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\title{The STG runtime system (revised)}
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\author{Simon Peyton Jones \\ Microsoft Research Ltd., Cambridge \and
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Simon Marlow \\ Microsoft Research Ltd., Cambridge \and
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Alastair Reid \\ Yale University}
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\Section{Overview}{overview}
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This document describes the GHC/Hugs run-time system. It serves as
72
a Glasgow/Yale/Nottingham ``contract'' about what the RTS does.
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\Subsection{New features compared to GHC 3.xx}{new-features}
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\item The RTS supports mixed compiled/interpreted execution, so
78
that a program can consist of a mixture of GHC-compiled and Hugs-interpreted
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\item The RTS supports concurrency by default.
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This has some costs (eg we can't do hardware stack checks) but
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reduces the number of different configurations we need to support.
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\item CAFs are only retained if they are
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reachable. Since they are referred to by implicit references buried
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in code, this means that the garbage collector must traverse the whole
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accessible code tree. This feature eliminates a whole class of painful
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\item A running thread has only one stack, which contains a mixture of
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pointers and non-pointers. \secref{TSO} describes how we find out
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which is which. (GHC has used two stacks for some while. Using one
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stack instead of two reduces register pressure, reduces the size of
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update frames, and eliminates ``stack-stubbing'' instructions.)
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\item The ``return in registers'' return convention has been dropped
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because it was complicated and doesn't work well on register-poor
99
architectures. It has been partly replaced by unboxed tuples
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(\secref{unboxed-tuples}) which allow the programmer to
101
explicitly state where results should be returned in registers (or on
102
the stack) instead of on the heap.
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\item Exceptions are supported by the RTS.
106
\item Weak Pointers generalise the previously available Foreign Object
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\item The garbage collector supports a number of new features,
110
including a dynamically resizable heap and multiple generations with
111
aging within a generation.
115
\Subsection{Wish list}{wish-list}
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Here's a list of things we'd like to support in the future.
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\item Interrupts, speculative computation.
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The SM could tune the size of the allocation arena, the number of
123
generations, etc taking into account residency, GC rate and page fault
127
We could trigger a GC when all threads are blocked waiting for IO if
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the allocation arena (or some of the generations) are nearly full.
132
\Subsection{Configuration}{configuration}
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Some of the above features are expensive or less portable, so we
135
envision building a number of different configurations supporting
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different subsets of the above features.
138
You can make the following choices:
141
Support for parallelism. There are three mutually-exclusive choices.
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\item[@SEQUENTIAL@] Support for concurrency but not for parallelism.
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\item[@GRANSIM@] Concurrency support and simulated parallelism.
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\item[@PARALLEL@] Concurrency support and real parallelism.
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\item @PROFILING@ adds cost-centre profiling.
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\item @TICKY@ gathers internal statistics (often known as ``ticky-ticky'' code).
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\item @DEBUG@ does internal consistency checks.
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\item Persistence. (well, not yet).
158
Which garbage collector to use. At the moment we
159
only anticipate one, however.
162
\Subsection{Glossary}{glossary}
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\ToDo{This terminology is not used consistently within the document.
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If you find something which disagrees with this terminology, fix the
168
In the type system, we have boxed and unboxed types.
172
\item A \emph{pointed} type is one that contains $\bot$. Variables with
173
pointed types are the only things which can be lazily evaluated. In
174
the STG machine, this means that they are the only things that can be
175
\emph{entered} or \emph{updated} and it requires that they be boxed.
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\item An \emph{unpointed} type is one that does not contain $\bot$.
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Variables with unpointed types are never delayed --- they are always
179
evaluated when they are constructed. In the STG machine, this means
180
that they cannot be \emph{entered} or \emph{updated}. Unpointed objects
181
may be boxed (like @Array#@) or unboxed (like @Int#@).
185
In the implementation, we have different kinds of objects:
189
\item \emph{boxed} objects are heap objects used by the evaluators
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\item \emph{unboxed} objects are not heap allocated
193
\item \emph{stack} objects are allocated on the stack
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\item \emph{closures} are objects which can be \emph{entered}.
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They are always boxed and always have boxed types.
197
They may be in WHNF or they may be unevaluated.
199
\item A \emph{thunk} is a (representation of) a value of a \emph{pointed}
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type which is \emph{not} in WHNF.
202
\item A \emph{value} is an object in WHNF. It can be pointed or unpointed.
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At the hardware level, we have \emph{word}s and \emph{pointer}s.
212
\item A \emph{word} is (at least) 32 bits and can hold either a signed
215
\item A \emph{pointer} is (at least) 32 bits and big enough to hold a
216
function pointer or a data pointer.
220
Occasionally, a field of a data structure must hold either a word or a
221
pointer. In such circumstances, it is \emph{not safe} to assume that
222
words and pointers are the same size. \ToDo{GHC currently makes words
223
the same size as pointers to reduce complexity in the code
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generator/RTS. It would be useful to relax this restriction, and have
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eg. 32-bit Ints on a 64-bit machine.}
227
% should define terms like SRT, CAF, PAP, etc. here? --KSW 1999-03
229
\subsection{Subtle Dependencies}
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Some decisions have very subtle consequences which should be written
232
down in case we want to change our minds.
238
If the garbage collector is allowed to shrink the stack of a thread,
239
we cannot omit the stack check in return continuations
240
(\secref{heap-and-stack-checks}).
244
When we return to the scheduler, the top object on the stack is a closure.
245
The scheduler restarts the thread by entering the closure.
247
\secref{hugs-return-convention} discusses how Hugs returns an
248
unboxed value to GHC and how GHC returns an unboxed value to Hugs.
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When we return to the scheduler, we need a few empty words on the stack
253
to store a closure to reenter. \secref{heap-and-stack-checks}
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discusses who does the stack check and how much space they need.
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Heap objects never contain slop --- this is required if we want to
259
support mostly-copying garbage collection.
261
This is a big problem when updating since the updatee is usually
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bigger than an indirection object. The fix is to overwrite the end of
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the updatee with ``slop objects'' (described in
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\secref{slop-objects}). This is hard to arrange if we do
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\emph{lazy} blackholing (\secref{lazy-black-holing}) so we
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currently plan to blackhole an object when we push the update frame.
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% Idea: have specialised update code for various common sizes of
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% updatee, the update frame hence encodes the length of the object.
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% Specialised indirections will also encode the length of the object. A
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% generic version of the update code will overwrite the slop with a slop
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% object. We can do the same thing for blackhole objects, or just have
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% a generic version that is the same size as an indirection and
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% overwrite the slop with a slop object when blackholing. So: does this
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% avoid the need to do eager black holing?
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Info tables for constructors contain enough information to decide which
280
return convention they use. This allows Hugs to use a single piece of
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entry code for all constructors and insulates Hugs from changes in the
282
choice of return convention.
286
\Section{Source Language}{source-language}
288
\Subsection{Explicit Allocation}{explicit-allocation}
290
As in the original STG machine, (almost) all heap allocation is caused
291
by executing a let(rec). Since we no longer support the return in
292
registers convention for data constructors, constructors now cause heap
293
allocation and so they should be let-bound.
295
For example, we now write
297
> cons = \ x xs -> let r = (:) x xs in r
301
> cons = \ x xs -> (:) x xs
304
\note{For historical reasons, GHC doesn't use this syntax --- but it should.}
306
\Subsection{Unboxed tuples}{unboxed-tuples}
308
Functions can take multiple arguments as easily as they can take one
309
argument: there's no cost for adding another argument. But functions
310
can only return one result: the cost of adding a second ``result'' is
311
that the function must construct a tuple of ``results'' on the heap.
312
The assymetry is rather galling and can make certain programming
313
styles quite expensive. For example, consider a simple state transformer
316
> type S a = State -> (a,State)
317
> bindS m k s0 = case m s0 of { (a,s1) -> k a s1 }
318
> returnS a s = (a,s)
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Here, every use of @returnS@, @getS@ or @setS@ constructs a new tuple
323
in the heap which is instantly taken apart (and becomes garbage) by
324
the case analysis in @bind@. Even a short state-transformer program
325
will construct a lot of these temporary tuples.
327
Unboxed tuples provide a way for the programmer to indicate that they
328
do not expect a tuple to be shared and that they do not expect it to
329
be allocated in the heap. Syntactically, unboxed tuples are just like
330
single constructor datatypes except for the annotation @unboxed@.
332
> data unboxed AAndState# a = AnS a State
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> type S a = State -> AAndState# a
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> bindS m k s0 = case m s0 of { AnS a s1 -> k a s1 }
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> returnS a s = AnS a s
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> setS s _ = AnS () s
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Semantically, unboxed tuples are just unlifted tuples and are subject
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to the same restrictions as other unpointed types.
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Operationally, unboxed tuples are never built on the heap. When
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an unboxed tuple is returned, it is returned in multiple registers
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or multiple stack slots. At first sight, this seems a little strange
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but it's no different from passing double precision floats in two
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Unboxed tuples can only have one constructor and that
352
thunks never have unboxed types --- so we'll never try to update an
353
unboxed constructor. The restriction to a single constructor is
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largely to avoid garbage collection complications.
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The core syntax does not allow variables to be bound to
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unboxed tuples (ie in default case alternatives or as function arguments)
359
and does not allow unboxed tuples to be fields of other constructors.
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However, there's no harm in allowing it in the source syntax as a
361
convenient, but easily removed, syntactic sugar.
364
The compiler generates a closure of the form
366
> c = \ x y z -> C x y z
368
for every constructor (whether boxed or unboxed).
370
This closure is normally used during desugaring to ensure that
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constructors are saturated and to apply any strictness annotations.
372
They are also used when returning unboxed constructors to the machine
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code evaluator from the bytecode evaluator and when a heap check fails
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in a return continuation for an unboxed-tuple scrutinee.
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\Subsection{STG Syntax}{stg-syntax}
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\ToDo{Insert STG syntax with appropriate changes.}
384
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
385
\part{System Overview}
386
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
388
This part is concerned with defining the external interfaces of the
389
major components of the system; the next part is concerned with their
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The major components of the system are:
397
The evaluators (\secref{sm-overview}) are responsible for
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evaluating heap objects. The system supports two evaluators: the
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machine code evaluator; and the bytecode evaluator.
403
The scheduler (\secref{scheduler-overview}) acts as the
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coordinator for the whole system. It is responsible for switching
405
between evaluators, switching between threads, garbage collection,
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communication between multiple processors, etc.
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The storage manager (\secref{evaluators-overview}) is
411
responsible for allocating blocks of contiguous memory and for garbage
416
The loader (\secref{loader-overview}) is responsible for
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loading machine code and bytecode files from the file system and for
418
resolving references between separately compiled modules.
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The compilers (\secref{compilers-overview}) generate machine
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code and bytecode files which can be loaded by the loader.
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\ToDo{Insert diagram showing all components underneath the scheduler
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and communicating only with the scheduler}
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\Section{The Evaluators}{evaluators-overview}
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There are two evaluators: a machine code evaluator and a bytecode
434
evaluator. The evaluators task is to evaluate code within a thread
435
until one of the following happens:
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\item it is preempted
441
\item it blocks in one of the concurrency primitives
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\item it performs a safe ccall
443
\item it needs to switch to the other evaluator.
446
The evaluators expect to find a closure on top of the thread's stack
447
and terminate with a closure on top of the thread's stack.
449
\Subsection{Evaluation Model}{evaluation-model}
451
Whilst the evaluators differ internally, they share a common
452
evaluation model and many object representations.
454
\Subsubsection{Heap objects}{heap-objects-overview}
456
The choice of heap and stack objects used by the evaluators is tightly
457
bound to the evaluation model. This section provides an overview of
458
the most important heap and stack objects; further details are given
461
All heap objects look like this:
464
\begin{tabular}{|l|l|l|l|}\hline
465
\emph{Header} & \emph{Payload} \\ \hline
469
The headers vary between different kinds of object but they all start
470
with a pointer to a pair consisting of an \emph{info table} and some
471
\emph{entry code}. The info table is used both by the evaluators and
472
by the storage manager and contains a @type@ field which identifies
473
which kind of heap object uses it and determines the interpretation of
474
the payload and of the other fields of the info table. The entry code
475
is some machine code used by the machine code evaluator to evaluate
476
closures and raises an error for other kinds of objects.
478
The major kinds of heap object used are as follows. (For simplicity,
479
this description omits certain optimisations and extra fields required
480
by the garbage collector.)
484
\item[Constructors] are used to represent data constructors. Their
485
payload consists of the fields of the constructor; the tag of the
486
constructor is stored in the info table.
489
\begin{tabular}{|l|l|l|l|}\hline
490
@CONSTR@ & \emph{Fields} \\ \hline
494
\item[Primitive objects] are used to represent objects with unlifted
495
types which are too large to fit in a register (or stack slot) or for
496
which sharing must be preserved. Primitive objects include large
497
objects such as multiple precision integers and immutable arrays and
498
mutable objects such as mutable arrays, mutable variables, MVar's,
499
IVar's and foreign object pointers. Since primitive objects are not
500
lifted, they cannot be entered. Their payload varies according to the
503
\item[Function closures] are used to represent functions. Their
504
payload (if any) consists of the free variables of the function.
507
\begin{tabular}{|l|l|l|l|}\hline
508
@FUN@ & \emph{Free Variables} \\ \hline
512
Function closures are only generated by the machine code compiler.
514
\item[Thunks] are used to represent unevaluated expressions which will
515
be updated with their result. Their payload (if any) consists of the
516
free variables of the function. The entry code for a thunk starts by
517
pushing an \emph{update frame} onto the stack. When evaluation of the
518
thunk completes, the update frame will cause the thunk to be
519
overwritten again with an \emph{indirection} to the result of the
520
thunk, which is always a constructor or a partial application.
523
\begin{tabular}{|l|l|l|l|}\hline
524
@THUNK@ & \emph{Free Variables} \\ \hline
528
Thunks are only generated by the machine code evaluator.
530
\item[Byte-code Objects (@BCO@s)] are generated by the bytecode
531
compiler. In conjunction with \emph{updatable applications} and
532
\emph{non-updatable applications} they are used to represent
533
functions, unevaluated expressions and return addresses.
536
\begin{tabular}{|l|l|l|l|}\hline
537
@BCO@ & \emph{Constant Pool} & \emph{Bytecodes} \\ \hline
541
\item[Non-updatable (Partial) Applications] are used to represent the
542
application of a function to an insufficient number of arguments.
543
Their payload consists of the function and the arguments received so far.
546
\begin{tabular}{|l|l|l|l|}\hline
547
@PAP@ & \emph{Function Closure} & \emph{Arguments} \\ \hline
551
@PAP@s are used when a function is applied to too few arguments and by
552
code generated by the lambda-lifting phase of the bytecode compiler.
554
\item[Updatable Applications] are used to represent the application of
555
a function to a sufficient number of arguments. Their payload
556
consists of the function and its arguments.
558
Updateable applications are like thunks: on entering an updateable
559
application, the evaluators push an \emph{update frame} onto the stack
560
and overwrite the application with a \emph{black hole}; when
561
evaluation completes, the evaluators overwrite the application with an
562
\emph{indirection} to the result of the application.
565
\begin{tabular}{|l|l|l|l|}\hline
566
@AP@ & \emph{Function Closure} & \emph{Arguments} \\ \hline
570
@AP@s are only generated by the bytecode compiler.
572
\item[Black holes] are used to mark updateable closures which are
573
currently being evaluated. ``Black holing'' an object cures a
574
potential space leak and detects certain classes of infinite loops.
575
More imporantly, black holes act as synchronisation objects between
576
separate threads: if a second thread tries to enter an updateable
577
closure which is already being evaluated, the second thread is added
578
to a list of blocked threads and the thread is suspended.
