN_SIZE_SHRUNKEN_STACK@ empty words on the
+stack when it does so.
-\item {\em Known function closure (function with free variables) and enough arguments.}
+\ToDo{Would it be useful for the storage manager to enlarge the stack?}
-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}.
+\end{itemize}
-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
+Very large objects (eg large arrays and TSOs) are not moved if
+possible.
-\item {\em Known function closure and insufficient arguments.}
+\end{itemize}
-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.
+\section{The Compilers}
+Need to describe interface files, format of bytecode files, symbols
+defined by machine code files.
-\item {\em Unknown function closure, thunk or constructor.}
+\subsection{Interface Files}
-Sometimes, the function being called is not statically identifiable.
-Consider, for example, the @compose@ function:
+Here's an example - but I don't know the grammar - ADR.
@
- compose f g x = f (g x)
+_interface_ Main 1
+_exports_
+Main main ;
+_declarations_
+1 main _:_ IOBase.IO PrelBase.();;
@
-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 {\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}.
+\subsection{Bytecode files}
-The {\em entry code} for a non-updateable closure is just the
-closure's slow entry point.
+(All that matters here is what the loader sees.)
-\end{enumerate}
+\subsection{Machine code files}
-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.
+(Again, all that matters is what the loader sees.)
-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{Bytecode files}
-\subsection{Case expressions and return conventions}
-\label{sect:return-conventions}
+(All that matters here is what the loader sees.)
-The {\em evaluation} of a thunk is always initiated by
-a @case@ expression. For example:
-@
- case x of (a,b) -> E
-@
+\subsection{Machine code files}
-The code for a @case@ expression looks like this:
+(Again, all that matters is what the loader sees.)
-\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@.
+\section{The Loader}
-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@:
+\ToDo{Is it ok to load code when threads are running?}
-\begin{itemize}
+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.
-\item
+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 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.
+A portable solution is to perform some of the linking ourselves. Each module
+should provide three global symbols:
+\begin{itemize}
+\item
+An initialisation routine. (Might also be used for finalisation.)
+\item
+A table of symbols it exports.
+Entries in this table consist of the symbol name and the address of the
+names value.
+\item
+A table of symbols it imports.
+Entries in this table consist of the symbol name and a list of references
+to that symbol.
+\end{itemize}
-\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}
+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.
-\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.
+References in import lists are of two types:
+\begin{description}
+\item[ References in machine code ]
-When passing an unboxed tuple to a function, the components are
-flattened out and passed in \Arg{1} \ldots \Arg{n} as usual.
+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.
-\end{itemize}
+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.
-\subsection{Vectored Returns}
+\item[ References in data structures (SRTs and static data constructors) ]
-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.
+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. Using an
+indirection is possible but tricky.
-\begin{itemize}
+Note: We could avoid patching machine code if all references to
+eternal 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.
-\item
+\end{description}
-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}.
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\part{Internal details}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-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}).
+This part is concerned with the internal details of the components
+described in the previous part.
-\item
+The major components of the system are:
+\begin{itemize}
+\item The scheduler
+\item The storage manager
+\item The evaluators
+\item The loader
+\item The compilers
+\end{itemize}
-The constructor @Just@ returns by jumping to the second element of the
-return vector with a pointer to the closure in \Arg{1}.
+\section{The Scheduler}
-\end{itemize}
+\section{The Storage Manager}
+\label{sect:storage-manager-internals}
-In this way no test need be made to see which constructor returns;
-instead, execution resumes immediately in the appropriate branch of
-the @case@.
+\ToDo{Fix this picture}
-\subsection{Direct Returns}
+\begin{figure}
+\begin{center}
+\input{closure}
+\end{center}
+\caption{A closure}
+\label{fig:closure}
+\end{figure}
-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}.
+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}.
-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.
+\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.}
-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}).
+The header consists of the following fields:
+\begin{itemize}
+\item A one-word {\em info pointer}, which points to
+the object's static {\em info table}.
+\item Zero or more {\em admin words} that support
+\begin{itemize}
+\item Profiling (notably a {\em cost centre} word).
+ \note{We could possibly omit the cost centre word from some
+ administrative objects.}
+\item Parallelism (e.g. GranSim keeps the object's global address here,
+though GUM keeps a separate hash table).
+\item Statistics (e.g. a word to track how many times a thunk is entered.).
-Single-constructor data types also use direct returns, although in
-that case there is no need to return a tag in \Arg{2}.
+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.
-\ToDo{Say whether we pop the return address before returning}
+NB: It is {\em not} an ``entry count'', it is an
+``entries-after-update count.'' The commoning up of @CONST@,
+@CHARLIKE@ and @INTLIKE@ closures is turned off(?) if this is
+required. This has only been done for 2s collection.
-\ToDo{Stack stubbing?}
+\end{itemize}
+\end{itemize}
-\subsection{Updates}
-\label{sect:data-updates}
+Most of the RTS is completely insensitive to the number of admin words.
+The total size of the fixed header is @FIXED_HS@.
-The entry code for an updatable thunk (which must be of arity 0):
+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 copies the free variables out of the thunk into registers or
- onto the stack.
-\item pushes an {\em update frame} onto the stack.
+\item 2 lists of threads: runnable threads and sleeping threads.
-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
-\end{tabular}
-\end{center}
+\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}.)
-\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 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
-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 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
-Start evaluating the body of the expression.
+\item The {\em Foreign Object list} is a list of all foreign objects
+ which have not yet been deallocated. (Section~\ref{sect:FOREIGN}.)
-\end{itemize}
+\item The {\em Spark pool} is a doubly(?) linked list of Spark objects
+maintained by the parallel system. (Section~\ref{sect:SPARK}.)
-When the expression finishes evaluation, it will enter the update
-frame on the top of the stack. Since the returner doesn't know
-whether it is entering a normal return address/vector or an update
-frame, we follow exactly the same conventions as return addresses and
-return vectors. That is, on entering the update frame:
+\item The {\em Blocked Fetch list} (or
+lists?). (Section~\ref{sect:BLOCKED_FETCH}.)
-\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 For each thread, there is a list of all update frames on the
+stack. (Section~\ref{sect:data-updates}.)
-\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:
+\ToDo{The links for these fields are usually inserted immediately
+after the fixed header except ...}
-@
- | ^ |
- | return vector |
- |---------------|
- | fixed-size |
- | info table |
- |---------------| <- update code pointer
- | update code |
- | v |
-@
+\subsection{Info Tables}
-Each entry in the return vector (which is large enough to cover the
-largest vectored-return type) points to the update code.
+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.
-The update code:
+\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
+\\ \hline entry code
+\\ \vdots
+\end{tabular}
+\end{center}
+An info table has the following contents (working backwards in memory
+addresses):
\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}
-
-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.
+\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.
-\subsection{Semi-tagging}
-\label{sect:semi-tagging}
+\item A one-word {\em closure type field}, @INFO_TYPE@, identifies what kind
+of closure the object is. The various types of closure are described
+in Section~\ref{sect:closures}.
+In some configurations, some useful properties of
+closures (is it a HNF? can it be sparked?)
+are represented as high-order bits so they can be tested quickly.
+
+\item A single pointer or word --- the {\em storage manager info field},
+@INFO_SM@, contains auxiliary information describing the closure's
+precise layout, for the benefit of the garbage collector and the code
+that stuffs graph into packets for transmission over the network.
+
+\item A one-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}.
+
+\item {\em Profiling info\/}
+
+\ToDo{The profiling info is completely bogus. I've not deleted it
+from the document but I've commented it all out.}
+
+% change to \iftrue to uncomment this section
+\iffalse
+
+Closure category records are attached to the info table of the
+closure. They are declared with the info table. We put pointers to
+these ClCat things in info tables. We need these ClCat things because
+they are mutable, whereas info tables are immutable. Hashing will map
+similar categories to the same hash value allowing statistics to be
+grouped by closure category.
+
+Cost Centres and Closure Categories are hashed to provide indexes
+against which arbitrary information can be stored. These indexes are
+memoised in the appropriate cost centre or category record and
+subsequent hashes avoided by the index routine (it simply returns the
+memoised index).
+
+There are different features which can be hashed allowing information
+to be stored for different groupings. Cost centres have the cost
+centre recorded (using the pointer), module and group. Closure
+categories have the closure description and the type
+description. Records with the same feature will be hashed to the same
+index value.