580
When evaluation of the black-holed closure completes, the black hole
581
is overwritten with an indirection to the result of the closure and
582
any blocked threads are restored to the runnable queue.
584
Closures are overwritten by black-holes during a ``lazy black-holing''
585
phase which runs on each thread when it returns to the scheduler.
586
\ToDo{section describing lazy black-holing}.
589
\begin{tabular}{|l|l|l|l|}\hline
590
@BLACKHOLE@ & \emph{Blocked threads} \\ \hline
594
\ToDo{In a single threaded system, it's trivial to detect infinite
595
loops: reentering a BLACKHOLE is always an error. How easy is it in a
596
multi-threaded system?}
598
\item[Indirections] are used to update an unevaluated closure with its
599
(usually fully evaluated) result in situations where it isn't possible
600
to perform an update in place. (In the current system, we always
601
update with an indirection to avoid duplicating the result when doing
605
\begin{tabular}{|l|l|l|l|}\hline
606
@IND@ & \emph{Closure} \\ \hline
610
Indirections needn't always point to a closure in WHNF. They can
611
point to a chain of indirections which point to an evaluated closure.
613
\item[Thread State Objects (@TSO@s)] represent Haskell threads. Their
614
payload consists of some per-thread information such as the Thread ID
615
and the status of the thread (runnable, blocked etc.), and the
616
thread's stack. See @TSO.h@ for the full story. @TSO@s may be
617
resized by the scheduler if its stack is too small or too large.
619
The thread stack grows downwards from higher to lower addresses.
622
\begin{tabular}{|l|l|l|l|}\hline
623
@TSO@ & \emph{Thread info} & \emph{Stack} \\ \hline
629
\Subsubsection{Stack objects}{stack-objects-overview}
631
The stack contains a mixture of \emph{pending arguments} and
632
\emph{stack objects}.
634
Pending arguments are arguments to curried functions which have not
635
yet been incorporated into an activation frame. For example, when
636
evaluating @let { g x y = x + y; f x = g{x} } in f{3,4}@, the
637
evaluator pushes both arguments onto the stack and enters @f@. @f@
638
only requires one argument so it leaves the second argument as a
639
\emph{pending argument}. The pending argument remains on the stack
640
until @f@ calls @g@ which requires two arguments: the argument passed
641
to it by @f@ and the pending argument which was passed to @f@.
643
Unboxed pending arguments are always preceeded by a ``tag'' which says
644
how large the argument is. This allows the garbage collector to
645
locate pointers within the stack.
647
There are three kinds of stack object: return addresses, update frames
648
and seq frames. All stack objects look like this
651
\begin{tabular}{|l|l|l|l|}\hline
652
\emph{Header} & \emph{Payload} \\ \hline
656
As with heap objects, the header starts with a pointer to a pair
657
consisting of an \emph{info table} and some \emph{entry code}.
661
\item[Return addresses] are used to cause selection and execution of
662
case alternatives when a constructor is returned. Return addresses
663
generated by the machine code compiler look like this:
666
\begin{tabular}{|l|l|l|l|}\hline
667
@RET_XXX@ & \emph{Free Variables of the case alternatives} \\ \hline
671
The free variables are a mixture of pointers and non-pointers whose
672
layout is described by a bitmask in the info table.
674
There are several kinds of @RET_XXX@ return address - see
675
\secref{activation-records} for the details.
677
Return addresses generated by the bytecode compiler look like this:
679
\begin{tabular}{|l|l|l|l|}\hline
680
@BCO_RET@ & \emph{BCO} & \emph{Free Variables of the case alternatives} \\ \hline
684
There is just one @BCO_RET@ info pointer. We avoid needing different
685
@BCO_RET@s for each stack layout by tagging unboxed free variables as
686
though they were pending arguments.
688
\item[Update frames] are used to trigger updates. When an update
689
frame is entered, it overwrites the updatee with an indirection to the
690
result, restarts any threads blocked on the @BLACKHOLE@ and returns to
691
the stack object underneath the update frame.
694
\begin{tabular}{|l|l|l|l|}\hline
695
@UPDATE_FRAME@ & \emph{Next Update Frame} & \emph{Updatee} \\ \hline
699
\item[Seq frames] are used to implement the polymorphic @seq@
700
primitive. They are a special kind of update frame, and are linked on
701
the update frame list.
704
\begin{tabular}{|l|l|l|l|}\hline
705
@SEQ_FRAME@ & \emph{Next Update Frame} \\ \hline
709
\item[Stop frames] are put on the bottom of each thread's stack, and
710
act as sentinels for the update frame list (i.e. the last update frame
711
points to the stop frame). Returning to a stop frame terminates the
712
thread. Stop frames have no payload:
715
\begin{tabular}{|l|l|l|l|}\hline
716
@SEQ_FRAME@ \\ \hline
722
\Subsubsection{Case expressions}{case-expr-overview}
724
In the STG language, all evaluation is triggered by evaluating a case
725
expression. When evaluating a case expression @case e of alts@, the
726
evaluators pushes a return address onto the stack and evaluate the
727
expression @e@. When @e@ eventually reduces to a constructor, the
728
return address on the stack is entered. The details of how the
729
constructor is passed to the return address and how the appropriate
730
case alternative is selected vary between evaluators.
732
Case expressions for unboxed data types are essentially the same: the
733
case expression pushes a return address onto the stack before
734
evaluating the scrutinee; when a function returns an unboxed value, it
735
enters the return address on top of the stack.
738
\Subsubsection{Function applications}{fun-app-overview}
740
In the STG language, all function calls are tail calls. The arguments
741
are pushed onto the stack and the function closure is entered. If any
742
arguments are unboxed, they must be tagged as unboxed pending
743
arguments. Entering a closure is just a special case of calling a
744
function with no arguments.
747
\Subsubsection{Let expressions}{let-expr-overview}
749
In the STG language, almost all heap allocation is caused by let
750
expressions. Filling in the contents of a set of mutually recursive
751
heap objects is simple enough; the only difficulty is that once the
752
heap space has been allocated, the thread must not return to the
753
scheduler until after the objects are filled in.
756
\Subsubsection{Primitive operations}{primop-overview}
760
Most primops are simple, some aren't.
767
\Section{Scheduler}{scheduler-overview}
769
The Scheduler is the heart of the run-time system. A running program
770
consists of a single running thread, and a list of runnable and
771
blocked threads. A thread is represented by a \emph{Thread Status
772
Object} (TSO), which contains a few words status information and a
773
stack. Except for the running thread, all threads have a closure on
774
top of their stack; the scheduler restarts a thread by entering an
775
evaluator which performs some reduction and returns to the scheduler.
777
\Subsection{The scheduler's main loop}{scheduler-main-loop}
779
The scheduler consists of a loop which chooses a runnable thread and
780
invokes one of the evaluators which performs some reduction and
783
The scheduler also takes care of system-wide issues such as heap
784
overflow or communication with other processors (in the parallel
785
system) and thread-specific problems such as stack overflow.
787
\Subsection{Creating a thread}{create-thread}
795
When the scheduler is first invoked.
799
When a message is received from another processor (I think). (Parallel
804
When a C program calls some Haskell code.
808
By @forkIO@, @takeMVar@ and (maybe) other Concurrent Haskell primitives.
813
\Subsection{Restarting a thread}{thread-restart}
815
When the scheduler decides to run a thread, it has to decide which
816
evaluator to use. It does this by looking at the type of the closure
819
\item @BCO@ $\Rightarrow$ bytecode evaluator
820
\item @FUN@ or @THUNK@ $\Rightarrow$ machine code evaluator
821
\item @CONSTR@ $\Rightarrow$ machine code evaluator
822
\item other $\Rightarrow$ either evaluator.
825
The only surprise in the above is that the scheduler must enter the
826
machine code evaluator if there's a constructor on top of the stack.
827
This allows the bytecode evaluator to return a constructor to a
828
machine code return address by pushing the constructor on top of the
829
stack and returning to the scheduler. If the return address under the
830
constructor is @HUGS_RET@, the entry code for @HUGS_RET@ will
831
rearrange the stack so that the return @BCO@ is on top of the stack
832
and return to the scheduler which will then call the bytecode
833
evaluator. There is little point in trying to shorten this slightly
834
indirect route since it is will happen very rarely if at all.
836
\note{As an optimisation, we could store the choice of evaluator in
837
the TSO status whenever we leave the evaluator. This is required for
838
any thread, no matter what state it is in (blocked, stack overflow,
839
etc). It isn't clear whether this would accomplish anything.}
841
\Subsection{Returning from a thread}{thread-return}
843
The evaluators return to the scheduler when any of the following
847
\item A heap check fails, and a garbage collection is required.
849
\item A stack check fails, and the scheduler must either enlarge the
850
current thread's stack, or flag an out of memory condition.
852
\item A thread enters a closure built by the other evaluator. That
853
is, when the bytecode interpreter enters a closure compiled by GHC or
854
when the machine code evaluator enters a BCO.
856
\item A thread returns to a return continuation built by the other
857
evaluator. That is, when the machine code evaluator returns to a
858
continuation built by Hugs or when the bytecode evaluator returns to a
859
continuation built by GHC.
861
\item The evaluator needs to perform a ``safe'' C call
864
\item The thread becomes blocked. This happens when a thread requires
865
the result of a computation currently being performed by another
866
thread, or it reads a synchronisation variable that is currently empty
869
\item The thread is preempted (the preemption mechanism is described
870
in \secref{thread-preemption}).
872
\item The thread terminates.
875
Except when the thread terminates, the thread always terminates with a
876
closure on the top of the stack. The mechanism used to trigger the
877
world switch and the choice of closure left on top of the stack varies
878
according to which world is being left and what is being returned.
880
\Subsubsection{Leaving the bytecode evaluator}{hugs-to-ghc-switch}
882
\paragraph{Entering a machine code closure}
884
When it enters a closure, the bytecode evaluator performs a switch
885
based on the type of closure (@AP@, @PAP@, @Ind@, etc). On entering a
886
machine code closure, it returns to the scheduler with the closure on
889
\paragraph{Returning a constructor}
891
When it enters a constructor, the bytecode evaluator tests the return
892
continuation on top of the stack. If it is a machine code
893
continuation, it returns to the scheduler with the constructor on top
896
\note{This is why the scheduler must enter the machine code evaluator
897
if it finds a constructor on top of the stack.}
899
\paragraph{Returning an unboxed value}
901
\note{Hugs doesn't support unboxed values in source programs but they
902
are used for a few complex primops.}
904
When it returns an unboxed value, the bytecode evaluator tests the
905
return continuation on top of the stack. If it is a machine code
906
continuation, it returns to the scheduler with the tagged unboxed
907
value and a special closure on top of the stack. When the closure is
908
entered (by the machine code evaluator), it returns the unboxed value
909
on top of the stack to the return continuation under it.
911
The runtime library for GHC provides one of these closures for each unboxed
912
type. Hugs cannot generate them itself since the entry code is really
915
\paragraph{Heap/Stack overflow and preemption}
917
The bytecode evaluator tests for heap/stack overflow and preemption
918
when entering a BCO and simply returns with the BCO on top of the
921
\Subsubsection{Leaving the machine code evaluator}{ghc-to-hugs-switch}
923
\paragraph{Entering a BCO}
925
The entry code for a BCO pushes the BCO onto the stack and returns to
928
\paragraph{Returning a constructor}
930
We avoid the need to test return addresses in the machine code
931
evaluator by pushing a special return address on top of a pointer to
932
the bytecode return continuation. \figref{hugs-return-stack1}
933
shows the state of the stack just before evaluating the scrutinee.
945
%\input{hugs_return1.pstex_t}
947
\caption{Stack layout for evaluating a scrutinee}
948
\label{fig:hugs-return-stack1}
951
This return address rearranges the stack so that the bco pointer is
952
above the constructor on the stack (as shown in
953
\figref{hugs-boxed-return}) and returns to the scheduler.
960
| con |--> Constructor
965
%\input{hugs_return2.pstex_t}
967
\caption{Stack layout for entering a Hugs return address}
968
\label{fig:hugs-boxed-return}
971
\paragraph{Returning an unboxed value}
973
We avoid the need to test return addresses in the machine code
974
evaluator by pushing a special return address on top of a pointer to
975
the bytecode return continuation. This return address rearranges the
976
stack so that the bco pointer is above the tagged unboxed value (as
977
shown in \figref{hugs-entering-unboxed-return}) and returns to the
992
%\input{hugs_return2.pstex_t}
994
\caption{Stack layout for returning an unboxed value}
995
\label{fig:hugs-entering-unboxed-return}
998
\paragraph{Heap/Stack overflow and preemption}
1003
\Subsection{Preempting a thread}{thread-preemption}
1005
Strictly speaking, threads cannot be preempted --- the scheduler
1006
merely sets a preemption request flag which the thread must arrange to
1007
test on a regular basis. When an evaluator finds that the preemption
1008
request flag is set, it pushes an appropriate closure onto the stack
1009
and returns to the scheduler.
1011
In the bytecode interpreter, the flag is tested whenever we enter a
1012
closure. If the preemption flag is set, it leaves the closure on top
1013
of the stack and returns to the scheduler.
1015
In the machine code evaluator, the flag is only tested when a heap or
1016
stack check fails. This is less expensive than testing the flag on
1017
entering every closure but runs the risk that a thread will enter an
1018
infinite loop which does not allocate any space. If the flag is set,
1019
the evaluator returns to the scheduler exactly as if a heap check had
1022
\Subsection{``Safe'' and ``unsafe'' C calls}{c-calls}
1024
There are two ways of calling C:
1028
\item[``Unsafe'' C calls] are used if the programer is certain that
1029
the C function will not do anything dangerous. Unsafe C calls are
1030
faster but must be hand-checked by the programmer.
1032
Dangerous things include:
1038
Call a system function such as @getchar@ which might block
1039
indefinitely. This is dangerous because we don't want the entire
1040
runtime system to block just because one thread blocks.
1044
Call an RTS function which will block on the RTS access semaphore.
1045
This would lead to deadlock.
1049
Call a Haskell function. This is just a special case of calling an
1054
Unsafe C calls are performed by pushing the arguments onto the C stack
1055
and jumping to the C function's entry point. On exit, the result of
1056
the function is in a register which is returned to the Haskell code as
1059
\item[``Safe'' C calls] are used if the programmer suspects that the
1060
thread may do something dangerous. Safe C calls are relatively slow
1061
but are less problematic.
1063
Safe C calls are performed by pushing the arguments onto the Haskell
1064
stack, pushing a return continuation and returning a \emph{C function
1065
descriptor} to the scheduler. The scheduler suspends the Haskell thread,
1066
spawns a new operating system thread which pops the arguments off the
1067
Haskell stack onto the C stack, calls the C function, pushes the
1068
function result onto the Haskell stack and informs the scheduler that
1069
the C function has completed and the Haskell thread is now runnable.
1073
The bytecode evaluator will probably treat all C calls as being safe.
1075
\ToDo{It might be good for the programmer to indicate how the program
1076
is unsafe. For example, if we distinguish between C functions which
1077
might call Haskell functions and those which might block, we could
1078
perform an unsafe call for blocking functions in a single-threaded
1079
system or, perhaps, in a multi-threaded system which only happens to
1080
have a single thread at the moment.}
1084
\Section{The Storage Manager}{sm-overview}
1086
The storage manager is responsible for managing the heap and all
1087
objects stored in it. It provides special support for lazy evaluation
1088
and for foreign function calls.
1090
\Subsection{SM support for lazy evaluation}{sm-lazy-evaluation}
1095
Indirections are shorted out.
1099
Update frames pointing to unreachable objects are squeezed out.
1101
\ToDo{Part IV suggests this doesn't happen.}
1105
Adjacent update frames (for different closures) are compressed to a
1106
single update frame pointing to a single black hole.
1111
\Subsection{SM support for foreign function calls}{sm-foreign-calls}
1117
Stable pointers allow other languages to access Haskell objects.
1121
Weak pointers and foreign objects provide finalisation support for
1122
Haskell references to external objects.
1126
\Subsection{Misc}{sm-misc}
1132
If the stack contains a large amount of free space, the storage
1133
manager may shrink the stack. If it shrinks the stack, it guarantees
1134
never to leave less than @MIN_SIZE_SHRUNKEN_STACK@ empty words on the
1135
stack when it does so.
1139
For efficiency reasons, very large objects (eg large arrays and TSOs)
1140
are not moved if possible.
1145
\Section{The Compilers}{compilers-overview}
1147
Need to describe interface files, format of bytecode files, symbols
1148
defined by machine code files.
1150
\Subsection{Interface Files}{interface-files}
1152
Here's an example - but I don't know the grammar - ADR.
1158
1 main _:_ IOBase.IO PrelBase.();;
1161
\Subsection{Bytecode files}{bytecode-files}
1163
(All that matters here is what the loader sees.)
1165
\Subsection{Machine code files}{asm-files}
1167
(Again, all that matters is what the loader sees.)
1169
\Section{The Loader}{loader-overview}
1171
In a batch mode system, we can statically link all the modules
1172
together. In an interactive system we need a loader which will
1173
explicitly load and unload individual modules (or, perhaps, blocks of
1174
mutually dependent modules) and resolve references between modules.
1176
While many operating systems provide support for dynamic loading and
1177
will automatically resolve cross-module references for us, we generally
1178
cannot rely on being able to load mutually dependent modules.
1180
A portable solution is to perform some of the linking ourselves. Each module
1181
should provide three global symbols:
1184
An initialisation routine. (Might also be used for finalisation.)
1186
A table of symbols it exports.
1187
Entries in this table consist of the symbol name and the address of the
1190
A table of symbols it imports.
1191
Entries in this table consist of the symbol name and a list of references
1195
On loading a group of modules, the loader adds the contents of the
1196
export lists to a symbol table and then fills in all the references in the
1199
References in import lists are of two types:
1201
\item[ References in machine code ]
1203
The most efficient approach is to patch the machine code directly, but
1204
this will be a lot of work, very painful to port and rather fragile.
1206
Alternatively, the loader could store the value of each symbol in the
1207
import table for each module and the compiled code can access all
1208
external objects through the import table. This requires that the
1209
import table be writable but does not require that the machine code or
1210
info tables be writable.
1212
\item[ References in data structures (SRTs and static data constructors) ]
1214
Either we patch the SRTs and constructors directly or we somehow use
1215
indirections through the symbol table. Patching the SRTs requires
1216
that we make them writable and prevents us from making effective use
1217
of virtual memories that use copy-on-write policies (this only makes a
1218
difference if we want to run several copies of the same program
1219
simultaneously). Using an indirection is possible but tricky.
1221
Note: We could avoid patching machine code if all references to
1222
external references went through the SRT --- then we just have one
1223
thing to patch. But the SRT always contains a pointer to the closure
1224
rather than the fast entry point (say), so we'd take a big performance
1229
Using the above scheme, all accesses to ``external'' objects involve a
1230
layer of indirection. To avoid this overhead, the machine code
1231
compiler might provide a way for the programmer to specify which
1232
modules will be statically linked and which will be dynamically linked
1233
--- the idea being that statically linked code and data will be
1237
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
1238
\part{Internal details}
1239
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
1241
This part is concerned with the internal details of the components
1242
described in the previous part.
1244
The major components of the system are:
1246
\item The scheduler (\secref{scheduler-internals})
1247
\item The storage manager (\secref{storage-manager-internals})
1248
\item The evaluators
1253
\Section{The Scheduler}{scheduler-internals}
1255
\ToDo{Detailed description of scheduler}
1257
Many heap objects contain fields allowing them to be inserted onto lists
1258
during evaluation or during garbage collection. The lists required by
1259
the evaluator and storage manager are as follows.
1263
\item 4 lists of threads: runnable threads, sleeping threads, threads
1264
waiting for timeout and threads waiting for I/O.
1266
\item The \emph{mutables list} is a list of all objects in the old
1267
generation which might contain pointers into the new generation. Most
1268
of the objects on this list are indirections (\secref{IND})
1269
or ``mutable.'' (\secref{mutables}.)
1271
\item The \emph{Foreign Object list} is a list of all foreign objects
1272
which have not yet been deallocated. (\secref{FOREIGN}.)
1274
\item The \emph{Spark pool} is a doubly(?) linked list of Spark objects
1275
maintained by the parallel system. (\secref{SPARK}.)
1277
\item The \emph{Blocked Fetch list} (or
1278
lists?). (\secref{BLOCKED_FETCH}.)
1280
\item For each thread, there is a list of all update frames on the
1281
stack. (\secref{data-updates}.)
1283
\item The Stable Pointer Table is a table of pointers to objects which
1284
are known to the outside world and must be retained by the garbage
1285
collector even if they are not accessible from within the heap.
1289
\ToDo{The links for these fields are usually inserted immediately
1290
after the fixed header except ...}
1294
\Section{The Storage Manager}{storage-manager-internals}
1296
\subsection{Misc Text looking for a home}
1298
A \emph{value} may be:
1300
\item \emph{Boxed}, i.e.~represented indirectly by a pointer to a heap object (e.g.~foreign objects, arrays); or
1301
\item \emph{Unboxed}, i.e.~represented directly by a bit-pattern in one or more registers (e.g.~@Int#@ and @Float#@).
1303
All \emph{pointed} values are \emph{boxed}.
1306
\Subsection{Heap Objects}{heap-objects}
1307
\label{sec:fixed-header}
1313
\ToDo{Fix this picture}
1318
Every \emph{heap object} is a contiguous block of memory, consisting
1319
of a fixed-format \emph{header} followed by zero or more \emph{data
1322
The header consists of the following fields:
1324
\item A one-word \emph{info pointer}, which points to
1325
the object's static \emph{info table}.
1326
\item Zero or more \emph{admin words} that support
1328
\item Profiling (notably a \emph{cost centre} word).
1329
\note{We could possibly omit the cost centre word from some
1330
administrative objects.}
1331
\item Parallelism (e.g. GranSim keeps the object's global address here,
1332
though GUM keeps a separate hash table).
1333
\item Statistics (e.g. a word to track how many times a thunk is entered.).
1335
We add a Ticky word to the fixed-header part of closures. This is
1336
used to indicate if a closure has been updated but not yet entered. It
1337
is set when the closure is updated and cleared when subsequently
1338
entered. \footnote{% NB: It is \emph{not} an ``entry count'', it is
1339
an ``entries-after-update count.'' The commoning up of @CONST@,
1340
@CHARLIKE@ and @INTLIKE@ closures is turned off(?) if this is
1341
required. This has only been done for 2s collection. }
1346
Most of the RTS is completely insensitive to the number of admin
1347
words. The total size of the fixed header is given by
1348
@sizeof(StgHeader)@.
1350
\Subsection{Info Tables}{info-tables}
1352
An \emph{info table} is a contiguous block of memory, laid out as follows:
1355
\begin{tabular}{|r|l|}
1356
\hline Parallelism Info & variable
1357
\\ \hline Profile Info & variable
1358
\\ \hline Debug Info & variable
1359
\\ \hline Static reference table & pointer word (optional)
1360
\\ \hline Storage manager layout info & pointer word
1361
\\ \hline Closure flags & 8 bits
1362
\\ \hline Closure type & 8 bits
1363
\\ \hline Constructor Tag / SRT length & 16 bits
1364
\\ \hline entry code
1369
On a 64-bit machine the tag, type and flags fields will all be doubled
1370
in size, so the info table is a multiple of 64 bits.
1372
An info table has the following contents (working backwards in memory
1377
\item The \emph{entry code} for the closure. This code appears
1378
literally as the (large) last entry in the info table, immediately
1379
preceded by the rest of the info table. An \emph{info pointer} always
1380
points to the first byte of the entry code.
1382
\item A 16-bit constructor tag / SRT length. For a constructor info
1383
table this field contains the tag of the constructor, in the range
1384
$0..n-1$ where $n$ is the number of constructors in the datatype.
1385
Otherwise, it contains the number of entries in this closure's Static
1386
Reference Table (\secref{srt}).
1388
\item An 8-bit {\em closure type field}, which identifies what kind of
1389
closure the object is. The various types of closure are described in
1392
\item an 8-bit flags field, which holds various flags pertaining to
1395
\item A single pointer or word --- the {\em storage manager info
1396
field}, contains auxiliary information describing the closure's
1397
precise layout, for the benefit of the garbage collector and the code
1398
that stuffs graph into packets for transmission over the network.
1399
There are three kinds of layout information:
1402
\item Standard layout information is for closures which place pointers
1403
before non-pointers in instances of the closure (this applies to most
1404
heap-based and static closures, but not activation records). The
1405
layout information for standard closures is
1408
\item Number of pointer fields (16 bits).
1409
\item Number of non-pointer fields (16 bits).
1412
\item Activation records don't have pointers before non-pointers,
1413
since stack-stubbing requires that the record has holes in it. The
1414
layout is therefore represented by a bitmap in which each '1' bit
1415
represents a non-pointer word. This kind of layout info is used for
1416
@RET_SMALL@ and @RET_VEC_SMALL@ closures.
1418
\item If an activation record is longer than 32 words, then the layout
1419
field contains a pointer to a bitmap record, consisting of a length
1420
field followed by two or more bitmap words. This layout information
1421
is used for @RET_BIG@ and @RET_VEC_BIG@ closures.
1423
\item Selector Thunks (\secref{THUNK_SELECTOR}) use the closure
1424
layout field to hold the selector index, since the layout is always
1425
known (the closure contains a single pointer field).
1428
\item A one-word {\em Static Reference Table} field. This field
1429
points to the static reference table for the closure (\secref{srt}),
1430
and is only present for the following closure types:
1438
\ToDo{Expand the following explanation.}
1440
An SRT is basically a vector of pointers to static closures. A
1441
top-level function or thunk will have an SRT (which might be empty),
1442
which points to all the static closures referenced by that function or
1443
thunk. Every non-top-level thunk or function also has an SRT, but
1444
it'll be a sub-sequence of the top-level SRT, so we just store a
1445
pointer and a length in the info table - the pointer points into the
1446
middle of the larger SRT.
1448
At GC time, the garbage collector traverses the transitive closure of
1449
all the SRTs reachable from the roots, and thereby discovers which
1452
\item \emph{Profiling info\/}
1454
\ToDo{The profiling info is completely bogus. I've not deleted it
1455
from the document but I've commented it all out.}
1457
% change to \iftrue to uncomment this section
1460
Closure category records are attached to the info table of the
1461
closure. They are declared with the info table. We put pointers to
1462
these ClCat things in info tables. We need these ClCat things because
1463
they are mutable, whereas info tables are immutable. Hashing will map
1464
similar categories to the same hash value allowing statistics to be
1465
grouped by closure category.
1467
Cost Centres and Closure Categories are hashed to provide indexes
1468
against which arbitrary information can be stored. These indexes are
1469
memoised in the appropriate cost centre or category record and
1470
subsequent hashes avoided by the index routine (it simply returns the
1473
There are different features which can be hashed allowing information
1474
to be stored for different groupings. Cost centres have the cost
1475
centre recorded (using the pointer), module and group. Closure
1476
categories have the closure description and the type
1477
description. Records with the same feature will be hashed to the same
1480
The initialisation routines, @init_index_<feature>@, allocate a hash
1481
table in which the cost centre / category records are stored. The
1482
lower bound for the table size is taken from @max_<feature>_no@. They
1483
return the actual table size used (the next power of 2). Unused
1484
locations in the hash table are indicated by a 0 entry. Successive
1485
@init_index_<feature>@ calls just return the actual table size.
1487
Calls to @index_<feature>@ will insert the cost centre / category
1488
record in the @<feature>@ hash table, if not already inserted. The hash
1489
index is memoised in the record and returned.
1491
CURRENTLY ONLY ONE MEMOISATION SLOT IS AVILABLE IN EACH RECORD SO
1492
HASHING CAN ONLY BE DONE ON ONE FEATURE FOR EACH RECORD. This can be
1493
easily relaxed at the expense of extra memoisation space or continued
1496
The initialisation routines must be called before initialisation of
1497
the stacks and heap as they require to allocate storage. It is also
1498
expected that the caller may want to allocate additional storage in
1499
which to store profiling information based on the return table size
1503
\begin{tabular}{|l|}
1507
\\ \hline Description String
1508
\\ \hline Type String
1514
\item[Hash Index] Memoised copy
1516
Is this category selected (-1 == not memoised, selected? 0 or 1)
1518
One of the following values (defined in CostCentre.lh):
1526
A partial application.
1528
A thunk, or suspension.
1533
\item[@ForeignObj_K@]
1534
A Foreign object (non-Haskell heap resident).
1536
The Stable Pointer table. (There should only be one of these but it
1537
represents a form of weak space leak since it can't shrink to meet
1538
non-demand so it may be worth watching separately? ADR)
1539
\item[@INTERNAL_KIND@]
1540
Something internal to the runtime system.
1544
\item[Description] Source derived string detailing closure description.
1545
\item[Type] Source derived string detailing closure type.
1548
\fi % end of commented out stuff
1550
\item \emph{Parallelism info\/}
1553
\item \emph{Debugging info\/}
1559
%-----------------------------------------------------------------------------
1560
\Subsection{Kinds of Heap Object}{closures}
1562
Heap objects can be classified in several ways, but one useful one is
1566
\emph{Static closures} occupy fixed, statically-allocated memory
1567
locations, with globally known addresses.
1570
\emph{Dynamic closures} are individually allocated in the heap.
1573
\emph{Stack closures} are closures allocated within a thread's stack
1574
(which is itself a heap object). Unlike other closures, there are
1575
never any pointers to stack closures. Stack closures are discussed in
1579
A second useful classification is this:
1582
\item \emph{Executive objects}, such as thunks and data constructors,
1583
participate directly in a program's execution. They can be subdivided
1584
into three kinds of objects according to their type: \begin{itemize}
1586
\item \emph{Pointed objects}, represent values of a \emph{pointed}
1587
type (<.pointed types launchbury.>) --i.e.~a type that includes
1588
$\bottom$ such as @Int@ or @Int# -> Int#@.
1590
\item \emph{Unpointed objects}, represent values of a \emph{unpointed}
1591
type --i.e.~a type that does not include $\bottom$ such as @Int#@ or
1594
\item \emph{Activation frames}, represent ``continuations''. They are
1595
always stored on the stack and are never pointed to by heap objects or
1596
passed as arguments. \note{It's not clear if this will still be true
1597
once we support speculative evaluation.}
1601
\item \emph{Administrative objects}, such as stack objects and thread
1602
state objects, do not represent values in the original program.
1605
Only pointed objects can be entered. If an unpointed object is
1606
entered the program will usually terminate with a fatal error.
1608
This section enumerates all the kinds of heap objects in the system.
1609
Each is identified by a distinct closure type field in its info table.