+
+The initialisation routines, @init_index_<feature>@, allocate a hash
+table in which the cost centre / category records are stored. The
+lower bound for the table size is taken from @max_<feature>_no@. They
+return the actual table size used (the next power of 2). Unused
+locations in the hash table are indicated by a 0 entry. Successive
+@init_index_<feature>@ calls just return the actual table size.
+
+Calls to @index_<feature>@ will insert the cost centre / category
+record in the @<feature>@ hash table, if not already inserted. The hash
+index is memoised in the record and returned.
+
+CURRENTLY ONLY ONE MEMOISATION SLOT IS AVILABLE IN EACH RECORD SO
+HASHING CAN ONLY BE DONE ON ONE FEATURE FOR EACH RECORD. This can be
+easily relaxed at the expense of extra memoisation space or continued
+rehashing.
+
+The initialisation routines must be called before initialisation of
+the stacks and heap as they require to allocate storage. It is also
+expected that the caller may want to allocate additional storage in
+which to store profiling information based on the return table size
+value(s).
+
+\begin{center}
+\begin{tabular}{|l|}
+ \hline Hash Index
+\\ \hline Selected
+\\ \hline Kind
+\\ \hline Description String
+\\ \hline Type String
+\\ \hline
+\end{tabular}
+\end{center}
+
+\begin{description}
+\item[Hash Index] Memoised copy
+\item[Selected]
+ Is this category selected (-1 == not memoised, selected? 0 or 1)
+\item[Kind]
+One of the following values (defined in CostCentre.lh):
+
+\begin{description}
+\item[@CON_K@]
+A constructor.
+\item[@FN_K@]
+A literal function.
+\item[@PAP_K@]
+A partial application.
+\item[@THK_K@]
+A thunk, or suspension.
+\item[@BH_K@]
+A black hole.
+\item[@ARR_K@]
+An array.
+\item[@ForeignObj_K@]
+A Foreign object (non-Haskell heap resident).
+\item[@SPT_K@]
+The Stable Pointer table. (There should only be one of these but it
+represents a form of weak space leak since it can't shrink to meet
+non-demand so it may be worth watching separately? ADR)
+\item[@INTERNAL_KIND@]
+Something internal to the runtime system.
+\end{description}
+
+
+\item[Description] Source derived string detailing closure description.
+\item[Type] Source derived string detailing closure type.
+\end{description}
+
+\fi % end of commented out stuff
+
+\item {\em Parallelism info\/}
+\ToDo{}
+
+\item {\em Debugging info\/}
+\ToDo{}
+
+\end{itemize}
+
+
+%-----------------------------------------------------------------------------
+\subsection{Kinds of Heap Object}
+\label{sect:closures}
+
+Heap objects can be classified in several ways, but one useful one is
+this:
+\begin{itemize}
+\item
+{\em Static closures} occupy fixed, statically-allocated memory
+locations, with globally known addresses.
+
+\item
+{\em Dynamic closures} are individually allocated in the heap.
+
+\item
+{\em Stack closures} are closures allocated within a thread's stack
+(which is itself a heap object). Unlike other closures, there are
+never any pointers to stack closures. Stack closures are discussed in
+Section~\ref{sect:stacks}.
+
+\end{itemize}
+A second useful classification is this:
+\begin{itemize}
+\item
+{\em Executive 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 {\em Activation frames}, represent ``continuations''. They are
+always stored on the stack and are never pointed to by heap objects or
+passed as arguments. \note{It's not clear if this will still be true
+once we support speculative evaluation.}
+
+\end{itemize}
+
+\item {\em Administrative objects}, such as stack objects and thread
+state objects, do not represent values in the original program.
+\end{itemize}
+
+Only pointed objects can be entered. All pointed objects share a
+common header format: the ``pointed header''; while all unpointed
+objects share a common header format: the ``unpointed header''.
+\ToDo{Describe the difference and update the diagrams to mention
+an appropriate header type.}
+
+This section enumerates all the kinds of heap objects in the system.
+Each is identified by a distinct @INFO_TYPE@ tag in its info table.
+
+\begin{tabular}{|l|l|l|l|l|l|l|l|l|l|l|}
+\hline
+
+closure kind & Section \\
+
+\hline
+{\em Pointed} \\
+\hline
+
+@CONSTR@ & \ref{sect:CONSTR} \\
+@CONSTR_STATIC@ & \ref{sect:CONSTR} \\
+@CONSTR_STATIC_NOCAF@ & \ref{sect:CONSTR} \\
+
+@FUN@ & \ref{sect:FUN} \\
+@FUN_STATIC@ & \ref{sect:FUN} \\
+
+@THUNK@ & \ref{sect:THUNK} \\
+@THUNK_STATIC@ & \ref{sect:THUNK} \\
+@THUNK_SELECTOR@ & \ref{sect:THUNK_SEL} \\
+
+@BCO@ & \ref{sect:BCO} \\
+@BCO_CAF@ & \ref{sect:BCO} \\
+
+@AP@ & \ref{sect:AP} \\
+@PAP@ & \ref{sect:PAP} \\
+
+@IND@ & \ref{sect:IND} \\
+@IND_OLDGEN@ & \ref{sect:IND} \\
+@IND_PERM@ & \ref{sect:IND} \\
+@IND_OLDGEN_PERM@ & \ref{sect:IND} \\
+@IND_STATIC@ & \ref{sect:IND} \\
+
+\hline
+{\em Unpointed} \\
+\hline
+
+@ARR_WORDS@ & \ref{sect:ARR_WORDS1},\ref{sect:ARR_WORDS2} \\
+@ARR_PTRS@ & \ref{sect:ARR_PTRS} \\
+@MUTVAR@ & \ref{sect:MUTVAR} \\
+@MUTARR_PTRS@ & \ref{sect:MUTARR_PTRS} \\
+@MUTARR_PTRS_FROZEN@ & \ref{sect:MUTARR_PTRS_FROZEN} \\
+
+@FOREIGN@ & \ref{sect:FOREIGN} \\
+
+@BH@ & \ref{sect:BH} \\
+@MVAR@ & \ref{sect:MVAR} \\
+@IVAR@ & \ref{sect:IVAR} \\
+@FETCHME@ & \ref{sect:FETCHME} \\
+\hline
+\end{tabular}
+
+Activation frames do not live (directly) on the heap --- but they have
+a similar organisation.
+
+\begin{tabular}{|l|l|}\hline
+closure kind & Section \\ \hline
+@RET_SMALL@ & \ref{sect:activation-records} \\
+@RET_VEC_SMALL@ & \ref{sect:activation-records} \\
+@RET_BIG@ & \ref{sect:activation-records} \\
+@RET_VEC_BIG@ & \ref{sect:activation-records} \\
+@UPDATE_FRAME@ & \ref{sect:activation-records} \\
+\hline
+\end{tabular}
+
+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} \\
+\hline
+\end{tabular}
+
+\ToDo{I guess the parallel system has something like a stable ptr
+table. Is there any opportunity for sharing code/data structures
+here?}
+
+
+\subsection{Predicates}
+
+\ToDo{The following is a first attempt at defining a useful set of
+predicates. Some (such as @isWHNF@ and @isSparkable@) may need their
+definitions tweaked a little.}
+
+The runtime system sometimes needs to be able to distinguish objects
+according to their properties: is the object updateable? is it in weak
+head normal form? etc. These questions can be answered by examining
+the @INFO_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 some @BCO@s.
+
+\ToDo{Need to distinguish between whnf BCOs and non-whnf BCOs in their
+@INFO_TYPE@}
+
+\item @isBOTTOM@ is true if the object is known to be $\bot$. It is
+true of @BH@s. \note{I suspect we'll want to add other kinds of
+infotype which are known to be bottom later.}
+
+\item @isUPDATEABLE@ is true if the object may be overwritten with an
+ indirection object.
+
+@isUPDATEABLE@ is true for @THUNK@s, @AP@s and @BH@s.
+
+\end{itemize}
+
+It is possible for a pointed object to be neither updatable nor in
+WHNF. For example, indirections.
+
+\item @isUNPOINTED@ is true if an object has an unpointed type.
+All such objects are boxed since only boxed objects have info pointers.
+
+It is true for @ARR_WORDS@, @ARR_PTRS@, @MUTVAR@, @MUTARR_PTRS@,
+@MUTARR_PTRS_FROZEN@, @FOREIGN@ objects, @MVAR@s and @IVAR@s.
+
+\item @isACTIVATION_FRAME@ is true for activation frames of all sorts.
+
+It is true for return addresses and update frames.
+\begin{itemize}
+\item @isVECTORED_RETADDR@ is true for vectored return addresses.
+\item @isDIRECT_RETADDR@ is true for direct return addresses.
+\end{itemize}
+
+\item @isADMINISTRATIVE@ is true for administrative objects:
+@TSO@s, the stable pointer table, spark objects and blocked fetches.
+
+\end{itemize}
+
+\begin{itemize}
+
+\item @isSTATIC@ is true for any statically allocated closure.
+
+\item @isMUTABLE@ is true for objects with mutable pointer fields:
+ @MUT_ARR@s, @MUTVAR@s, @MVAR@s and @IVAR@s.
+
+\item @isSparkable@ is true if the object can (and should) be sparked.
+It is true of updateable objects which are not in WHNF with the
+exception of @THUNK_SELECTOR@s and black holes.
+
+\end{itemize}
+
+As a minor optimisation, we might use the top bits of the @INFO_TYPE@
+field to ``cache'' the answers to some of these predicates.
+
+An indirection either points to HNF (post update); or is result of
+overwriting a FetchMe, in which case the thing fetched is either
+under evaluation (BH), or by now an HNF. Thus, indirections get NoSpark flag.
+
+
+\iffalse
+@
+#define _NF 0x0001 /* Normal form */
+#define _NS 0x0002 /* Don't spark */
+#define _ST 0x0004 /* Is static */
+#define _MU 0x0008 /* Is mutable */
+#define _UP 0x0010 /* Is updatable (but not mutable) */
+#define _BM 0x0020 /* Is a "primitive" array */
+#define _BH 0x0040 /* Is a black hole */
+#define _IN 0x0080 /* Is an indirection */
+#define _TH 0x0100 /* Is a thunk */
+
+
+
+SPEC
+SPEC_N SPEC | _NF | _NS
+SPEC_S SPEC | _TH
+SPEC_U SPEC | _UP | _TH
+
+GEN
+GEN_N GEN | _NF | _NS
+GEN_S GEN | _TH
+GEN_U GEN | _UP | _TH
+
+DYN _NF | _NS
+TUPLE _NF | _NS | _BM
+DATA _NF | _NS | _BM
+MUTUPLE _NF | _NS | _MU | _BM
+IMMUTUPLE _NF | _NS | _BM
+STATIC _NS | _ST
+CONST _NF | _NS
+CHARLIKE _NF | _NS
+INTLIKE _NF | _NS
+
+BH _NS | _BH
+BH_N BH
+BH_U BH | _UP
+
+BQ _NS | _MU | _BH
+IND _NS | _IN
+CAF _NF | _NS | _ST | _IN
+
+FM
+FETCHME FM | _MU
+FMBQ FM | _MU | _BH
+
+TSO _MU
+
+STKO
+STKO_DYNAMIC STKO | _MU
+STKO_STATIC STKO | _ST
+
+SPEC_RBH _NS | _MU | _BH
+GEN_RBH _NS | _MU | _BH
+BF _NS | _MU | _BH
+INTERNAL
+
+@
+\fi
+
+
+\subsection{Pointed Objects}
+
+All pointed objects can be entered.
+
+\subsubsection{Function closures}\label{sect:FUN}
+
+Function closures represent lambda abstractions. For example,
+consider the top-level declaration:
+@
+ f = \x -> let g = \y -> x+y
+ in g x
+@
+Both @f@ and @g@ are represented by function closures. The closure
+for @f@ is {\em static} while that for @g@ is {\em dynamic}.
+
+The layout of a function closure is as follows:
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+{\em Fixed header} & {\em Pointers} & {\em Non-pointers} \\ \hline
+\end{tabular}
+\end{center}
+The data words (pointers and non-pointers) are the free variables of
+the function closure.
+The number of pointers
+and number of non-pointers are stored in the @INFO_SM@ word, in the least significant
+and most significant half-word respectively.
+
+There are several different sorts of function closure, distinguished
+by their @INFO_TYPE@ field:
+\begin{itemize}
+\item @FUN@: a vanilla, dynamically allocated on the heap.
+
+\item $@FUN_@p@_@np$: to speed up garbage collection a number of
+specialised forms of @FUN@ are provided, for particular $(p,np)$ pairs,
+where $p$ is the number of pointers and $np$ the number of non-pointers.
+
+\item @FUN_STATIC@. Top-level, static, function closures (such as
+@f@ above) have a different
+layout than dynamic ones:
+\begin{center}
+\begin{tabular}{|l|l|l|}\hline
+{\em Fixed header} & {\em 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
+been scavenged.
+\note{Static function closures that have no static references, and hence
+a null SRT pointer, don't need the static object link field. Is it worth
+taking advantage of this? See @CONSTR_STATIC_NOCAF@.}
+\end{itemize}
+
+Each lambda abstraction, $f$, in the STG program has its own private info table.
+The following labels are relevant:
+\begin{itemize}
+\item $f$@_info@ is $f$'s info table.
+\item $f$@_entry@ is $f$'s slow entry point (i.e. the entry code of its
+info table; so it will label the same byte as $f$@_info@).
+\item $f@_fast_@k$ is $f$'s fast entry point. $k$ is the number of arguments
+$f$ takes; encoding this number in the fast-entry label occasionally catches some nasty
+code-generation errors.
+\end{itemize}
+
+\subsubsection{Data Constructors}\label{sect:CONSTR}
+
+Data-constructor closures represent values constructed with
+algebraic data type constructors.
+The general layout of data constructors is the same as that for function
+closures. That is
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+{\em Fixed header} & {\em Pointers} & {\em 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_CHARLIKE@: same deal, but for @Char@.
+
+\item @CONSTR_STATIC@ is similar to @FUN_STATIC@, with the complication that
+the layout of the constructor must mimic that of a dynamic constructor,
+because a static constructor might be returned to some code that unpacks it.
+So its layout is like this:
+\begin{center}
+\begin{tabular}{|l|l|l|l|l|}\hline
+{\em Fixed header} & {\em Pointers} & {\em Non-pointers} & {\em Static object link}\\ \hline
+\end{tabular}
+\end{center}
+The static object link, at the end of the closure, serves the same purpose
+as that for @FUN_STATIC@. The pointers in the static constructor can point
+only to other static closures.
+
+The static object link occurs last in the closure so that static
+constructors can store their data fields in exactly the same place as
+dynamic constructors.
+
+\item @CONSTR_STATIC_NOCAF@. A statically allocated data constructor
+that guarantees not to point (directly or indirectly) to any CAF
+(section~\ref{sect:CAF}). This means it does not need a static object
+link field. Since we expect that there might be quite a lot of static
+constructors this optimisation makes sense. Furthermore, the @NOCAF@
+tag allows the compiler to indicate that no CAFs can be reached
+anywhere {\em even indirectly}.
+
+
+\end{itemize}
+
+For each data constructor $Con$, two
+info tables are generated:
+\begin{itemize}
+\item $Con$@_info@ labels $Con$'s dynamic info table,
+shared by all dynamic instances of the constructor.
+\item $Con$@_static@ labels $Con$'s static info table,
+shared by all static instances of the constructor.
+\end{itemize}
+
+\subsubsection{Thunks}
+\label{sect:THUNK}
+\label{sect:THUNK_SEL}
+
+A thunk represents an expression that is not obviously in head normal
+form. For example, consider the following top-level definitions:
+@
+ range = between 1 10
+ f = \x -> let ys = take x range
+ in sum ys
+@
+Here the right-hand sides of @range@ and @ys@ are both thunks; the former
+is static while the latter is dynamic.
+
+The layout of a thunk is the same as that for a function closure,
+except that it may have some words of ``slop'' at the end to make sure
+that it has
+at least @MIN_UPD_PAYLOAD@ words in addition to its
+fixed header.
+\begin{center}
+\begin{tabular}{|l|l|l|l|l|}\hline
+{\em Fixed header} & {\em Pointers} & {\em Non-pointers} & {\em Slop} \\ \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).
+
+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}.
+
+\begin{center}
+\begin{tabular}{|l|l|l|l|l|}\hline
+{\em Fixed header} & {\em Pointers} & {\em Non-pointers} & {\em Slop} & {\em Static object link}\\ \hline
+\end{tabular}
+\end{center}
+
+\item @THUNK_SELECTOR@ is a (dynamically allocated) thunk
+whose entry code performs a simple selection operation from
+a data constructor drawn from a single-constructor type. For example,
+the thunk
+@
+ x = case y of (a,b) -> a
+@
+is a selector thunk. A selector thunk is laid out like this:
+\begin{center}
+\begin{tabular}{|l|l|l|l|}\hline
+{\em Fixed header} & {\em 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 only label associated with a thunk is its info table:
+\begin{description}
+\item[$f$@_info@] is $f$'s info table.