1611
\begin{tabular}{|l|l|l|l|l|l|l|l|l|l|l|}
1614
closure type & Section \\
1620
@CONSTR@ & \ref{sec:CONSTR} \\
1621
@CONSTR_p_n@ & \ref{sec:CONSTR} \\
1622
@CONSTR_STATIC@ & \ref{sec:CONSTR} \\
1623
@CONSTR_NOCAF_STATIC@ & \ref{sec:CONSTR} \\
1625
@FUN@ & \ref{sec:FUN} \\
1626
@FUN_p_n@ & \ref{sec:FUN} \\
1627
@FUN_STATIC@ & \ref{sec:FUN} \\
1629
@THUNK@ & \ref{sec:THUNK} \\
1630
@THUNK_p_n@ & \ref{sec:THUNK} \\
1631
@THUNK_STATIC@ & \ref{sec:THUNK} \\
1632
@THUNK_SELECTOR@ & \ref{sec:THUNK_SELECTOR} \\
1634
@BCO@ & \ref{sec:BCO} \\
1636
@AP_UPD@ & \ref{sec:AP_UPD} \\
1637
@PAP@ & \ref{sec:PAP} \\
1639
@IND@ & \ref{sec:IND} \\
1640
@IND_OLDGEN@ & \ref{sec:IND} \\
1641
@IND_PERM@ & \ref{sec:IND} \\
1642
@IND_OLDGEN_PERM@ & \ref{sec:IND} \\
1643
@IND_STATIC@ & \ref{sec:IND} \\
1645
@CAF_UNENTERED@ & \ref{sec:CAF} \\
1646
@CAF_ENTERED@ & \ref{sec:CAF} \\
1647
@CAF_BLACKHOLE@ & \ref{sec:CAF} \\
1653
@BLACKHOLE@ & \ref{sec:BLACKHOLE} \\
1654
@BLACKHOLE_BQ@ & \ref{sec:BLACKHOLE_BQ} \\
1656
@MVAR@ & \ref{sec:MVAR} \\
1658
@ARR_WORDS@ & \ref{sec:ARR_WORDS} \\
1660
@MUTARR_PTRS@ & \ref{sec:MUT_ARR_PTRS} \\
1661
@MUTARR_PTRS_FROZEN@ & \ref{sec:MUT_ARR_PTRS_FROZEN} \\
1663
@MUT_VAR@ & \ref{sec:MUT_VAR} \\
1665
@WEAK@ & \ref{sec:WEAK} \\
1666
@FOREIGN@ & \ref{sec:FOREIGN} \\
1667
@STABLE_NAME@ & \ref{sec:STABLE_NAME} \\
1671
Activation frames do not live (directly) on the heap --- but they have
1672
a similar organisation.
1674
\begin{tabular}{|l|l|}\hline
1675
closure type & Section \\ \hline
1676
@RET_SMALL@ & \ref{sec:activation-records} \\
1677
@RET_VEC_SMALL@ & \ref{sec:activation-records} \\
1678
@RET_BIG@ & \ref{sec:activation-records} \\
1679
@RET_VEC_BIG@ & \ref{sec:activation-records} \\
1680
@UPDATE_FRAME@ & \ref{sec:activation-records} \\
1681
@CATCH_FRAME@ & \ref{sec:activation-records} \\
1682
@SEQ_FRAME@ & \ref{sec:activation-records} \\
1683
@STOP_FRAME@ & \ref{sec:activation-records} \\
1687
There are also a number of administrative objects. It is an error to
1688
enter one of these objects.
1690
\begin{tabular}{|l|l|}\hline
1691
closure type & Section \\ \hline
1692
@TSO@ & \ref{sec:TSO} \\
1693
@SPARK_OBJECT@ & \ref{sec:SPARK} \\
1694
@BLOCKED_FETCH@ & \ref{sec:BLOCKED_FETCH} \\
1695
@FETCHME@ & \ref{sec:FETCHME} \\
1699
\Subsection{Predicates}{closure-predicates}
1701
The runtime system sometimes needs to be able to distinguish objects
1702
according to their properties: is the object updateable? is it in weak
1703
head normal form? etc. These questions can be answered by examining
1704
the closure type field of the object's info table.
1706
We define the following predicates to detect families of related
1707
info types. They are mutually exclusive and exhaustive.
1710
\item @isCONSTR@ is true for @CONSTR@s.
1711
\item @isFUN@ is true for @FUN@s.
1712
\item @isTHUNK@ is true for @THUNK@s.
1713
\item @isBCO@ is true for @BCO@s.
1714
\item @isAP@ is true for @AP@s.
1715
\item @isPAP@ is true for @PAP@s.
1716
\item @isINDIRECTION@ is true for indirection objects.
1717
\item @isBH@ is true for black holes.
1718
\item @isFOREIGN_OBJECT@ is true for foreign objects.
1719
\item @isARRAY@ is true for array objects.
1720
\item @isMVAR@ is true for @MVAR@s.
1721
\item @isIVAR@ is true for @IVAR@s.
1722
\item @isFETCHME@ is true for @FETCHME@s.
1723
\item @isSLOP@ is true for slop objects.
1724
\item @isRET_ADDR@ is true for return addresses.
1725
\item @isUPD_ADDR@ is true for update frames.
1726
\item @isTSO@ is true for @TSO@s.
1727
\item @isSTABLE_PTR_TABLE@ is true for the stable pointer table.
1728
\item @isSPARK_OBJECT@ is true for spark objects.
1729
\item @isBLOCKED_FETCH@ is true for blocked fetch objects.
1730
\item @isINVALID_INFOTYPE@ is true for all other info types.
1734
The following predicates detect other interesting properties:
1738
\item @isPOINTED@ is true if an object has a pointed type.
1740
If an object is pointed, the following predicates may be true
1741
(otherwise they are false). @isWHNF@ and @isUPDATEABLE@ are
1745
\item @isWHNF@ is true if the object is in Weak Head Normal Form.
1746
Note that unpointed objects are (arbitrarily) not considered to be in WHNF.
1748
@isWHNF@ is true for @PAP@s, @CONSTR@s, @FUN@s and all @BCO@s.
1750
\ToDo{Need to distinguish between whnf BCOs and non-whnf BCOs in their
1753
\item @isUPDATEABLE@ is true if the object may be overwritten with an
1756
@isUPDATEABLE@ is true for @THUNK@s, @AP@s and @BH@s.
1760
It is possible for a pointed object to be neither updatable nor in
1761
WHNF. For example, indirections.
1763
\item @isUNPOINTED@ is true if an object has an unpointed type.
1764
All such objects are boxed since only boxed objects have info pointers.
1766
It is true for @ARR_WORDS@, @ARR_PTRS@, @MUTVAR@, @MUTARR_PTRS@,
1767
@MUTARR_PTRS_FROZEN@, @FOREIGN@ objects, @MVAR@s and @IVAR@s.
1769
\item @isACTIVATION_FRAME@ is true for activation frames of all sorts.
1771
It is true for return addresses and update frames.
1773
\item @isVECTORED_RETADDR@ is true for vectored return addresses.
1774
\item @isDIRECT_RETADDR@ is true for direct return addresses.
1777
\item @isADMINISTRATIVE@ is true for administrative objects:
1778
@TSO@s, the stable pointer table, spark objects and blocked fetches.
1780
\item @hasSRT@ is true if the info table for the object contains an
1783
@hasSRT@ is true for @THUNK@s, @FUN@s, and @RET@s.
1789
\item @isSTATIC@ is true for any statically allocated closure.
1791
\item @isMUTABLE@ is true for objects with mutable pointer fields:
1792
@MUT_ARR@s, @MUTVAR@s, @MVAR@s and @IVAR@s.
1794
\item @isSparkable@ is true if the object can (and should) be sparked.
1795
It is true of updateable objects which are not in WHNF with the
1796
exception of @THUNK_SELECTOR@s and black holes.
1800
As a minor optimisation, we might use the top bits of the @INFO_TYPE@
1801
field to ``cache'' the answers to some of these predicates.
1803
An indirection either points to HNF (post update); or is result of
1804
overwriting a FetchMe, in which case the thing fetched is either under
1805
evaluation (BLACKHOLE), or by now an HNF. Thus, indirections get
1808
\subsection{Closures (aka Pointed Objects)}
1810
An object can be entered iff it is a closure.
1812
\Subsubsection{Function closures}{FUN}
1814
Function closures represent lambda abstractions. For example,
1815
consider the top-level declaration:
1817
f = \x -> let g = \y -> x+y
1820
Both @f@ and @g@ are represented by function closures. The closure
1821
for @f@ is \emph{static} while that for @g@ is \emph{dynamic}.
1823
The layout of a function closure is as follows:
1825
\begin{tabular}{|l|l|l|l|}\hline
1826
\emph{Fixed header} & \emph{Pointers} & \emph{Non-pointers} \\ \hline
1830
The data words (pointers and non-pointers) are the free variables of
1831
the function closure. The number of pointers and number of
1832
non-pointers are stored in @info->layout.ptrs@ and
1833
@info->layout.nptrs@ respecively.
1835
There are several different sorts of function closure, distinguished
1836
by their closure type field:
1840
\item @FUN@: a vanilla, dynamically allocated on the heap.
1842
\item $@FUN_@p@_@np$: to speed up garbage collection a number of
1843
specialised forms of @FUN@ are provided, for particular $(p,np)$
1844
pairs, where $p$ is the number of pointers and $np$ the number of
1847
\item @FUN_STATIC@. Top-level, static, function closures (such as @f@
1848
above) have a different layout than dynamic ones:
1851
\begin{tabular}{|l|l|l|}\hline
1852
\emph{Fixed header} & \emph{Static object link} \\ \hline
1856
Static function closures have no free variables. (However they may
1857
refer to other static closures; these references are recorded in the
1858
function closure's SRT.) They have one field that is not present in
1859
dynamic closures, the \emph{static object link} field. This is used
1860
by the garbage collector in the same way that to-space is, to gather
1861
closures that have been determined to be live but that have not yet
1864
\note{Static function closures that have no static references, and
1865
hence a null SRT pointer, don't need the static object link field. We
1866
don't take advantage of this at the moment, but we could. See
1867
@CONSTR\_NOCAF\_STATIC@.}
1870
Each lambda abstraction, $f$, in the STG program has its own private
1871
info table. The following labels are relevant:
1875
\item $f$@_info@ is $f$'s info table.
1877
\item $f$@_entry@ is $f$'s slow entry point (i.e. the entry code of
1878
its info table; so it will label the same byte as $f$@_info@).
1880
\item $f@_fast_@k$ is $f$'s fast entry point. $k$ is the number of
1881
arguments $f$ takes; encoding this number in the fast-entry label
1882
occasionally catches some nasty code-generation errors.
1886
\Subsubsection{Data constructors}{CONSTR}
1888
Data-constructor closures represent values constructed with algebraic
1889
data type constructors. The general layout of data constructors is
1890
the same as that for function closures. That is
1893
\begin{tabular}{|l|l|l|l|}\hline
1894
\emph{Fixed header} & \emph{Pointers} & \emph{Non-pointers} \\ \hline
1898
There are several different sorts of constructor:
1902
\item @CONSTR@: a vanilla, dynamically allocated constructor.
1904
\item @CONSTR_@$p$@_@$np$: just like $@FUN_@p@_@np$.
1906
\item @CONSTR_INTLIKE@. A dynamically-allocated heap object that
1907
looks just like an @Int@. The garbage collector checks to see if it
1908
can common it up with one of a fixed set of static int-like closures,
1909
thus getting it out of the dynamic heap altogether.
1911
\item @CONSTR_CHARLIKE@: same deal, but for @Char@.
1913
\item @CONSTR_STATIC@ is similar to @FUN_STATIC@, with the
1914
complication that the layout of the constructor must mimic that of a
1915
dynamic constructor, because a static constructor might be returned to
1916
some code that unpacks it. So its layout is like this:
1919
\begin{tabular}{|l|l|l|l|l|}\hline
1920
\emph{Fixed header} & \emph{Pointers} & \emph{Non-pointers} & \emph{Static object link}\\ \hline
1924
The static object link, at the end of the closure, serves the same purpose
1925
as that for @FUN_STATIC@. The pointers in the static constructor can point
1926
only to other static closures.
1928
The static object link occurs last in the closure so that static
1929
constructors can store their data fields in exactly the same place as
1930
dynamic constructors.
1932
\item @CONSTR_NOCAF_STATIC@. A statically allocated data constructor
1933
that guarantees not to point (directly or indirectly) to any CAF
1934
(\secref{CAF}). This means it does not need a static object
1935
link field. Since we expect that there might be quite a lot of static
1936
constructors this optimisation makes sense. Furthermore, the @NOCAF@
1937
tag allows the compiler to indicate that no CAFs can be reached
1938
anywhere \emph{even indirectly}.
1942
For each data constructor $Con$, two info tables are generated:
1945
\item $Con$@_con_info@ labels $Con$'s dynamic info table,
1946
shared by all dynamic instances of the constructor.
1947
\item $Con$@_static@ labels $Con$'s static info table,
1948
shared by all static instances of the constructor.
1951
Each constructor also has a \emph{constructor function}, which is a
1952
curried function which builds an instance of the constructor. The
1953
constructor function has an info table labelled as @$Con$_info@, and
1954
entry code pointed to by @$Con$_entry@.
1956
Nullary constructors are represented by a single static info table,
1957
which everyone points to. Thus for a nullary constructor we can omit
1958
the dynamic info table and the constructor function.
1960
\subsubsection{Thunks}
1962
\label{sec:THUNK_SELECTOR}
1964
A thunk represents an expression that is not obviously in head normal
1965
form. For example, consider the following top-level definitions:
1967
range = between 1 10
1968
f = \x -> let ys = take x range
1971
Here the right-hand sides of @range@ and @ys@ are both thunks; the former
1972
is static while the latter is dynamic.
1974
The layout of a thunk is the same as that for a function closure.
1975
However, thunks must have a payload of at least @MIN_UPD_SIZE@
1976
words to allow it to be overwritten with a black hole and an
1977
indirection. The compiler may have to add extra non-pointer fields to
1978
satisfy this constraint.
1981
\begin{tabular}{|l|l|l|l|l|}\hline
1982
\emph{Fixed header} & \emph{Pointers} & \emph{Non-pointers} \\ \hline
1986
The layout word in the info table contains the same information as for
1987
function closures; that is, number of pointers and number of
1990
A thunk differs from a function closure in that it can be updated.
1992
There are several forms of thunk:
1996
\item @THUNK@ and $@THUNK_@p@_@np$: vanilla, dynamically allocated
1997
thunks. Dynamic thunks are overwritten with normal indirections
1998
(@IND@), or old generation indirections (@IND_OLDGEN@): see
2001
\item @THUNK_STATIC@. A static thunk is also known as a
2002
\emph{constant applicative form}, or \emph{CAF}. Static thunks are
2003
overwritten with static indirections.
2006
\begin{tabular}{|l|l|}\hline
2007
\emph{Fixed header} & \emph{Static object link}\\ \hline
2011
\item @THUNK_SELECTOR@ is a (dynamically allocated) thunk whose entry
2012
code performs a simple selection operation from a data constructor
2013
drawn from a single-constructor type. For example, the thunk
2015
x = case y of (a,b) -> a
2017
is a selector thunk. A selector thunk is laid out like this:
2020
\begin{tabular}{|l|l|l|l|}\hline
2021
\emph{Fixed header} & \emph{Selectee pointer} \\ \hline
2025
The layout word contains the byte offset of the desired word in the
2026
selectee. Note that this is different from all other thunks.
2028
The garbage collector ``peeks'' at the selectee's tag (in its info
2029
table). If it is evaluated, then it goes ahead and does the
2030
selection, and then behaves just as if the selector thunk was an
2031
indirection to the selected field. If it is not evaluated, it treats
2032
the selector thunk like any other thunk of that shape.
2033
[Implementation notes. Copying: only the evacuate routine needs to be
2034
special. Compacting: only the PRStart (marking) routine needs to be
2037
There is a fixed set of pre-compiled selector thunks built into the
2038
RTS, representing offsets from 0 to @MAX_SPEC_SELECTOR_THUNK@. The
2039
info tables are labelled @__sel_$n$_upd_info@ where $n$ is the offset.
2040
Non-updating versions are also built in, with info tables labelled
2041
@__sel_$n$_noupd_info@.
2045
The only label associated with a thunk is its info table:
2048
\item[$f$@\_info@] is $f$'s info table.
2052
\Subsubsection{Byte-code objects}{BCO}
2054
A Byte-Code Object (BCO) is a container for a a chunk of byte-code,
2055
which can be executed by Hugs. The byte-code represents a
2056
supercombinator in the program: when Hugs compiles a module, it
2057
performs lambda lifting and each resulting supercombinator becomes a
2058
byte-code object in the heap.
2060
BCOs are not updateable; the bytecode compiler represents updatable
2061
thunks using a combination of @AP@s and @BCO@s.
2063
The semantics of BCOs are described in \secref{hugs-heap-objects}. A
2064
BCO has the following structure:
2067
\begin{tabular}{|l|l|l|l|l|l|}
2069
\emph{Fixed Header} & \emph{Layout} & \emph{Offset} & \emph{Size} &
2070
\emph{Literals} & \emph{Byte code} \\
2077
\item The entry code is a static code fragment/info table that returns
2078
to the scheduler to invoke Hugs (\secref{ghc-to-hugs-switch}).
2079
\item \emph{Layout} contains the number of pointer literals in the
2080
\emph{Literals} field.
2081
\item \emph{Offset} is the offset to the byte code from the start of
2083
\item \emph{Size} is the number of words of byte code in the object.
2084
\item \emph{Literals} contains any pointer and non-pointer literals used in
2085
the byte-codes (including jump addresses), pointers first.
2086
\item \emph{Byte code} contains \emph{Size} words of non-pointer byte
2091
\Subsubsection{Partial applications}{PAP}
2093
A partial application (PAP) represents a function applied to too few
2094
arguments. It is only built as a result of updating after an
2095
argument-satisfaction check failure. A PAP has the following shape:
2098
\begin{tabular}{|l|l|l|l|}\hline
2099
\emph{Fixed header} & \emph{No of words of stack} & \emph{Function closure} & \emph{Stack chunk ...} \\ \hline
2103
The ``Stack chunk'' is a copy of the chunk of stack above the update
2104
frame; ``No of words of stack'' tells how many words it consists of.
2105
The function closure is (a pointer to) the closure for the function
2106
whose argument-satisfaction check failed.
2108
In the normal case where a PAP is built as a result of an argument
2109
satisfaction check failure, the stack chunk will just contain
2110
``pending arguments'', ie. pointers and tagged non-pointers. It may
2111
in fact also contain activation records, but not update frames, seq
2112
frames, or catch frames. The reason is the garbage collector uses the
2113
same code to scavenge a stack as it does to scavenge the payload of a
2114
PAP, but an update frame contains a link to the next update frame in
2115
the chain and this link would need to be relocated during garbage
2116
collection. Revertible black holes and asynchronous exceptions use
2117
the more general form of PAPs (see Section \ref{revertible-bh}).
2119
There is just one standard form of PAP. There is just one info table
2120
too, called @PAP_info@. Its entry code simply copies the arg stack
2121
chunk back on top of the stack and enters the function closure. (It
2122
has to do a stack overflow test first.)
2124
There is just one way to build a PAP: by calling @stg_update_PAP@ with
2125
the function closure in register @R1@ and the pending arguments on the
2126
stack. The @stg_update_PAP@ function will build the PAP, perform the
2127
update, and return to the next activation record on the stack. If
2128
there are \emph{no} pending arguments on the stack, then no PAP need
2129
be built: in this case @stg_update_PAP@ just overwrites the updatee
2130
with an indirection to the function closure.
2132
PAPs are also used to implement Hugs functions (where the arguments
2133
are free variables). PAPs generated by Hugs can be static so we need
2134
both @PAP@ and @PAP_STATIC@.
2136
\Subsubsection{\texttt{AP\_UPD} objects}{AP_UPD}
2138
@AP_UPD@ objects are used to represent thunks built by Hugs, and to
2139
save the currently-active computations when performing @raiseAsync()@.
2141
distinction between an @AP_UPD@ and a @PAP@ is that an @AP_UPD@ is
2145
\begin{tabular}{|l|l|l|l|}
2147
\emph{Fixed Header} & \emph{No of stack words} & \emph{Function closure} & \emph{Stack chunk} \\
2152
The entry code pushes an update frame, copies the arg stack chunk on
2153
top of the stack, and enters the function closure. (It has to do a
2154
stack overflow test first.)
2156
The ``stack chunk'' is a block of stack not containing update frames,
2157
seq frames or catch frames (just like a PAP). In the case of Hugs,
2158
the stack chunk will contain the free variables of the thunk, and the
2159
function closure is (a pointer to) the closure for the thunk. The
2160
argument stack may be empty if the thunk has no free variables.
2162
\note{Since @AP\_UPD@s are updateable, the @MIN\_UPD\_SIZE@ constraint applies here too.}
2164
\Subsubsection{Indirections}{IND}
2166
Indirection closures just point to other closures. They are introduced
2167
when a thunk is updated to point to its value. The entry code for all
2168
indirections simply enters the closure it points to.
2170
There are several forms of indirection:
2173
\item[@IND@] is the vanilla, dynamically-allocated indirection.
2174
It is removed by the garbage collector. It has the following
2177
\begin{tabular}{|l|l|l|}\hline
2178
\emph{Fixed header} & \emph{Target closure} \\ \hline
2182
An @IND@ only exists in the youngest generation. In older
2183
generations, we have @IND_OLDGEN@s. The update code
2184
(@Upd_frame_$n$_entry@) checks whether the updatee is in the youngest
2185
generation before deciding which kind of indirection to use.
2187
\item[@IND\_OLDGEN@] is the vanilla, dynamically-allocated indirection.
2188
It is removed by the garbage collector. It has the following
2191
\begin{tabular}{|l|l|l|}\hline
2192
\emph{Fixed header} & \emph{Target closure} & \emph{Mutable link field} \\ \hline
2195
It contains a \emph{mutable link field} that is used to string together
2196
mutable objects in each old generation.
2199
For lexical profiling, it is necessary to maintain cost centre
2200
information in an indirection, so ``permanent indirections'' are
2201
retained forever. Otherwise they are just like vanilla indirections.
2202
\note{If a permanent indirection points to another permanent
2203
indirection or a @CONST@ closure, it is possible to elide the indirection
2204
since it will have no effect on the profiler.}
2206
\note{Do we still need @IND@ in the profiling build, or do we just
2207
need @IND@ but its behaviour changes when profiling is on?}
2209
\item[@IND\_OLDGEN\_PERM@]
2210
Just like an @IND_OLDGEN@, but sticks around like an @IND_PERM@.
2212
\item[@IND\_STATIC@] is used for overwriting CAFs when they have been
2213
evaluated. Static indirections are not removed by the garbage
2214
collector; and are statically allocated outside the heap (and should
2215
stay there). Their static object link field is used just as for
2216
@FUN_STATIC@ closures.
2219
\begin{tabular}{|l|l|l|}
2221
\emph{Fixed header} & \emph{Target closure} & \emph{Static link field} \\
2228
\subsubsection{Black holes and blocking queues}
2229
\label{sec:BLACKHOLE}
2230
\label{sec:BLACKHOLE_BQ}
2232
Black hole closures are used to overwrite closures currently being
2233
evaluated. They inform the garbage collector that there are no live
2234
roots in the closure, thus removing a potential space leak.
2236
Black holes also become synchronization points in the concurrent
2237
world. When a thread attempts to enter a blackhole, it must wait for
2238
the result of the computation, which is presumably in progress in
2241
\note{In a single-threaded system, entering a black hole indicates an
2242
infinite loop. In a concurrent system, entering a black hole
2243
indicates an infinite loop only if the hole is being entered by the
2244
same thread that originally entered the closure. It could also bring
2245
about a deadlock situation where several threads are waiting
2246
circularly on computations in progress.}
2248
There are two types of black hole:
2253
A straightforward blackhole just consists of an info pointer and some
2254
padding to allow updating with an @IND_OLDGEN@ if necessary. This
2255
type of blackhole has no waiting threads.
2258
\begin{tabular}{|l|l|l|}
2260
\emph{Fixed header} & \emph{Padding} & \emph{Padding} \\
2265
If we're doing \emph{eager blackholing} then a thunk's info pointer is
2266
overwritten with @BLACKHOLE_info@ at the time of entry; hence the need
2267
for blackholes to be small, otherwise we'd be overwriting part of the
2270
\item[@BLACKHOLE\_BQ@]
2271
When a thread enters a @BLACKHOLE@, it is turned into a @BLACKHOLE_BQ@
2272
(blocking queue), which contains a linked list of blocked threads in
2273
addition to the info pointer.
2276
\begin{tabular}{|l|l|l|}
2278
\emph{Fixed header} & \emph{Blocked thread link} & \emph{Mutable link field} \\
2283
The \emph{Blocked thread link} points to the TSO of the first thread
2284
waiting for the value of this thunk. All subsequent TSOs in the list
2285
are linked together using their @tso->link@ field, ending in
2286
@END_TSO_QUEUE_closure@.
2288
Because new threads can be added to the \emph{Blocked thread link}, a
2289
blocking queue is \emph{mutable}, so we need a mutable link field in
2290
order to chain it on to a mutable list for the generational garbage
2295
\Subsubsection{FetchMes}{FETCHME}
2297
In the parallel systems, FetchMes are used to represent pointers into
2298
the global heap. When evaluated, the value they point to is read from
2301
\ToDo{Describe layout}
2303
Because there may be offsets into these arrays, a primitive array
2304
cannot be handled as a FetchMe in the parallel system, but must be
2305
shipped in its entirety if its parent closure is shipped.
2309
\Subsection{Unpointed Objects}{unpointed-objects}
2311
A variable of unpointed type is always bound to a \emph{value}, never
2312
to a \emph{thunk}. For this reason, unpointed objects cannot be
2315
\subsubsection{Immutable objects}
2316
\label{sec:ARR_WORDS}
2319
\item[@ARR\_WORDS@] is a variable-sized object consisting solely of
2320
non-pointers. It is used for arrays of all sorts of things (bytes,
2321
words, floats, doubles... it doesn't matter).
2323
Strictly speaking, an @ARR_WORDS@ could be mutable, but because it
2324
only contains non-pointers we don't need to track this fact.
2327
\begin{tabular}{|c|c|c|c|}
2329
\emph{Fixed Hdr} & \emph{No of non-pointers} & \emph{Non-pointers\ldots} \\ \hline
2334
\subsubsection{Mutable objects}
2335
\label{sec:mutables}
2337
\label{sec:MUT_ARR_PTRS}
2338
\label{sec:MUT_ARR_PTRS_FROZEN}
2341
Some of these objects are \emph{mutable}; they represent objects which
2342
are explicitly mutated by Haskell code through the @ST@ or @IO@
2343
monads. They're not used for thunks which are updated precisely once.
2344
Depending on the garbage collector, mutable closures may contain extra
2345
header information which allows a generational collector to implement
2346
the ``write barrier.''
2348
Notice that mutable objects all have the same general layout: there is
2349
a mutable link field as the second word after the header. This is so
2350
that code to process old-generation mutable lists doesn't need to look
2351
at the type of the object to determine where its link field is.
2355
\item[@MUT\_VAR@] is a mutable variable.
2357
\begin{tabular}{|c|c|c|}
2359
\emph{Fixed Hdr} \emph{Pointer} & \emph{Mutable link} & \\ \hline
2363
\item[@MUT\_ARR\_PTRS@] is a mutable array of pointers. Such an array
2364
may be \emph{frozen}, becoming an @MUT_ARR_PTRS_FROZEN@, with a
2365
different info-table.
2368
\begin{tabular}{|c|c|c|c|}
2370
\emph{Fixed Hdr} & \emph{No of ptrs} & \emph{Mutable link} & \emph{Pointers\ldots} \\ \hline
2374
\item[@MUT\_ARR\_PTRS\_FROZEN@] This is the immutable version of
2375
@MUT_ARR_PTRS@. It still has a mutable link field for two reasons: we
2376
need to keep it on the mutable list for an old generation at least
2377
until the next garbage collection, and it may become mutable again via
2381
\begin{tabular}{|c|c|c|c|}
2383
\emph{Fixed Hdr} & \emph{No of ptrs} & \emph{Mutable link} & \emph{Pointers\ldots} \\ \hline
2390
\begin{tabular}{|l|l|l|l|l|}
2392
\emph{Fixed header} & \emph{Head} & \emph{Mutable link} & \emph{Tail}
2403
\Subsubsection{Foreign objects}{FOREIGN}
2405
Here's what a ForeignObj looks like:
2408
\begin{tabular}{|l|l|l|l|}
2410
\emph{Fixed header} & \emph{Data} \\
2415
A foreign object is simple a boxed pointer to an address outside the
2416
Haskell heap, possible to @malloc@ed data. The only reason foreign
2417
objects exist is so that we can track the lifetime of one using weak
2418
pointers (see \secref{WEAK}) and run a finaliser when the foreign
2419
object is unreachable.
2421
\subsubsection{Weak pointers}
2425
\begin{tabular}{|l|l|l|l|l|}
2427
\emph{Fixed header} & \emph{Key} & \emph{Value} & \emph{Finaliser}
2433
\ToDo{Weak poitners}
2435
\subsubsection{Stable names}
2436
\label{sec:STABLE_NAME}
2439
\begin{tabular}{|l|l|l|l|}
2441
\emph{Fixed header} & \emph{Index} \\
2448
The remaining objects types are all administrative --- none of them
2451
\subsection{Other weird objects}
2453
\label{sec:BLOCKED_FETCH}
2456
\item[@BlockedFetch@ heap objects (`closures')] (parallel only)
2458
@BlockedFetch@s are inbound fetch messages blocked on local closures.
2459
They arise as entries in a local blocking queue when a fetch has been
2460
received for a local black hole. When awakened, we look at their
2461
contents to figure out where to send a resume.
2463
A @BlockedFetch@ closure has the form:
2465
\begin{tabular}{|l|l|l|l|l|l|}\hline
2466
\emph{Fixed header} & link & node & gtid & slot & weight \\ \hline
2470
\item[Spark Closures] (parallel only)
2472
Spark closures are used to link together all closures in the spark pool. When
2473
the current processor is idle, it may choose to speculatively evaluate some of
2474
the closures in the pool. It may also choose to delete sparks from the pool.
2476
\begin{tabular}{|l|l|l|l|l|l|}\hline
2477
\emph{Fixed header} & \emph{Spark pool link} & \emph{Sparked closure} \\ \hline
2481
\item[Slop Objects]\label{sec:slop-objects}
2483
Slop objects are used to overwrite the end of an updatee if it is
2484
larger than an indirection. Normal slop objects consist of an info
2485
pointer a size word and a number of slop words.