+\end{description}
+
+
+\subsubsection{Byte-Code Objects}
+\label{sect: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.
+
+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-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 (PAPs)}\label{sect:PAP}
+
+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
+\end{tabular}
+\end{center}
+The ``arg stack'' is a copy of the chunk of stack above the update
+frame; ``no of arg words'' tells how many words it consists of. The
+function closure is (a pointer to) the closure for the function whose
+argument-satisfaction check failed.
+
+There is just one standard form of PAP 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.)
+
+PAPs are also used to implement Hugs functions (where the arguments are free variables).
+PAPs generated by Hugs can be static.
+
+\subsubsection{@AP@ objects}
+\label{sect: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} & {\em No of arg words} & {\em Function closure} & {\em 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.
+
+
+\subsubsection{Indirections}
+\label{sect:IND}
+\label{sect:IND_OLDGEN}
+
+Indirection closures just point to other closures. They are introduced
+when a thunk is updated to point to its value.
+The entry code for all indirections simply enters the closure it points to.
+
+There are several forms of indirection:
+\begin{description}
+\item[@IND@] is the vanilla, dynamically-allocated indirection.
+It is removed by the garbage collector. It has the following
+shape:
+\begin{center}
+\begin{tabular}{|l|l|l|}\hline
+{\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
+\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.
+
+
+\item[@IND_PERMANENT@ and @IND_OLDGEN_PERMANENT@.]
+for lexical profiling, it is necessary to maintain cost centre
+information in an indirection, so ``permanent indirections'' are
+retained forever. Otherwise they are just like vanilla indirections.
+\note{If a permanent indirection points to another permanent
+indirection or a @CONST@ closure, it is possible to elide the indirection
+since it will have no effect on the profiler.}
+\note{Do we still need @IND@ and @IND_OLDGEN@
+in the profiling build, or can we just make
+do with one pair whose behaviour changes when profiling is built?}
+
+\item[@IND_STATIC@] is used for overwriting CAFs when they have been
+evaluated. Static indirections are not removed by the garbage
+collector; and are statically allocated outside the heap (and should
+stay there). Their static object link field is used just as for
+@FUN_STATIC@ closures.
+
+\begin{center}
+\begin{tabular}{|l|l|l|}
+\hline
+{\em Fixed header} & {\em Target closure} & {\em 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}
+
+Black hole closures are used to overwrite closures currently being
+evaluated. They inform the garbage collector that there are no live
+roots in the closure, thus removing a potential space leak.
+
+Black holes also become synchronization points in the threaded world.
+They contain a pointer to a list of blocked threads to be awakened
+when the black hole is updated (or @NULL@ if the list is empty).
+\begin{center}
+\begin{tabular}{|l|l|l|}
+\hline
+{\em Fixed header} & {\em Mutable link} & {\em Blocked thread link} \\
+\hline
+\end{tabular}
+\end{center}
+The {\em Blocked thread link} points to the TSO of the first thread
+waiting for the value of this thunk. All subsequent TSOs in the list
+are linked together using their @TSO_LINK@ field.
+
+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.
+
+\ToDo{MVars}
+
+
+\subsubsection{FetchMes}\label{sect:FETCHME}
+
+In the parallel systems, FetchMes are used to represent pointers into
+the global heap. When evaluated, the value they point to is read from
+the global heap.
+
+\ToDo{Describe layout}
+
+Because there may be offsets into these arrays, a primitive array
+cannot be handled as a FetchMe in the parallel system, but must be
+shipped in its entirety if its parent closure is shipped.
+
+
+
+\subsection{Unpointed Objects}
+
+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}.
+
+\subsubsection{Immutable Objects}
+\label{sect:ARR_WORDS1}
+\label{sect:ARR_PTRS}
+
+\begin{description}
+\item[@ARR_WORDS@] is a variable-sized object consisting solely of
+non-pointers. It is used for arrays of all
+sorts of things (bytes, words, floats, doubles... it doesn't matter).
+\begin{center}
+\begin{tabular}{|c|c|c|c|}
+\hline
+{\em Fixed Hdr} & {\em No of non-pointers} & {\em Non-pointers\ldots} \\ \hline
+\end{tabular}
+\end{center}
+
+\item[@ARR_PTRS@] is an immutable, variable sized array of pointers.
+\begin{center}
+\begin{tabular}{|c|c|c|c|}
+\hline
+{\em Fixed Hdr} & {\em Mutable link} & {\em No of pointers} & {\em Pointers\ldots} \\ \hline
+\end{tabular}
+\end{center}
+The mutable link is present so that we can easily freeze and thaw an
+array (by changing the header and adding/removing the array to the
+mutables list).
+
+\end{description}
+
+\subsubsection{Mutable Objects}
+\label{sect:mutables}
+\label{sect:ARR_WORDS2}
+\label{sect:MUTVAR}
+\label{sect:MUTARR_PTRS}
+\label{sect:MUTARR_PTRS_FROZEN}
+
+Some of these objects are {\em mutable}; they represent objects which
+are explicitly mutated by Haskell code through the @ST@ monad.
+They're not used for thunks which are updated precisely once.
+Depending on the garbage collector, mutable closures may contain extra
+header information which allows a generational collector to implement
+the ``write barrier.''
+
+\begin{description}
+
+\item[@ARR_WORDS@] is also used to represent {\em mutable} arrays of
+bytes, words, floats, doubles, etc. It's possible to use the same
+object type because even generational collectors don't need to
+distinguish them.
+
+\item[@MUTVAR@] is a mutable variable.
+\begin{center}
+\begin{tabular}{|c|c|c|}
+\hline
+{\em Fixed Hdr} & {\em Mutable link} & {\em 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
+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
+\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}
+
+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} \\
+\hline
+\end{tabular}
+\end{center}
+
+The @FreeRoutine@ is a reference to the finalisation routine to call
+when the @ForeignObj@ becomes garbage. If @freeForeignObject@ is
+called on a Foreign Object, the @FreeRoutine@ is set to zero and the
+garbage collector will not attempt to call @FreeRoutine@ when the
+object becomes garbage.
+
+The Foreign object link is a link to the next foreign object in the
+list. This list is traversed at the end of garbage collection: if an
+object is about to be deallocated (e.g.~it was not marked or
+evacuated), the free routine is called and the object is deleted from
+the list.
+
+
+The remaining objects types are all administrative --- none of them may be entered.
+
+\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}
+
+\item[Slop Objects]\label{sect: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
+{\em Info Pointer} & {\em Size} & {\em Slop Words} \\ \hline
+\end{tabular}
+\end{center}
+
+This is too large for single word slop objects which consist of a
+single info table.
+
+Note that slop objects only contain an info pointer, not a standard
+fixed header. This doesn't cause problems because slop objects are
+always unreachable --- they can only be accessed by linearly scanning
+the heap.
+
+\end{description}
+
+\subsection{Thread State Objects (TSOs)}\label{sect:TSO}
+
+\ToDo{This is very out of date. We now embed a single stack object
+within the TSO. TSOs include an ID number which can be used to
+generate a hash value. The gransim, profiling and ticky info is
+surely bogus.}
+
+In the multi-threaded system, the state of a suspended thread is
+packed up into a Thread State Object (TSO) which contains all the
+information needed to restart the thread and for the garbage collector
+to find all reachable objects. When a thread is running, it may be
+``unpacked'' into machine registers and various other memory locations
+to provide faster access.
+
+Single-threaded systems don't really {\em need\/} TSOs --- but they do
+need some way to tell the storage manager about live roots so it is
+convenient to use a single TSO to store the mutator state even in
+single-threaded systems.
+
+Rather than manage TSOs' alloc/dealloc, etc., in some {\em ad hoc}
+way, we instead alloc/dealloc/etc them in the heap; then we can use
+all the standard garbage-collection/fetching/flushing/etc machinery on
+them. So that's why TSOs are ``heap objects,'' albeit very special
+ones.
+\begin{center}
+\begin{tabular}{|l|l|}
+ \hline {\em Fixed header}
+\\ \hline @TSO_LINK@
+\\ \hline @TSO_WHATNEXT@
+\\ \hline @TSO_WHATNEXT_INFO@
+\\ \hline @TSO_STACK@
+\\ \hline {\em Exception Handlers}
+\\ \hline {\em Ticky Info}
+\\ \hline {\em Profiling Info}
+\\ \hline {\em Parallel Info}
+\\ \hline {\em GranSim Info}
+\\ \hline
+\end{tabular}
+\end{center}
+The contents of a TSO are:
+\begin{itemize}
+
+\item A pointer (@TSO_LINK@) used to maintain a list of threads with a similar
+ state (e.g.~all runnable, all sleeping, all blocked on the same black
+ hole, all blocked on the same MVar, etc.)