2488
\begin{tabular}{|l|l|l|l|l|l|}\hline
2489
\emph{Info Pointer} & \emph{Size} & \emph{Slop Words} \\ \hline
2493
This is too large for single word slop objects which consist of a
2496
Note that slop objects only contain an info pointer, not a standard
2497
fixed header. This doesn't cause problems because slop objects are
2498
always unreachable --- they can only be accessed by linearly scanning
2501
\note{Currently we don't use slop objects because the storage manager
2502
isn't reliant on objects being adjacent, but if we move to a ``mostly
2503
copying'' style collector, this will become an issue.}
2507
\Subsection{Thread State Objects (TSOs)}{TSO}
2509
In the multi-threaded system, the state of a suspended thread is
2510
packed up into a Thread State Object (TSO) which contains all the
2511
information needed to restart the thread and for the garbage collector
2512
to find all reachable objects. When a thread is running, it may be
2513
``unpacked'' into machine registers and various other memory locations
2514
to provide faster access.
2516
Single-threaded systems don't really \emph{need\/} TSOs --- but they do
2517
need some way to tell the storage manager about live roots so it is
2518
convenient to use a single TSO to store the mutator state even in
2519
single-threaded systems.
2521
Rather than manage TSOs' alloc/dealloc, etc., in some \emph{ad hoc}
2522
way, we instead alloc/dealloc/etc them in the heap; then we can use
2523
all the standard garbage-collection/fetching/flushing/etc machinery on
2524
them. So that's why TSOs are ``heap objects,'' albeit very special
2527
\begin{tabular}{|l|l|}
2528
\hline \emph{Fixed header}
2529
\\ \hline \emph{Link field}
2530
\\ \hline \emph{Mutable link field}
2531
\\ \hline \emph{What next}
2532
\\ \hline \emph{State}
2533
\\ \hline \emph{Thread Id}
2534
\\ \hline \emph{Exception Handlers}
2535
\\ \hline \emph{Ticky Info}
2536
\\ \hline \emph{Profiling Info}
2537
\\ \hline \emph{Parallel Info}
2538
\\ \hline \emph{GranSim Info}
2539
\\ \hline \emph{Stack size}
2540
\\ \hline \emph{Max Stack size}
2543
\\ \hline \emph{SpLim}
2551
The contents of a TSO are:
2554
\item[\emph{Link field}] This is a pointer used to maintain a list of
2555
threads with a similar state (e.g.~all runnable, all sleeping, all
2556
blocked on the same black hole, all blocked on the same MVar,
2559
\item[\emph{Mutable link field}] Because the stack is mutable by
2560
definition, the generational collector needs to track TSOs in older
2561
generations that may point into younger ones (which is just about any
2562
TSO for a thread that has run recently). Hence the need for a mutable
2563
link field (see \secref{mutables}).
2565
\item[\emph{What next}]
2566
This field has five values:
2568
\item[@ThreadEnterGHC@] The thread can be started by entering the
2569
closure pointed to by the word on the top of the stack.
2570
\item[@ThreadRunGHC@] The thread can be started by jumping to the
2571
address on the top of the stack.
2572
\item[@ThreadEnterHugs@] The stack has a pointer to a Hugs-built
2573
closure on top of the stack: enter the closure to run the thread.
2574
\item[@ThreadKilled@] The thread has been killed (by @killThread#@).
2575
It is probably still around because it is on some queue somewhere and
2576
hasn't been garbage collected yet.
2577
\item[@ThreadComplete@] The thread has finished. Its @TSO@ hasn't
2578
been garbage collected yet.
2581
\item[\emph{Thread Id}]
2582
This field contains a (not necessarily unique) integer that identifies
2583
the thread. It can be used eg. for hashing.
2585
\item[\emph{Ticky Info}] Optional information for ``Ticky Ticky''
2586
statistics: @TSO_STK_HWM@ is the maximum number of words allocated to
2589
\item[\emph{Profiling Info}] Optional information for profiling:
2590
@TSO_CCC@ is the current cost centre.
2592
\item[\emph{Parallel Info}]
2593
Optional information for parallel execution.
2597
% \item The types of threads (@TSO_TYPE@):
2598
% \begin{description}
2599
% \item[@T_MAIN@] Must be executed locally.
2600
% \item[@T_REQUIRED@] A required thread -- may be exported.
2601
% \item[@T_ADVISORY@] An advisory thread -- may be exported.
2602
% \item[@T_FAIL@] A failure thread -- may be exported.
2605
% \item I've no idea what else
2609
\item[\emph{GranSim Info}]
2610
Optional information for gransim execution.
2612
% \item Optional information for GranSim execution:
2618
% \item basic blocks
2625
% \item global sparks
2626
% \item local sparks
2629
% \item clock (gransim light only)
2633
% Here are the various queues for GrAnSim-type events.
2642
\item[\emph{Stack Info}] Various fields contain information on the
2643
stack: its current size, its maximum size (to avoid infinite loops
2644
overflowing the memory), the current stack pointer (\emph{Sp}), the
2645
current stack update frame pointer (\emph{Su}), and the stack limit
2646
(\emph{SpLim}). The latter three fields are loaded into the relevant
2647
registers when the thread is run.
2649
\item[\emph{Stack}] This is the actual stack for the thread,
2650
\emph{Stack size} words long. It grows downwards from higher
2651
addresses to lower addresses. When the stack overflows, it will
2652
generally be relocated into larger premises unless \emph{Max stack
2657
The garbage collector needs to be able to find all the
2658
pointers in a stack. How does it do this?
2662
\item Within the stack there are return addresses, pushed
2663
by @case@ expressions. Below a return address (i.e. at higher
2664
memory addresses, since the stack grows downwards) is a chunk
2665
of stack that the return address ``knows about'', namely the
2666
activation record of the currently running function.
2668
\item Below each such activation record is a \emph{pending-argument
2669
section}, a chunk of
2670
zero or more words that are the arguments to which the result
2671
of the function should be applied. The return address does not
2673
``know'' how many pending arguments there are, or their types.
2674
(For example, the function might return a result of type $\alpha$.)
2676
\item Below each pending-argument section is another return address,
2677
and so on. Actually, there might be an update frame instead, but we
2678
can consider update frames as a special case of a return address with
2679
a well-defined activation record.
2683
The game plan is this. The garbage collector walks the stack from the
2684
top, traversing pending-argument sections and activation records
2685
alternately. Next we discuss how it finds the pointers in each of
2686
these two stack regions.
2689
\Subsubsection{Activation records}{activation-records}
2691
An \emph{activation record} is a contiguous chunk of stack,
2692
with a return address as its first word, followed by as many
2693
data words as the return address ``knows about''. The return
2694
address is actually a fully-fledged info pointer. It points
2695
to an info table, replete with:
2698
\item entry code (i.e. the code to return to).
2700
\item closure type is either @RET_SMALL/RET_VEC_SMALL@ or
2701
@RET_BIG/RET_VEC_BIG@, depending on whether the activation record has
2702
more than 32 data words (\note{64 for 8-byte-word architectures}) and
2703
on whether to use a direct or a vectored return.
2705
\item the layout info for @RET_SMALL@ is a bitmap telling the layout
2706
of the activation record, one bit per word. The least-significant bit
2707
describes the first data word of the record (adjacent to the fixed
2708
header) and so on. A ``@1@'' indicates a non-pointer, a ``@0@''
2709
indicates a pointer. We don't need to indicate exactly how many words
2710
there are, because when we get to all zeros we can treat the rest of
2711
the activation record as part of the next pending-argument region.
2713
For @RET_BIG@ the layout field points to a block of bitmap words,
2714
starting with a word that tells how many words are in the block.
2716
\item the info table contains a Static Reference Table pointer for the
2717
return address (\secref{srt}).
2720
The activation record is a fully fledged closure too. As well as an
2721
info pointer, it has all the other attributes of a fixed header
2722
(\secref{fixed-header}) including a saved cost centre which
2723
is reloaded when the return address is entered.
2725
In other words, all the attributes of closures are needed for
2726
activation records, so it's very convenient to make them look alike.
2729
\Subsubsection{Pending arguments}{pending-args}
2731
So that the garbage collector can correctly identify pointers in
2732
pending-argument sections we explicitly tag all non-pointers. Every
2733
non-pointer in a pending-argument section is preceded (at the next
2734
lower memory word) by a one-word byte count that says how many bytes
2735
to skip over (excluding the tag word).
2737
The garbage collector traverses a pending argument section from the
2738
top (i.e. lowest memory address). It looks at each word in turn:
2741
\item If it is less than or equal to a small constant @ARGTAG_MAX@
2742
then it treats it as a tag heralding zero or more words of
2743
non-pointers, so it just skips over them.
2745
\item If it points to the code segment, it must be a return
2746
address, so we have come to the end of the pending-argument section.
2748
\item Otherwise it must be a bona fide heap pointer.
2752
\Subsection{The Stable Pointer Table}{STABLEPTR_TABLE}
2754
A stable pointer is a name for a Haskell object which can be passed to
2755
the external world. It is ``stable'' in the sense that the name does
2756
not change when the Haskell garbage collector runs---in contrast to
2757
the address of the object which may well change.
2759
A stable pointer is represented by an index into the
2760
@StablePointerTable@. The Haskell garbage collector treats the
2761
@StablePointerTable@ as a source of roots for GC.
2763
In order to provide efficient access to stable pointers and to be able
2764
to cope with any number of stable pointers (eg $0 \ldots 100000$), the
2765
table of stable pointers is an array stored on the heap and can grow
2766
when it overflows. (Since we cannot compact the table by moving
2767
stable pointers about, it seems unlikely that a half-empty table can
2768
be reduced in size---this could be fixed if necessary by using a
2769
hash table of some sort.)
2771
In general a stable pointer table closure looks like this:
2774
\begin{tabular}{|l|l|l|l|l|l|l|l|l|l|l|}
2776
\emph{Fixed header} & \emph{No of pointers} & \emph{Free} & $SP_0$ & \ldots & $SP_{n-1}$
2784
\item[@NPtrs@:] number of (stable) pointers.
2786
\item[@Free@:] the byte offset (from the first byte of the object) of the first free stable pointer.
2788
\item[$SP_i$:] A stable pointer slot. If this entry is in use, it is
2789
an ``unstable'' pointer to a closure. If this entry is not in use, it
2790
is a byte offset of the next free stable pointer slot.
2794
When a stable pointer table is evacuated
2796
\item the free list entries are all set to @NULL@ so that the evacuation
2797
code knows they're not pointers;
2799
\item The stable pointer slots are scanned linearly: non-@NULL@ slots
2800
are evacuated and @NULL@-values are chained together to form a new free list.
2803
There's no need to link the stable pointer table onto the mutable
2804
list because we always treat it as a root.
2806
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
2807
\Subsection{Garbage Collecting CAFs}{CAF}
2808
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
2810
% begin{direct quote from current paper}
2811
A CAF (constant applicative form) is a top-level expression with no
2812
arguments. The expression may need a large, even unbounded, amount of
2813
storage when it is fully evaluated.
2815
CAFs are represented by closures in static memory that are updated
2816
with indirections to objects in the heap space once the expression is
2817
evaluated. Previous version of GHC maintained a list of all evaluated
2818
CAFs and traversed them during GC, the result being that the storage
2819
allocated by a CAF would reside in the heap until the program ended.
2820
% end{direct quote from current paper}
2822
% begin{elaboration on why CAFs are very very bad}
2823
Treating CAFs this way has two problems:
2826
It can cause a very large space leak. For example, this program
2827
should run in constant space but, instead, will run out of memory.
2833
> nats = [0..maxInt]
2837
Expressions with no arguments have very different space behaviour
2838
depending on whether or not they occur at the top level. For example,
2839
if we make \verb+nats+ a local definition, the space leak goes away
2840
and the resulting program runs in constant space, as expected.
2846
> nats = [0..maxInt]
2849
This huge change in the operational behaviour of the program
2850
is a problem for optimising compilers and for programmers.
2851
For example, GHC will normally flatten a set of let bindings using
2852
this transformation:
2854
let x1 = let x2 = e2 in e1 ==> let x2 = e2 in let x1 = e1
2856
but it does not do so if this would raise \verb+x2+ to the top level
2857
since that may create a CAF. Many Haskell programmers avoid creating
2858
large CAFs by adding a dummy argument to a CAF or by moving a CAF away
2862
% end{elaboration on why CAFs are very very bad}
2864
Solving the CAF problem requires different treatment in interactive
2865
systems such as Hugs than in batch-mode systems such as GHC
2868
In a batch-mode the program the runtime system is terminated
2869
after every execution of the runtime system. In such systems,
2870
the garbage collector can completely ``destroy'' a CAF when it
2871
is no longer live --- in much the same way as it ``destroys''
2872
normal closures when they are no longer live.
2875
In an interactive system, many expressions are evaluated without
2876
restarting the runtime system between each evaluation. In such
2877
systems, the garbage collector cannot completely ``destroy'' a CAF
2878
when it is no longer live because, whilst it might not be required in
2879
the evaluation of the current expression, it might be required in the
2882
There are two possible behaviours we might want:
2885
When a CAF is no longer required for the current evaluation, the CAF
2886
should be reverted to its original form. This behaviour ensures that
2887
the operational behaviour of the interactive system is a reasonable
2888
predictor of the operational behaviour of the batch-mode system. This
2889
allows us to use Hugs for performance debugging (in particular, trying
2890
to understand and reduce the heap usage of a program) --- an area of
2891
increasing importance as Haskell is used more and more to solve ``real
2892
problems'' in ``real problem domains''.
2895
Even if a CAF is no longer required for the current evaluation, we might
2896
choose to hang onto it by collecting it in the normal way. This keeps
2897
the space leak but might be useful in a teaching environment when
2898
trying to teach the difference between call by name evaluation (which
2899
doesn't share work) and lazy evaluation (which does share work).
2903
It turns out that it is easy to support both styles of use, so the
2904
runtime system provides a switch which lets us turn this on and off
2905
during execution. \ToDo{What is this switch called?} It would also
2906
be easy to provide a function \verb+RevertCAF+ to let the interpreter
2907
revert any CAF it wanted between (but not during) executions, if we so
2908
desired. Running \verb+RevertCAF+ during execution would lose some sharing
2909
but is otherwise harmless.
2913
% % begin{even more pointless observation?}
2914
% The simplest fix would be to remove the special treatment of
2915
% top level variables. This works but is very inefficient.
2917
% (Note: delete this paragraph from final version.)
2918
% % end{even more pointless observation?}
2920
% begin{pointless observation?}
2921
An easy but inefficient fix to the CAF problem would be to make a
2922
complete copy of the heap before every evaluation and discard the copy
2923
after evaluation. This works but is inefficient.
2924
% end{pointless observation?}
2926
An efficient way to achieve a similar effect is to revert all
2927
updatable thunks to their original form as they become unnecessary for
2928
the current evaluation. To do this, we modify the compiler to ensure
2929
that the only updatable thunks generated by the compiler are CAFs and
2930
we modify the garbage collector to revert entered CAFs to unentered
2931
CAFs as their value becomes unnecessary.
2934
\subsubsection{New Heap Objects}
2936
We add three new kinds of heap object: unentered CAF closures, entered
2937
CAF objects and CAF blackholes. We first describe how they are
2938
evaluated and then how they are garbage collected.
2941
Unentered CAF closures contain a pointer to closure representing the
2942
body of the CAF. The ``body closure'' is not updatable.
2944
Unentered CAF closures contain two unused fields to make them the same
2945
size as entered CAF closures --- which allows us to perform an inplace
2946
update. \ToDo{Do we have to add another kind of inplace update operation
2947
to the storage manager interface or do we consider this to be internal
2950
\begin{tabular}{|l|l|l|l|}\hline
2951
\verb+CAF_unentered+ & \emph{body closure} & \emph{unused} & \emph{unused} \\ \hline
2954
When an unentered CAF is entered, we do the following:
2957
allocate a CAF black hole;
2960
push an update frame (to update the CAF black hole) onto the stack;
2963
overwrite the CAF with an entered CAF object (see below) with the same
2964
body and whose value field points to the black hole;
2967
add the CAF to a list of all entered CAFs (called ``the CAF list'');
2971
the closure representing the value of the CAF is entered.