+
+\item A word (@TSO_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 Optional information for ``Ticky Ticky'' statistics: @TSO_STK_HWM@ is
+ the maximum number of words allocated to this thread.
+
+\item Optional information for profiling:
+ @TSO_CCC@ is the current cost centre.
+
+\item Optional information for parallel execution:
+\begin{itemize}
+
+\item The types of threads (@TSO_TYPE@):
+\begin{description}
+\item[@T_MAIN@] Must be executed locally.
+\item[@T_REQUIRED@] A required thread -- may be exported.
+\item[@T_ADVISORY@] An advisory thread -- may be exported.
+\item[@T_FAIL@] A failure thread -- may be exported.
+\end{description}
+
+\item I've no idea what else
+
+\end{itemize}
+
+\item Optional information for GranSim execution:
+\begin{itemize}
+\item locked
+\item sparkname
+\item started at
+\item exported
+\item basic blocks
+\item allocs
+\item exectime
+\item fetchtime
+\item fetchcount
+\item blocktime
+\item blockcount
+\item global sparks
+\item local sparks
+\item queue
+\item priority
+\item clock (gransim light only)
+\end{itemize}
+
+
+Here are the various queues for GrAnSim-type events.
+@
+Q_RUNNING
+Q_RUNNABLE
+Q_BLOCKED
+Q_FETCHING
+Q_MIGRATING
+@
+
+\end{itemize}
+
+\subsection{Stack Objects}
+\label{sect:STACK_OBJECT}
+\label{sect:stacks}
+
+These are ``stack objects,'' which are used in the threaded world as
+the stack for each thread is allocated from the heap in smallish
+chunks. (The stack in the sequential world is allocated outside of
+the heap.)
+
+Each reduction thread has to have its own stack space. As there may
+be many such threads, and as any given one may need quite a big stack,
+a naive give-'em-a-big-stack-and-let-'em-run approach will cost a {\em
+lot} of memory.
+
+Our approach is to give a thread a small stack space, and then link
+on/off extra ``chunks'' as the need arises. Again, this is a
+storage-management problem, and, yet again, we choose to graft the
+whole business onto the existing heap-management machinery. So stack
+objects will live in the heap, be garbage collected, etc., etc..
+
+A stack object is laid out like this:
+
+\begin{center}
+\begin{tabular}{|l|}
+\hline
+{\em Fixed header}
+\\ \hline
+{\em Link to next stack object (0 for last)}
+\\ \hline
+{\em N, the payload size in words}
+\\ \hline
+{\em @Sp@ (byte offset from head of object)}
+\\ \hline
+{\em @Su@ (byte offset from head of object)}
+\\ \hline
+{\em Payload (N words)}
+\\ \hline
+\end{tabular}
+\end{center}
+
+\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
+at the lowest address of the chunk of stack it ``knows about''
+just like an info pointer on a closure.
+
+The garbage collector needs to be able to find all the
+pointers in a stack. How does it do this?
+
+\begin{itemize}
+
+\item Within the stack there are return addresses, pushed
+by @case@ expressions. Below a return address (i.e. at higher
+memory addresses, since the stack grows downwards) is a chunk
+of stack that the return address ``knows about'', namely the
+activation record of the currently running function.
+
+\item Below each such activation record is a {\em pending-argument
+section}, a chunk of
+zero or more words that are the arguments to which the result
+of the function should be applied. The return address does not
+statically
+``know'' how many pending arguments there are, or their types.
+(For example, the function might return a result of type $\alpha$.)
+
+\item Below each pending-argument section is another return address,
+and so on. Actually, there might be an update frame instead, but we
+can consider update frames as a special case of a return address with
+a well-defined activation record.
+
+\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
+walks the stack from the top, traversing pending-argument sections and
+activation records alternately. Next we discuss how it finds
+the pointers in each of these two stack regions.
+
+
+\subsubsection{Activation records}\label{sect:activation-records}
+
+An {\em activation record} is a contiguous chunk of stack,
+with a return address as its first word, followed by as many
+data words as the return address ``knows about''. The return
+address is actually a fully-fledged info pointer. It points
+to an info table, replete with:
+
+\begin{itemize}
+\item entry code (i.e. the code to return to).
+\item @INFO_TYPE@ is either @RET_SMALL/RET_VEC_SMALL@ or @RET_BIG/RET_VEC_BIG@, depending
+on whether the activation record has more than 32 data words (\note{64 for 8-byte-word architectures}) and on whether
+to use a direct or a vectored return.
+\item @INFO_SM@ for @RET_SMALL@ is a bitmap telling the layout
+of the activation record, one bit per word. The least-significant bit
+describes the first data word of the record (adjacent to the fixed
+header) and so on. A ``@1@'' indicates a non-pointer, a ``@0@''
+indicates
+a pointer. We don't need to indicate exactly how many words there
+are,
+because when we get to all zeros we can treat the rest of the
+activation record as part of the next pending-argument region.
+
+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.
+
+\item @INFO_SRT@ is the Static Reference Table for the return
+address (Section~\ref{sect: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.
+
+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}
+
+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:
+
+\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.
+
+\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.
+
+\item Otherwise it must be a bona fide heap pointer.
+\end{itemize}
+
+
+\subsection{The Stable Pointer Table}\label{sect: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
+not change when the Haskell garbage collector runs---in contrast to
+the address of the object which may well change.
+
+A stable pointer is represented by an index into the
+@StablePointerTable@. The Haskell garbage collector treats the
+@StablePointerTable@ as a source of roots for GC.
+
+In order to provide efficient access to stable pointers and to be able
+to cope with any number of stable pointers (eg $0 \ldots 100000$), the
+table of stable pointers is an array stored on the heap and can grow
+when it overflows. (Since we cannot compact the table by moving
+stable pointers about, it seems unlikely that a half-empty table can
+be reduced in size---this could be fixed if necessary by using a
+hash table of some sort.)
+
+In general a stable pointer table closure looks like this:
+
+\begin{center}
+\begin{tabular}{|l|l|l|l|l|l|l|l|l|l|l|}
+\hline
+{\em Fixed header} & {\em No of pointers} & {\em Free} & $SP_0$ & \ldots & $SP_{n-1}$
+\\\hline
+\end{tabular}
+\end{center}
+
+The fields are:
+\begin{description}
+
+\item[@NPtrs@:] number of (stable) pointers.
+
+\item[@Free@:] the byte offset (from the first byte of the object) of the first free stable pointer.
+
+\item[$SP_i$:] A stable pointer slot. If this entry is in use, it is
+an ``unstable'' pointer to a closure. If this entry is not in use, it
+is a byte offset of the next free stable pointer slot.
+
+\end{description}
+
+When a stable pointer table is evacuated
+\begin{enumerate}
+\item the free list entries are all set to @NULL@ so that the evacuation
+ code knows they're not pointers;
+
+\item The stable pointer slots are scanned linearly: non-@NULL@ slots
+are evacuated and @NULL@-values are chained together to form a new free list.
+\end{enumerate}
+
+There's no need to link the stable pointer table onto the mutable
+list because we always treat it as a root.
+
+
+
+\section{The Bytecode Evaluator}
+
+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.
+
+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.
+
+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.
+
+We achieve this simplicity by forgoing some of the optimisations used
+by compiled code:
+\begin{itemize}
+\item
+
+Whereas compiled code has five different ways of entering a closure
+(section~\ref{sect:entering-closures}), interpreted code has only one.
+The entry point for interpreted code behaves like slow entry points for
+compiled code.
+
+\item
+
+We use just one info table for {\em all\/} direct returns.
+This introduces two problems:
+\begin{enumerate}
+\item How does the interpreter know what code to execute?
+
+Instead of pushing just a return address, we push a return BCO and a
+trivial return address which just enters the return BCO.
+
+(In a purely interpreted system, we could avoid pushing the trivial
+return address.)
+
+\item How can the garbage collector follow pointers within the
+activation record?
+
+We could push a third word ---a bitmask describing the location of any
+pointers within the record--- but, since we're already tagging unboxed
+function arguments on the stack, we use the same mechanism for unboxed
+values within the activation record.
+
+\ToDo{Do we have to stub out dead variables in the activation frame?}
+
+\end{enumerate}
+
+\item
+
+We trivially support vectored returns by pushing a return vector whose
+entries are all the same.
+
+\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 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 (section~\label{sect: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}
+\label{sect: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 Section \ref{sect: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 - 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.
+
+\ToDo{The phrase "all entry points..." suggests that BCOs have multiple
+entry points. If so, we need to say a lot more about it...}
+
+\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.
+
+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 Section \ref{sect:AP}.
+
+Partial applications are represented by @PAP@ objects, which are
+non-updatable.
+
+\ToDo{Hugs Constructors}.
+
+\subsection{Calling conventions}
+\label{sect:hugs-calling-conventions}
+\label{sect:standard-closures}
+
+The calling convention for any byte-code function is straightforward:
+\begin{itemize}
+\item Push any arguments on the stack.
+\item Push a pointer to the BCO.
+\item Begin interpreting the byte code.
+\end{itemize}
+
+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
+Section~\ref{sect: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:
+
+\begin{description}
+\item[Constructor]
+To enter a constructor, we simply return (see Section
+\ref{sect:hugs-return-convention}).
+
+\item[Indirection]
+To enter an indirection, we simply enter the object it points to
+after possibly adjusting the current cost centre.
+
+\item[@AP@]
+
+To enter an @AP@, we push an update frame, push the
+arguments, push the function and enter the function.
+(Not forgetting a stack check at the start.)
+
+\item[@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, 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 Section
+\ref{sect:hugs-to-ghc-switch}).
+
+
+\subsection{Return convention}
+\label{sect:hugs-return-convention}
+
+When Hugs pushes a return address, it pushes both a pointer to the BCO
+to return to, and a pointer to a static code fragment @HUGS_RET@ (this
+is described in Section \ref{sect:ghc-to-hugs-switch}). The
+stack layout is shown in Figure \ref{fig: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}
+
+\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}
+
+\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}
+
+\begin{figure}[ht]
+\begin{center}
+@
+| stack |
++----------+
+| ghc_ret |
++----------+
+| con |--> CON
++----------+
+@
+%\input{hugs_ret3.pstex_t}
+\end{center}
+\caption{Stack layout on enterings a GHC return address}
+\label{fig:hugs-return3}
+\end{figure}
+
+\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}
+
+When a Hugs byte-code sequence enters a closure, it examines the
+return address on top of the stack.
+
+\begin{itemize}
+
+\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
+(Figure \ref{fig:hugs-return2}).
+
+\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-switch}.
+
+\end{itemize}
+
+\ToDo{This duplicates what we say about switching worlds
+(section~\ref{sect:switching-worlds}) - kill one or t'other.}
+
+
+\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.}
+
+\subsection{Addressing Modes}
+
+To avoid potential alignment problems and simplify garbage collection,
+all literal constants are stored in two tables (one boxed, the other
+unboxed) within each BCO and are referred to by offsets into the tables.
+Slots in the constant tables are word aligned.
+
+\ToDo{How big can the offsets be? Is the offset specified in the
+address field or in the instruction?}
+
+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}
+
+
+\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
+> 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}
+
+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}
+
+\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}
-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.
+Similar rules exist for floats, doubles, chars, etc.
-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}).
+\item[ Delete environment from stack --- ready for tail call ].
-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>
+\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 $\size{\bs} = n$.
-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
-@
-ind: \Arg{1} = \Arg{1}->next;
- goto ind_loop;
-@
-where @ind_loop@ is the second line of code.
+\item[ Push a return address ].
-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.
+\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[ Push a BCO ].
-\subsection{Heap and Stack Checks}
-\label{sect:heap-and-stack-checks}
+\begin{tabular}{|llrrrrr|}
+\hline
+ & PUSHBCO $o$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $\sigma!o : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\hline
+\end{tabular}
-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.
+\end{description}
-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.
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\subsection{Heap manipulation}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-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.
+\begin{description}
-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.
+\item[ Allocate a heap object ].
-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.
+\begin{tabular}{|llrrrrr|}
+\hline
+ & ALLOC $m$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
+\next & $\is$ & $hp:s$ & $su$ & $h$ & $hp+m$ & $\sigma$ \\
+\hline
+\end{tabular}
-\ToDo{Be more precise about how much space is required - document it
-in the calling convention section.}
+\item[ Build a constructor ].
-\subsection{Handling interrupts/signals}
+\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 $\size{\ws} = \arity{C}$.
-@
-May have to keep C stack pointer in register to placate OS?
-May have to revert black holes - ouch!
-@
+\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 $\size{\ws} = m$.
-\section{Interpreted Execution}
+\item[ Unpacking a constructor ].
-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.
+\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}
+
+\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}
-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.
+\item[Returning a constructor to GHC].
-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{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}
-We achieve this simplicity by forgoing some of the optimisations used
-by compiled code:
-\begin{itemize}
-\item
+\end{description}
-Whereas compiled code has five different ways of entering a closure
-(section~\ref{sect:entering-closures}), interpreted code has only one.
-The entry point for interpreted code behaves like slow entry points for
-compiled code.
-\item
+\subsection{Updates}
-We use just one info table for {\em all\/} direct returns.
-This introduces two problems:
-\begin{enumerate}
-\item How does the interpreter know what code to execute?
+\begin{description}
-Instead of pushing just a return address, we push a return BCO and a
-trivial return address which just enters the return BCO.
+\item[ Updating with a constructor].
-(In a purely interpreted system, we could avoid pushing the trivial
-return address.)
+\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 How can the garbage collector follow pointers within the
-activation record?
+\item[ Argument checks].
-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.
+\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 $m \ge (su - sp)$
-\ToDo{Do we have to stub out dead variables in the activation frame?}
+\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
+ $h' = h[ua \mapsto \Ind{a'}, a' \mapsto \PAP(a,\reverse\ \as) ]$
-\end{enumerate}
+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.
-\item
+\end{description}
-We trivially support vectored returns by pushing a return vector whose
-entries are all the same.
+\subsection{Branches}
-\item
+\begin{description}
-We avoid the need to build SRTs by putting bytecode objects on the
-heap and restricting BCOs to a single basic block.
+\item[ Testing a constructor ].
-\end{itemize}
+\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 $C.\tag = tag$
-\subsubsection{Hugs Info Tables}
+\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 $C.\tag \neq tag$
-Hugs requires the following info tables and closures:
-\begin{description}
-\item [@HUGS_RET@].
+\end{description}
-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 (section~{sect: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.
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\subsection{Heap and stack checks}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-\item [@UPD_RET@].
+\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.
-\item [Constructors].
+\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.
-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.
+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.
-\item [@AP@ and @PAP@].
-\item [Indirections].
+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}
-\item [Selectors].
--- doesn't generate them itself but it ought to recognise them
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+\subsection{Primops}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-\item [Complex primops].
+\ToDo{primops take m words and return n words. The expect boxed arguments on the stack.}
-\end{description}
+\section{The Machine Code Evaluator}
-\subsection{Hugs Heap Objects}
-\label{sect:hugs-heap-objects}
+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}.
-\subsubsection{Byte-Code Objects}
+\subsection{Calling conventions}
-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.
+\subsubsection{The call/return registers}
-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 collectr. 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.
+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 $n=0$.
-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.
+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{The phrase "all entry points..." suggests that BCOs have multiple
-entry points. If so, we need to say a lot more about it...}
+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{Thunks and partial applications}
+\subsubsection{Entering closures}
+\label{sect:entering-closures}
-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.
+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.
-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 Section \ref{sect:AP}.
+\subsubsection{Function call}
-Partial applications are represented by @PAP@ objects, which are
-non-updatable.
+The function-call mechanism is obviously crucial. There are five different
+cases to consider:
+\begin{enumerate}
-\ToDo{Hugs Constructors}.
+\item {\em Known combinator (function with no free variables) and enough arguments.}
-\subsection{Calling conventions}
-\label{sect:hugs-calling-conventions}
-\label{sect:standard-closures}
+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}.
-The calling convention for any byte-code function is straightforward:
-\begin{itemize}
-\item Push any arguments on the stack.
-\item Push a pointer to the BCO.
-\item Begin interpreting the byte code.
-\end{itemize}
+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.
-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
-Section~\ref{sect:hugs-to-ghc-closure}) and entering the closure
-there.
+\item {\em Known combinator and insufficient arguments.}
-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 slow call can be made: push all arguments onto stack and jump to
+function's {\em slow entry point}.
-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:
+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.
-\begin{description}
-\item[Constructor]
-To enter a constructor, we simply return (see Section
-\ref{sect:hugs-return-convention}).