2975
When evaluation of the CAF body returns a value, the update frame
2976
causes the CAF black hole to be updated with the value in the normal
2979
\ToDo{Add a picture}
2982
Entered CAF closures contain two pointers: a pointer to the CAF body
2983
(the same as for unentered CAF closures); a pointer to the CAF value
2984
(this is initialised with a CAF blackhole, as previously described);
2985
and a link to the next CAF in the CAF list
2987
\ToDo{How is the end of the list marked? Null pointer or sentinel value?}.
2990
\begin{tabular}{|l|l|l|l|}\hline
2991
\verb+CAF_entered+ & \emph{body closure} & \emph{value} & \emph{link} \\ \hline
2994
When an entered CAF is entered, it enters its value closure.
2997
CAF blackholes are identical to normal blackholes except that they
2998
have a different infotable. The only reason for having CAF blackholes
2999
is to allow an optimisation of lazy blackholing where we stop scanning
3000
the stack when we see the first {\em normal blackhole} but not
3001
when we see a {\em CAF blackhole.}
3002
\ToDo{The optimisation we want to allow should be described elsewhere
3003
so that all we have to do here is describe the difference.}
3005
Instead of allocating a blackhole to update with the value of the CAF,
3006
it might seem simpler to update the CAF directly. This would require
3007
a new kind of update frame which would update the value field of the
3008
CAF with a pointer to the value and wouldn't catch blackholes caused
3009
by CAFs that depend on themselves so we chose not to do so.
3013
\subsubsection{Garbage Collection}
3015
To avoid the space leak, each run of the garbage collector must revert
3016
the entered CAFs which are not required to complete the current
3017
evaluation (that is all the closures reachable from the set of
3018
runnable threads and the stable pointer table).
3020
It does this by performing garbage collection in three phases:
3023
During the first phase, we ``mark'' all closures reachable from the
3026
How we ``mark'' closures depends on the garbage collector. For
3027
example, in a 2-space collector, closures are ``marked'' by copying
3028
them into ``to-space'', overwriting them with a forwarding node and
3029
``marking'' all the closures reachable from the copy. The only
3030
requirements are that we can test whether a closure is marked and if a
3031
closure is marked then so are all closures reachable from it.
3033
\ToDo{At present we say that the scheduler state includes any state
3034
that Hugs may have. This is not true anymore.}
3036
Performing this phase first provides us with a cheap test for
3037
execution closures: at this stage in execution, the execution closures
3038
are precisely the marked closures.
3041
During the second phase, we revert all unmarked CAFs on the CAF list
3042
and remove them from the CAF list.
3044
Since the CAF list is exactly the set of all entered CAFs, this reverts
3045
all entered CAFs which are not execution closures.
3048
During the third phase, we mark all top level objects (including CAFs)
3049
by calling \verb+MarkHugsRoots+ which will call \verb+MarkRoot+ for
3050
each top level object known to Hugs.
3054
To implement the second style of interactive behaviour (where we
3055
deliberately keep the CAF-related space leak), we simply omit the
3056
second phase. Omitting the second phase causes the third phase to
3057
mark any unmarked CAF value closures.
3059
So far, we have been describing a pure Hugs system which contains no
3060
machine generated code. The main difference in a hybrid system is
3061
that GHC-generated code is statically allocated in memory instead of
3062
being dynamically allocated on the heap. We split both
3063
\verb+CAF_unentered+ and \verb+CAF_entered+ into two versions: a
3064
static and a dynamic version. The static and dynamic versions of each
3065
CAF differ only in whether they are moved during garbage collection.
3066
When reverting CAFs, we revert dynamic entered CAFs to dynamic
3067
unentered CAFs and static entered CAFs to static unentered CAFs.
3072
\Section{The Bytecode Evaluator}{bytecode-evaluator}
3074
This section describes how the Hugs interpreter interprets code in the
3075
same environment as compiled code executes. Both evaluation models
3076
use a common garbage collector, so they must agree on the form of
3077
objects in the heap.
3079
Hugs interprets code by converting it to byte-code and applying a
3080
byte-code interpreter to it. Wherever possible, we try to ensure that
3081
the byte-code is all that is required to interpret a section of code.
3082
This means not dynamically generating info tables, and hence we can
3083
only have a small number of possible heap objects each with a statically
3084
compiled info table. Similarly for stack objects: in fact we only
3085
have one Hugs stack object, in which all information is tagged for the
3088
There is, however, one exception to this rule. Hugs must generate
3089
info tables for any constructors it is asked to compile, since the
3090
alternative is to force a context-switch each time compiled code
3091
enters a Hugs-built constructor, which would be prohibitively
3094
We achieve this simplicity by forgoing some of the optimisations used
3099
Whereas compiled code has five different ways of entering a closure
3100
(\secref{ghc-fun-call}), interpreted code has only one.
3101
The entry point for interpreted code behaves like slow entry points for
3106
We use just one info table for \emph{all\/} direct returns.
3107
This introduces two problems:
3109
\item How does the interpreter know what code to execute?
3111
Instead of pushing just a return address, we push a return BCO and a
3112
trivial return address which just enters the return BCO.
3114
(In a purely interpreted system, we could avoid pushing the trivial
3117
\item How can the garbage collector follow pointers within the
3120
We could push a third word ---a bitmask describing the location of any
3121
pointers within the record--- but, since we're already tagging unboxed
3122
function arguments on the stack, we use the same mechanism for unboxed
3123
values within the activation record.
3125
\ToDo{Do we have to stub out dead variables in the activation frame?}
3131
We trivially support vectored returns by pushing a return vector whose
3132
entries are all the same.
3136
We avoid the need to build SRTs by putting bytecode objects on the
3137
heap and restricting BCOs to a single basic block.
3141
\Subsection{Hugs Info Tables}{hugs-info-tables}
3143
Hugs requires the following info tables and closures:
3145
\item [@HUGS\_RET@].
3147
Contains both a vectored return table and a direct entry point. All
3148
entry points are the same: they rearrange the stack to match the Hugs
3149
return convention (\secref{hugs-return-convention}) and return to the
3150
scheduler. When the scheduler restarts the thread, it will find a BCO
3151
on top of the stack and will enter the Hugs interpreter.
3155
This is just the standard info table for an update frame.
3157
\item [Constructors].
3159
The entry code for a constructor jumps to a generic entry point in the
3160
runtime system which decides whether to do a vectored or unvectored
3161
return depending on the shape of the constructor/type. This implies that
3162
info tables must have enough info to make that decision.
3164
\item [@AP@ and @PAP@].
3166
\item [Indirections].
3170
Hugs doesn't generate them itself but it ought to recognise them
3172
\item [Complex primops].
3174
Some of the primops are too complex for GHC to generate inline.
3175
Instead, these primops are hand-written and called as normal functions.
3176
Hugs only needs to know their names and types but doesn't care whether
3177
they are generated by GHC or by hand. Two things to watch:
3181
Hugs must be able to enter these primops even if it is working on a
3182
standalone system that does not support genuine GHC generated code.
3184
\item The complex primops often involve unboxed tuple types (which
3185
Hugs does not support at the source level) so we cannot specify their
3186
types in a Haskell source file.
3192
\Subsection{Hugs Heap Objects}{hugs-heap-objects}
3194
\subsubsection{Byte-code objects}
3196
Compiled byte code lives on the global heap, in objects called
3197
Byte-Code Objects (or BCOs). The layout of BCOs is described in
3198
detail in \secref{BCO}, in this section we will describe
3201
Since byte-code lives on the heap, it can be garbage collected just
3202
like any other heap-resident data. Hugs arranges that any BCO's
3203
referred to by the Hugs symbol tables are treated as live objects by
3204
the garbage collector. When a module is unloaded, the pointers to its
3205
BCOs are removed from the symbol table, and the code will be garbage
3206
collected some time later.
3208
A BCO represents a basic block of code --- the (only) entry points is
3209
at the beginning of a BCO, and it is impossible to jump into the
3210
middle of one. A BCO represents not only the code for a function, but
3211
also its closure; a BCO can be entered just like any other closure.
3212
Hugs performs lambda-lifting during compilation to byte-code, and each
3213
top-level combinator becomes a BCO in the heap.
3216
\subsubsection{Thunks and partial applications}
3218
A thunk consists of a code pointer, and values for the free variables
3219
of that code. Since Hugs byte-code is lambda-lifted, free variables
3220
become arguments and are expected to be on the stack by the called
3223
Hugs represents updateable thunks with @AP_UPD@ objects applying a closure
3224
to a list of arguments. (As for @PAP@s, unboxed arguments should be
3225
preceded by a tag.) When it is entered, it pushes an update frame
3226
followed by its payload on the stack, and enters the first word (which
3227
will be a pointer to a BCO). The layout of @AP_UPD@ objects is described
3228
in more detail in \secref{AP_UPD}.
3230
Partial applications are represented by @PAP@ objects, which are
3233
\ToDo{Hugs Constructors}.
3235
\Subsection{Calling conventions}{hugs-calling-conventions}
3237
The calling convention for any byte-code function is straightforward:
3239
\item Push any arguments on the stack.
3240
\item Push a pointer to the BCO.
3241
\item Begin interpreting the byte code.
3244
In a system containing both GHC and Hugs, the bytecode interpreter
3245
only has to be able to enter BCOs: everything else can be handled by
3246
returning to the compiled world (as described in
3247
\secref{hugs-to-ghc-switch}) and entering the closure
3250
This would work but it would obviously be very inefficient if we
3251
entered a @AP@ by switching worlds, entering the @AP@, pushing the
3252
arguments and function onto the stack, and entering the function
3253
which, likely as not, will be a byte-code object which we will enter
3254
by \emph{returning} to the byte-code interpreter. To avoid such
3255
gratuitious world switching, we choose to recognise certain closure
3256
types as being ``standard'' --- and duplicate the entry code for the
3257
``standard closures'' in the bytecode interpreter.
3259
A closure is said to be ``standard'' if its entry code is entirely
3260
determined by its info table. \emph{Standard Closures} have the
3261
desirable property that the byte-code interpreter can enter the
3262
closure by simply ``interpreting'' the info table instead of switching
3263
to the compiled world. The standard closures include:
3266
\item[Constructor] To enter a constructor, we simply return (see
3267
\secref{hugs-return-convention}).
3270
To enter an indirection, we simply enter the object it points to
3271
after possibly adjusting the current cost centre.
3275
To enter an @AP@, we push an update frame, push the
3276
arguments, push the function and enter the function.
3277
(Not forgetting a stack check at the start.)
3281
To enter a @PAP@, we push the arguments, push the function and enter
3282
the function. (Not forgetting a stack check at the start.)
3286
To enter a selector (\secref{THUNK_SELECTOR}), we test whether the
3287
selectee is a value. If so, we simply select the appropriate
3288
component; if not, it's simplest to treat it as a GHC-built closure
3289
--- though we could interpret it if we wanted.
3293
The most obvious omissions from the above list are @BCO@s (which we
3294
dealt with above) and GHC-built closures (which are covered in
3295
\secref{hugs-to-ghc-switch}).
3298
\Subsection{Return convention}{hugs-return-convention}
3300
When Hugs pushes a return address, it pushes both a pointer to the BCO
3301
to return to, and a pointer to a static code fragment @HUGS_RET@ (this
3302
is described in \secref{ghc-to-hugs-switch}). The
3303
stack layout is shown in \figref{hugs-return-stack}.
3315
%\input{hugs_ret.pstex_t}
3317
\caption{Stack layout for a Hugs return address}
3318
\label{fig:hugs-return-stack}
3319
% this figure apparently duplicates {fig:hugs-return-stack1} earlier.
3330
%\input{hugs_ret2.pstex_t}
3332
\caption{Stack layout on enterings a Hugs return address}
3333
\label{fig:hugs-return2}
3346
%\input{hugs_ret2.pstex_t}
3348
\caption{Stack layout on entering a Hugs return address with an unboxed value}
3349
\label{fig:hugs-return-int1}
3362
%\input{hugs_ret3.pstex_t}
3364
\caption{Stack layout on enterings a GHC return address}
3365
\label{fig:hugs-return3}
3379
| restart |--> id_Int#_closure
3382
%\input{hugs_ret2.pstex_t}
3384
\caption{Stack layout on enterings a GHC return address with an unboxed value}
3385
\label{fig:hugs-return-int}
3388
When a Hugs byte-code sequence enters a closure, it examines the
3389
return address on top of the stack.
3393
\item If the return address is @HUGS_RET@, pop the @HUGS_RET@ and the
3394
bco for the continuation off the stack, push a pointer to the constructor onto
3395
the stack and enter the BCO with the current object pointer set to the BCO
3396
(\figref{hugs-return2}).
3398
\item If the top of the stack is not @HUGS_RET@, we need to do a world
3399
switch as described in \secref{hugs-to-ghc-switch}.
3403
\ToDo{This duplicates what we say about switching worlds
3404
(\secref{switching-worlds}) - kill one or t'other.}
3407
\ToDo{This was in the evaluation model part but it really belongs in
3408
this part which is about the internal details of each of the major
3411
\Subsection{Addressing Modes}{hugs-addressing-modes}
3413
To avoid potential alignment problems and simplify garbage collection,
3414
all literal constants are stored in two tables (one boxed, the other
3415
unboxed) within each BCO and are referred to by offsets into the tables.
3416
Slots in the constant tables are word aligned.
3418
\ToDo{How big can the offsets be? Is the offset specified in the
3419
address field or in the instruction?}
3421
Literals can have the following types: char, int, nat, float, double,
3422
and pointer to boxed object. There is no real difference between
3423
char, int, nat and float since they all occupy 32 bits --- but it
3424
costs almost nothing to distinguish them and may improve portability
3425
and simplify debugging.
3427
\Subsection{Compilation}{hugs-compilation}
3430
\def\is{\mbox{\it is}}
3431
\def\ts{\mbox{\it ts}}
3432
\def\as{\mbox{\it as}}
3433
\def\bs{\mbox{\it bs}}
3434
\def\cs{\mbox{\it cs}}
3435
\def\rs{\mbox{\it rs}}
3436
\def\us{\mbox{\it us}}
3437
\def\vs{\mbox{\it vs}}
3438
\def\ws{\mbox{\it ws}}
3439
\def\xs{\mbox{\it xs}}
3441
\def\e{\mbox{\it e}}
3442
\def\alts{\mbox{\it alts}}
3443
\def\fail{\mbox{\it fail}}
3444
\def\panic{\mbox{\it panic}}
3445
\def\ua{\mbox{\it ua}}
3446
\def\obj{\mbox{\it obj}}
3447
\def\bco{\mbox{\it bco}}
3448
\def\tag{\mbox{\it tag}}
3449
\def\entry{\mbox{\it entry}}
3450
\def\su{\mbox{\it su}}
3452
\def\Ind#1{{\mbox{\it Ind}\ {#1}}}
3453
\def\update#1{{\mbox{\it update}\ {#1}}}
3455
\def\next{$\Longrightarrow$}
3456
\def\append{\mathrel{+\mkern-6mu+}}
3457
\def\reverse{\mbox{\it reverse}}
3458
\def\size#1{{\vert {#1} \vert}}
3459
\def\arity#1{{\mbox{\it arity}{#1}}}
3461
\def\AP{\mbox{\it AP}}
3462
\def\PAP{\mbox{\it PAP}}
3463
\def\GHCRET{\mbox{\it GHCRET}}
3464
\def\GHCOBJ{\mbox{\it GHCOBJ}}
3466
To make sense of the instructions, we need a sense of how they will be
3467
used. Here is a small compiler for the STG language.