+%Todo: forward ref about tagging?
+%Todo: picture?
-\item[Indirection]
-To enter an indirection, we simply enter the object it points to
-after possibly adjusting the current cost centre.
+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[@AP@]
+\begin{itemize}
-To enter an @AP@, we push an update frame, push the
-arguments, push the function and enter the function.
-(Not forgetting a stack check at the start.)
+\item 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}.
-\item[@PAP@]
+\item If the argument satisfaction check succeeds, we jump to the fast
+entry point with the arguments in \Arg{1} \ldots \Arg{arity}.
-To enter a @PAP@, we push the arguments, push the function and enter
-the function. (Not forgetting a stack check at the start.)
+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.
-\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.
+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{description}
+\end{itemize}
-The most obvious omissions from the above list are @BCO@s (which we
-dealt with above) and GHC-built closures (which are covered in Section
-\ref{sect:hugs-to-ghc-closure}).
+\item {\em Known function closure (function with free variables) and enough arguments.}
-\subsection{Return convention}
-\label{sect:hugs-return-convention}
+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}.
-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}.
+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.
-\begin{figure}
-\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}
-\begin{figure}
-\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}
+\item {\em Known function closure and insufficient arguments.}
-\begin{figure}
-\begin{center}
-@
-| stack |
-+----------+
-| 3# |
-+----------+
-| I# |
-+----------+
-@
-%\input{hugs_ret2.pstex_t}
-\end{center}
-\caption{Stack layout on enterings a Hugs return address with an unboxed value}
-\label{fig:hugs-return-int}
-\end{figure}
+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{figure}
-\begin{center}
-@
-| stack |
-+----------+
-| ghc_ret |
-+----------+
-| con |--> CON
-+----------+
-@
-%\input{hugs_ret3.pstex_t}
-\end{center}
-\caption{Stack layout on enterings a GHC return address}
-\label{fig:hugs-return3}
-\end{figure}
+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.
-\begin{figure}
-\begin{center}
+
+\item {\em Unknown function closure, thunk or constructor.}
+
+Sometimes, the function being called is not statically identifiable.
+Consider, for example, the @compose@ function:
@
-| stack |
-+----------+
-| ghc_ret |
-+----------+
-| 3# |
-+----------+
-| I# |
-+----------+
-| restart |--> id_Int#_closure
-+----------+
+ compose f g x = f (g x)
@
-%\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}
+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)@.
-When a Hugs byte-code sequence enters a closure, it examines the
-return address on top of the stack.
+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{itemize}
+The {\em entry code} for a non-updateable closure is just the
+closure's slow entry point.
-\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
-(Figure \ref{fig:hugs-return2}).
+\end{enumerate}
-\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}.
+In addition to the above considerations, if there are \emph{too many}
+arguments then the extra arguments are simply pushed on the stack with
+appropriate tags.
-\end{itemize}
+To summarise, a closure's standard (slow) entry point performs the following:
+\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}
-\part{Implementation}
+\subsection{Case expressions and return conventions}
+\label{sect:return-conventions}
-\section{Overview}
+The {\em evaluation} of a thunk is always initiated by
+a @case@ expression. For example:
+@
+ case x of (a,b) -> E
+@
-This part describes the inner workings of the major components of the system.
-Their external interfaces are described in the previous part.
+The code for a @case@ expression looks like this:
-The major components of the system are:
\begin{itemize}
-\item The scheduler
-\item The loader
-\item The storage manager
-\item The machine code evaluator (compiled code)
-\item The bytecode evaluator (interpreted code)
+\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@:
-\section{Hugs Bytecodes}
-\label{sect:hugs-bytecodes}
+\begin{itemize}
-\ToDo{This was in the evaluation model part but it really belongs in
-this part which is about the internal details of each of the major
-sections.}
+\item
-\subsubsection{Addressing Modes}
+(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.
-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 If @x@ has a boxed type (e.g.~a data constructor or a function),
+a pointer to @x@ is returned in \Arg{1}.
-\ToDo{How big can the offsets be? Is the offset specified in the
-address field or in the instruction?}
+\ToDo{Warn that components of E should be extracted as soon as
+possible to avoid a space leak.}
-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.
+\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.
-\subsubsection{Compilation}
+\end{itemize}
+\subsection{Vectored Returns}
-\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}}
+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.
-\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}}
+\begin{itemize}
-\def\Ind#1{{\mbox{\it Ind}\ {#1}}}
-\def\update#1{{\mbox{\it update}\ {#1}}}
+\item
-\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}}}
+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}.
-\def\AP{\mbox{\it AP}}
-\def\PAP{\mbox{\it PAP}}
-\def\GHCRET{\mbox{\it GHCRET}}
-\def\GHCOBJ{\mbox{\it GHCOBJ}}
+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}).
-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.
+\item
-@
-> 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
+The constructor @Just@ returns by jumping to the second element of the
+return vector with a pointer to the closure in \Arg{1}.
-> 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
+\end{itemize}
-> build x (C{a1, ... am}) = do
-> pushUntaggedAtom am; ... pushUntaggedAtom a1
-> PACK x C
-> 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
+In this way no test need be made to see which constructor returns;
+instead, execution resumes immediately in the appropriate branch of
+the @case@.
-> cgRhs (vs \ xs. e) = do
-> ARGCHECK (xs ++ vs) -- can be omitted if xs == {}
-> STACKCHECK min(stackUse e,heapOverflowSlop)
-> HEAPCHECK (heapUse e)
-> cg e
+\subsection{Direct Returns}
-> pushAtom x = pushVar x
-> pushAtom i# = PUSHINT i#
+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}.
-> pushVar x = if isGlobalVar x then PUSHGLOBAL x else PUSHLOCAL x
+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.
-> pushUntaggedAtom x = pushVar x
-> pushUntaggedAtom i# = PUSHUNTAGGEDINT i#
+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}).
-> pushVar x = if isGlobalVar x then PUSHGLOBAL x else PUSHLOCAL x
-@
+Single-constructor data types also use direct returns, although in
+that case there is no need to return a tag in \Arg{2}.
-\ToDo{Is there an easy way to add semi-tagging? Would it be that different?}
+\ToDo{Say whether we pop the return address before returning}
-\ToDo{Optimise thunks of the form @f{x1,...xm}@ so that we build an AP directly}
+\ToDo{Stack stubbing?}
-\subsubsection{Instructions}
+\subsection{Updates}
+\label{sect:data-updates}
-We specify the semantics of instructions using transition rules of
-the form:
+The entry code for an updatable thunk (which must be of arity 0):
-\begin{tabular}{|llrrrrr|}
+\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.
+
+An update frame is a small activation record consisting of
+\begin{center}
+\begin{tabular}{|l|l|l|}
\hline
- & $\is$ & $s$ & $\su$ & $h$ & $hp$ & $\sigma$ \\
-\next & $\is'$ & $s'$ & $\su'$ & $h'$ & $hp'$ & $\sigma$ \\
+{\em Fixed header} & {\em Update Frame link} & {\em Updatee} \\
\hline
\end{tabular}
+\end{center}
-where $\is$ is an instruction stream, $s$ is the stack, $\su$ is the
-update frame pointer and $h$ is the heap.
+\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.
-\subsubsection{Stack manipulation}
+\item
+Start evaluating the body of the expression.
-\begin{description}
+\end{itemize}
-\item[ Push a global variable ].
+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{tabular}{|llrrrrr|}
-\hline
- & PUSHGLOBAL $o$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
-\next & $\is$ & $\sigma!o:s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
-\hline
-\end{tabular}
+\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[ Push a local variable ].
+\item If the data type is a direct-return type rather than a
+vectored-return type, then the tag is in \Arg{2}.
-\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 The update frame is still on the stack.
+\end{itemize}
-\item[ Push an unboxed int ].
+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:
-\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}
+@
+ | ^ |
+ | return vector |
+ |---------------|
+ | fixed-size |
+ | info table |
+ |---------------| <- update code pointer
+ | update code |
+ | v |
+@
-The $I\#$ is a tag included for the benefit of the garbage collector.
-Similar rules exist for floats, doubles, chars, etc.
+Each entry in the return vector (which is large enough to cover the
+largest vectored-return type) points to the update code.
-\item[ Push an unboxed int ].
+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}
-\begin{tabular}{|llrrrrr|}
-\hline
- & PUSHUNTAGGEDINT $o$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
-\next & $\is$ & $\sigma!o : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
-\hline
-\end{tabular}
+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.
-Similar rules exist for floats, doubles, chars, etc.
+\subsection{Semi-tagging}
+\label{sect:semi-tagging}
-\item[ Delete environment from stack --- ready for tail call ].
+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.
+
+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>
-\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 $\size{\bs} = n$.
+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
+@
+ind: \Arg{1} = \Arg{1}->next;
+ goto ind_loop;
+@
+where @ind_loop@ is the second line of code.
-\item[ Push a return address ].
+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.
-\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[ Push a BCO ].
+\subsection{Heap and Stack Checks}
+\label{sect:heap-and-stack-checks}
-\begin{tabular}{|llrrrrr|}
-\hline
- & PUSHBCO $o$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
-\next & $\is$ & $\sigma!o : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
-\hline
-\end{tabular}
+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.
-\end{description}
+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.
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-\subsubsection{Heap manipulation}
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+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.
-\begin{description}
+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.
-\item[ Allocate a heap object ].
+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.
-\begin{tabular}{|llrrrrr|}
-\hline
- & ALLOC $m$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
-\next & $\is$ & $hp:s$ & $su$ & $h$ & $hp+m$ & $\sigma$ \\
-\hline
-\end{tabular}
+\ToDo{Be more precise about how much space is required - document it
+in the calling convention section.}
-\item[ Build a constructor ].
+\subsection{Handling interrupts/signals}
-\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 $\size{\ws} = \arity{C}$.
+@
+May have to keep C stack pointer in register to placate OS?
+May have to revert black holes - ouch!
+@
-\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 $\size{\ws} = m$.
+\section{The Loader}
+\section{The Compilers}
-\item[ Unpacking a constructor ].
+\iffalse
+\part{Old stuff - needs to be mined for useful info}
-\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}
+\section{The Scheduler}
-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.
+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:
-\end{description}
+\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
-\subsubsection{Entering a closure}
+\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}
-\begin{description}
+Each thread is represented by a Thread State Object (TSO), which is
+described in detail in Section \ref{sect:TSO}.
-\item[ Enter a BCO ].
+The following is pseudo-code for the inner loop of the scheduler
+itself.
-\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}
+@
+while (threads_exist) {
+ // handle global problems: GC, parallelism, etc
+ if (need_gc) gc();
+ if (external_message) service_message();
+ // deal with other urgent stuff
-\item[ Enter a PAP closure ].
+ 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);
+}
+@
-\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}
+\subsection{Invoking the garbage collector}
+\subsection{Putting the thread to sleep}
-\item[ Entering an AP closure ].
+\subsection{Calling C from Haskell}
-\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}
+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.
-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}
+Safe calls are performed without returning to the scheduler and are
+discussed elsewhere (\ToDo{discuss elsewhere}).
+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.
-\item[ Returning a constructor ].
+\ToDo{Describe this in more detail.}
-\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}
+\subsection{Calling Haskell from C}
-\item[ Entering an indirection node ].
+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.
-\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}
+When the closure returns, the scheduler sends back a message which
+awakens the (C) thread.
-\item[Entering GHC closure].
+\ToDo{Do we need to worry about the garbage collector deallocating the
+thread if it gets blocked?}
-\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}
+\subsection{Switching Worlds}
+\label{sect:switching-worlds}
-\item[Returning a constructor to GHC].
+\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.}
-\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}
+Because this is a combined compiled/interpreted system, the
+interpreter will sometimes encounter compiled code, and vice-versa.
-\end{description}
+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.
+\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}
-\subsubsection{Updates}
+There are four cases we need to consider:
-\begin{description}
+\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}
-\item[ Updating with a constructor].
+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.
-\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}
+\ToDo{Hugs-built constructors?}
-\item[ Argument checks].
+We now examine the various cases one by one and describe how the
+switch happens in each situation.
-\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 $m \ge (su - sp)$
+\subsection{A GHC thread enters a Hugs-built closure}
+\label{sect:ghc-to-hugs-switch}
-\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
- $h' = h[ua \mapsto \Ind{a'}, a' \mapsto \PAP(a,\reverse\ \as) ]$
+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.
-Again, we reverse the list of values as we transfer them from the
-stack to the heap --- reflecting the fact that the stack and heap grow
-in different directions.
+The entry code for a BCO does the following:
-\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}
-\subsubsection{Branches}
+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.
-\begin{description}
+\subsection{A GHC thread returns to a Hugs-compiled return address}
+\label{sect:ghc-to-hugs-switch}
-\item[ Testing a constructor ].
+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:
-\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{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}
-\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 $C.\tag \neq tag$
+\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.
-\end{description}
+\subsection{A Hugs thread enters a GHC-compiled closure}
+\label{sect:hugs-to-ghc-switch}
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-\subsubsection{Heap and stack checks}
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+Hugs can recognise a GHC-built closure as not being one of the
+following types of object:
-\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{itemize}
+\item A @BCO@,
+\item A @AP@,
+\item A @PAP@,
+\item An indirection, or
+\item A constructor.
+\end{itemize}
-\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.
+When Hugs is called on to enter a GHC closure, it executes the
+following sequence of instructions:
-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.
+\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-switch}
+
+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:
-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.
+\item save the state of the thread in the TSO.
+\item return to the scheduler, setting @whatNext@ to @EnterGHC@.
\end{itemize}
+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.
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-\subsubsection{Primops}
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-\ToDo{primops take m words and return n words. The expect boxed arguments on the stack.}
+\fi
+\part{Implementation}
+
+\section{Overview}
+This part describes the inner workings of the major components of the system.
+Their external interfaces are described in the previous part.
+The major components of the system are:
+\begin{itemize}
+\item The scheduler
+\item The loader
+\item The storage manager
+\item The machine code evaluator (compiled code)
+\item The bytecode evaluator (interpreted code)
+\end{itemize}
+\iffalse
\section{Heap objects}
+\label{sect:heap-objects}
\label{sect:fixed-header}
\ToDo{Fix this picture}
\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}).
+\ref{sect: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
There's no need to link the stable pointer table onto the mutable
list because we always treat it as a root.
-
+\fi
\section{The Storage Manager}
\end{description}
-
%************************************************************************
%* *
\subsection{``What really happens in a garbage collection?''}
%* *
%************************************************************************
+\ToDo{I commented out this long, out of date section - ADR}
+
+\iffalse
+
This is a brief tutorial on ``what really happens'' going to/from the
storage manager in a garbage collection.
enough space before continuing.
\end{description}
-
+\fi % end of commented out part
\subsection{Static Reference Tables (SRTs)}
\label{sect:srt}
\subsection{Squeezing identical updates}
+\Note{This can also be done by testing whether @Sp == Su@ when we push
+an update frame. If so, we can overwrite the updatee with an
+indirection to the existing updatee (and some slop objects) and avoid
+pushing an update frame.}
+
\iffalse
\ToDo{Insert text stolen from update paper}
\subsection{Internal workings of the Generational Collector}
-\section{Dynamic Linking}
-
\section{Profiling}
Registering costs centres looks awkward - can we tidy it up?
%************************************************************************
%* *
-\subsubsection[ticky-stk-heap-use]{Stack and heap usage}
+\subsection[ticky-stk-heap-use]{Stack and heap usage}
%* *
%************************************************************************
%************************************************************************
%* *
-\subsubsection[ticky-allocs]{Allocations}
+\subsection[ticky-allocs]{Allocations}
%* *
%************************************************************************
%************************************************************************
%* *
-\subsubsection[ticky-enters]{Enters}
+\subsection[ticky-enters]{Enters}
%* *
%************************************************************************
%************************************************************************
%* *
-\subsubsection[ticky-returns]{Returns}
+\subsection[ticky-returns]{Returns}
%* *
%************************************************************************
%************************************************************************
%* *
-\subsubsection[ticky-update-frames]{Update frames}
+\subsection[ticky-update-frames]{Update frames}
%* *
%************************************************************************
%************************************************************************
%* *
-\subsubsection[ticky-updates]{Updates}
+\subsection[ticky-updates]{Updates}
%* *
%************************************************************************
%************************************************************************
%* *
-\subsubsection[ticky-selectors]{Doing selectors at GC time}
+\subsection[ticky-selectors]{Doing selectors at GC time}
%* *
%************************************************************************
\section{Old stuff about SRTs}
+\ToDo{Commented out}
+
+\iffalse
+
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
This scheme has not been adopted but has been implemented. The code is
commented out with @#if 0@.
+\fi
+
\subsection{The virtual register set}
+\ToDo{Commented out}
+
+\iffalse
+
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.
allow faster access to all the other non-machine registers.
@
+\fi
\end{document}
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