3470
> cg (f{a1, ... am}) = do
3471
> pushAtom am; ... pushAtom a1
3475
> cg (let {x1=rhs1; ... xm=rhsm} in e) = do
3476
> ALLOC x1 |rhs1|, ... ALLOC xm |rhsm|
3477
> build x1 rhs1, ... build xm rhsm
3479
> cg (case e of alts) = do
3480
> PUSHALTS (cgAlts alts)
3483
> cgAlts { alt1; ... altm } = cgAlt alt1 $ ... $ cgAlt altm pmFail
3485
> cgAlt (x@C{xs} -> e) fail = do
3487
> HEAPCHECK (heapUse e)
3491
> build x (C{a1, ... am}) = do
3492
> pushUntaggedAtom am; ... pushUntaggedAtom a1
3494
> -- A useful optimisation
3495
> build x ({v1, ... vm} \ {}. f{a1, ... am}) = do
3496
> pushVar am; ... pushVar a1
3499
> build x ({v1, ... vm} \ {}. e) = do
3500
> pushVar vm; ... pushVar v1
3501
> PUSHBCO (cgRhs ({v1, ... vm} \ {}. e))
3503
> build x ({v1, ... vm} \ {x1, ... xm}. e) = do
3504
> pushVar vm; ... pushVar v1
3505
> PUSHBCO (cgRhs ({v1, ... vm} \ {x1, ... xm}. e))
3508
> cgRhs (vs \ xs. e) = do
3509
> ARGCHECK (xs ++ vs) -- can be omitted if xs == {}
3510
> STACKCHECK min(stackUse e,heapOverflowSlop)
3511
> HEAPCHECK (heapUse e)
3514
> pushAtom x = pushVar x
3515
> pushAtom i# = PUSHINT i#
3517
> pushVar x = if isGlobalVar x then PUSHGLOBAL x else PUSHLOCAL x
3519
> pushUntaggedAtom x = pushVar x
3520
> pushUntaggedAtom i# = PUSHUNTAGGEDINT i#
3522
> pushVar x = if isGlobalVar x then PUSHGLOBAL x else PUSHLOCAL x
3525
\ToDo{Is there an easy way to add semi-tagging? Would it be that different?}
3527
\ToDo{Optimise thunks of the form @f{x1,...xm}@ so that we build an AP directly}
3529
\Subsection{Instructions}{hugs-instructions}
3531
We specify the semantics of instructions using transition rules of
3534
\begin{tabular}{|llrrrrr|}
3536
& $\is$ & $s$ & $\su$ & $h$ & $hp$ & $\sigma$ \\
3537
\next & $\is'$ & $s'$ & $\su'$ & $h'$ & $hp'$ & $\sigma$ \\
3541
where $\is$ is an instruction stream, $s$ is the stack, $\su$ is the
3542
update frame pointer and $h$ is the heap.
3545
\Subsection{Stack manipulation}{hugs-stack-manipulation}
3549
\item[ Push a global variable ].
3551
\begin{tabular}{|llrrrrr|}
3553
& PUSHGLOBAL $o$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3554
\next & $\is$ & $\sigma!o:s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3558
\item[ Push a local variable ].
3560
\begin{tabular}{|llrrrrr|}
3562
& PUSHLOCAL $o$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3563
\next & $\is$ & $s!o : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3567
\item[ Push an unboxed int ].
3569
\begin{tabular}{|llrrrrr|}
3571
& PUSHINT $o$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3572
\next & $\is$ & $I\# : \sigma!o : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3576
The $I\#$ is a tag included for the benefit of the garbage collector.
3577
Similar rules exist for floats, doubles, chars, etc.
3579
\item[ Push an unboxed int ].
3581
\begin{tabular}{|llrrrrr|}
3583
& PUSHUNTAGGEDINT $o$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3584
\next & $\is$ & $\sigma!o : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3588
Similar rules exist for floats, doubles, chars, etc.
3590
\item[ Delete environment from stack --- ready for tail call ].
3592
\begin{tabular}{|llrrrrr|}
3594
& SLIDE $m$ $n$ : $\is$ & $\as \append \bs \append \cs$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3595
\next & $\is$ & $\as \append \cs$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3599
where $\size{\as} = m$ and $\size{\bs} = n$.
3602
\item[ Push a return address ].
3604
\begin{tabular}{|llrrrrr|}
3606
& PUSHALTS $o$:$\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3607
\next & $\is$ & $@HUGS_RET@:\sigma!o:s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3611
\item[ Push a BCO ].
3613
\begin{tabular}{|llrrrrr|}
3615
& PUSHBCO $o$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3616
\next & $\is$ & $\sigma!o : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3622
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
3623
\Subsection{Heap manipulation}{hugs-heap-manipulation}
3624
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
3628
\item[ Allocate a heap object ].
3630
\begin{tabular}{|llrrrrr|}
3632
& ALLOC $m$ : $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3633
\next & $\is$ & $hp:s$ & $su$ & $h$ & $hp+m$ & $\sigma$ \\
3637
\item[ Build a constructor ].
3639
\begin{tabular}{|llrrrrr|}
3641
& PACK $o$ $o'$ : $\is$ & $\ws \append s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3642
\next & $\is$ & $s$ & $su$ & $h[s!o \mapsto Pack C\{\ws\}]$ & $hp$ & $\sigma$ \\
3646
where $C = \sigma!o'$ and $\size{\ws} = \arity{C}$.
3648
\item[ Build an AP or PAP ].
3650
\begin{tabular}{|llrrrrr|}
3652
& MKAP $o$ $m$:$\is$ & $f : \ws \append s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3653
\next & $\is$ & $s$ & $su$ & $h[s!o \mapsto \AP(f,\ws)]$ & $hp$ & $\sigma$ \\
3657
where $\size{\ws} = m$.
3659
\begin{tabular}{|llrrrrr|}
3661
& MKPAP $o$ $m$:$\is$ & $f : \ws \append s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3662
\next & $\is$ & $s$ & $su$ & $h[s!o \mapsto \PAP(f,\ws)]$ & $hp$ & $\sigma$ \\
3666
where $\size{\ws} = m$.
3668
\item[ Unpacking a constructor ].
3670
\begin{tabular}{|llrrrrr|}
3672
& UNPACK : $is$ & $a : s$ & $su$ & $h[a \mapsto C\ \ws]$ & $hp$ & $\sigma$ \\
3673
\next & $is'$ & $(\reverse\ \ws) \append a : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3677
The $\reverse\ \ws$ looks expensive but, since the stack grows down
3678
and the heap grows up, that's actually the cheap way of copying from
3679
heap to stack. Looking at the compilation rules, you'll see that we
3680
always push the args in reverse order.
3685
\Subsection{Entering a closure}{hugs-entering}
3689
\item[ Enter a BCO ].
3691
\begin{tabular}{|llrrrrr|}
3693
& [ENTER] & $a : s$ & $su$ & $h[a \mapsto BCO\{\is\} ]$ & $hp$ & $\sigma$ \\
3694
\next & $\is$ & $a : s$ & $su$ & $h$ & $hp$ & $a$ \\
3698
\item[ Enter a PAP closure ].
3700
\begin{tabular}{|llrrrrr|}
3702
& [ENTER] & $a : s$ & $su$ & $h[a \mapsto \PAP(f,\ws)]$ & $hp$ & $\sigma$ \\
3703
\next & [ENTER] & $f : \ws \append s$ & $su$ & $h$ & $hp$ & $???$ \\
3707
\item[ Entering an AP closure ].
3709
\begin{tabular}{|llrrrrr|}
3711
& [ENTER] & $a : s$ & $su$ & $h[a \mapsto \AP(f,ws)]$ & $hp$ & $\sigma$ \\
3712
\next & [ENTER] & $f : \ws \append @UPD_RET@:\su:a:s$ & $su'$ & $h$ & $hp$ & $???$ \\
3718
\item Instead of blindly pushing an update frame for $a$, we can first test whether there's already
3719
an update frame there. If so, overwrite the existing updatee with an indirection to $a$ and
3720
overwrite the updatee field with $a$. (Overwriting $a$ with an indirection to the updatee also
3721
works.) This results in update chains of maximum length 2.
3725
\item[ Returning a constructor ].
3727
\begin{tabular}{|llrrrrr|}
3729
& [ENTER] & $a : @HUGS_RET@ : \alts : s$ & $su$ & $h[a \mapsto C\{\ws\}]$ & $hp$ & $\sigma$ \\
3730
\next & $\alts.\entry$ & $a:s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3735
\item[ Entering an indirection node ].
3737
\begin{tabular}{|llrrrrr|}
3739
& [ENTER] & $a : s$ & $su$ & $h[a \mapsto \Ind{a'}]$ & $hp$ & $\sigma$ \\
3740
\next & [ENTER] & $a' : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3744
\item[Entering GHC closure].
3746
\begin{tabular}{|llrrrrr|}
3748
& [ENTER] & $a : s$ & $su$ & $h[a \mapsto \GHCOBJ]$ & $hp$ & $\sigma$ \\
3749
\next & [ENTERGHC] & $a : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3753
\item[Returning a constructor to GHC].
3755
\begin{tabular}{|llrrrrr|}
3757
& [ENTER] & $a : \GHCRET : s$ & $su$ & $h[a \mapsto C \ws]$ & $hp$ & $\sigma$ \\
3758
\next & [ENTERGHC] & $a : \GHCRET : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3765
\Subsection{Updates}{hugs-updates}
3769
\item[ Updating with a constructor].
3771
\begin{tabular}{|llrrrrr|}
3773
& [ENTER] & $a : @UPD_RET@ : ua : s$ & $su$ & $h[a \mapsto C\{\ws\}]$ & $hp$ & $\sigma$ \\
3774
\next & [ENTER] & $a \append s$ & $su$ & $h[au \mapsto \Ind{a}$ & $hp$ & $\sigma$ \\
3778
\item[ Argument checks].
3780
\begin{tabular}{|llrrrrr|}
3782
& ARGCHECK $m$:$\is$ & $a : \as \append s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3783
\next & $\is$ & $a : \as \append s$ & $su$ & $h'$ & $hp$ & $\sigma$ \\
3787
where $m \ge (su - sp)$
3789
\begin{tabular}{|llrrrrr|}
3791
& ARGCHECK $m$:$\is$ & $a : \as \append @UPD_RET@:su:ua:s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3792
\next & $\is$ & $a : \as \append s$ & $su$ & $h'$ & $hp$ & $\sigma$ \\
3796
where $m < (su - sp)$ and
3797
$h' = h[ua \mapsto \Ind{a'}, a' \mapsto \PAP(a,\reverse\ \as) ]$
3799
Again, we reverse the list of values as we transfer them from the
3800
stack to the heap --- reflecting the fact that the stack and heap grow
3801
in different directions.
3805
\Subsection{Branches}{hugs-branches}
3809
\item[ Testing a constructor ].
3811
\begin{tabular}{|llrrrrr|}
3813
& TEST $tag$ $is'$ : $is$ & $a : s$ & $su$ & $h[a \mapsto C\ \ws]$ & $hp$ & $\sigma$ \\
3814
\next & $is$ & $a : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3818
where $C.\tag = tag$
3820
\begin{tabular}{|llrrrrr|}
3822
& TEST $tag$ $is'$ : $is$ & $a : s$ & $su$ & $h[a \mapsto C\ \ws]$ & $hp$ & $\sigma$ \\
3823
\next & $is'$ & $a : s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3827
where $C.\tag \neq tag$
3831
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
3832
\Subsection{Heap and stack checks}{hugs-heap-stack-checks}
3833
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
3835
\begin{tabular}{|llrrrrr|}
3837
& STACKCHECK $stk$:$\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3838
\next & $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3842
if $s$ has $stk$ free slots.
3844
\begin{tabular}{|llrrrrr|}
3846
& HEAPCHECK $hp$:$\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3847
\next & $\is$ & $s$ & $su$ & $h$ & $hp$ & $\sigma$ \\
3851
if $h$ has $hp$ free slots.
3853
If either check fails, we push the current bco ($\sigma$) onto the
3854
stack and return to the scheduler. When the scheduler has fixed the
3855
problem, it pops the top object off the stack and reenters it.
3860
\item The bytecode CHECK1000 conservatively checks for 1000 words of heap space and 1000 words of stack space.
3861
We use it to reduce code space and instruction decoding time.
3862
\item The bytecode HEAPCHECK1000 conservatively checks for 1000 words of heap space.
3863
It is used in case alternatives.
3867
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
3868
\Subsection{Primops}{hugs-primops}
3869
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
3871
\ToDo{primops take m words and return n words. The expect boxed arguments on the stack.}
3874
\Section{The Machine Code Evaluator}{asm-evaluator}
3876
This section describes the framework in which compiled code evaluates
3877
expressions. Only at certain points will compiled code need to be
3878
able to talk to the interpreted world; these are discussed in
3879
\secref{switching-worlds}.
3881
\Subsection{Calling conventions}{ghc-calling-conventions}
3883
\Subsubsection{The call/return registers}{ghc-regs}
3885
One of the problems in designing a virtual machine is that we want it
3886
abstract away from tedious machine details but still reveal enough of
3887
the underlying hardware that we can make sensible decisions about code
3888
generation. A major problem area is the use of registers in
3889
call/return conventions. On a machine with lots of registers, it's
3890
cheaper to pass arguments and results in registers than to pass them
3891
on the stack. On a machine with very few registers, it's cheaper to
3892
pass arguments and results on the stack than to use ``virtual
3893
registers'' in memory. We therefore use a hybrid system: the first
3894
$n$ arguments or results are passed in registers; and the remaining
3895
arguments or results are passed on the stack. For register-poor
3896
architectures, it is important that we allow $n=0$.
3898
We'll label the arguments and results \Arg{1} \ldots \Arg{m} --- with
3899
the understanding that \Arg{1} \ldots \Arg{n} are in registers and
3900
\Arg{n+1} \ldots \Arg{m} are on top of the stack.
3902
Note that the mapping of arguments \Arg{1} \ldots \Arg{n} to machine
3903
registers depends on the \emph{kinds} of the arguments. For example,
3904
if the first argument is a Float, we might pass it in a different
3905
register from if it is an Int. In fact, we might find that a given
3906
architecture lets us pass varying numbers of arguments according to
3907
their types. For example, if a CPU has 2 Int registers and 2 Float
3908
registers then we could pass between 2 and 4 arguments in machine
3909
registers --- depending on whether they all have the same kind or they
3910
have different kinds.
3912
\Subsubsection{Entering closures}{entering-closures}
3914
To evaluate a closure we jump to the entry code for the closure
3915
passing a pointer to the closure in \Arg{1} so that the entry code can
3916
access its environment.
3918
\Subsubsection{Function call}{ghc-fun-call}
3920
The function-call mechanism is obviously crucial. There are five different
3924
\item \emph{Known combinator (function with no free variables) and
3927
A fast call can be made: push excess arguments onto stack and jump to
3928
function's \emph{fast entry point} passing arguments in \Arg{1} \ldots
3931
The \emph{fast entry point} is only called with exactly the right
3932
number of arguments (in \Arg{1} \ldots \Arg{m}) so it can instantly
3933
start doing useful work without first testing whether it has enough
3934
registers or having to pop them off the stack first.
3936
\item \emph{Known combinator and insufficient arguments.}
3938
A slow call can be made: push all arguments onto stack and jump to
3939
function's \emph{slow entry point}.
3941
Any unpointed arguments which are pushed on the stack must be tagged.
3942
This means pushing an extra word on the stack below the unpointed
3943
words, containing the number of unpointed words above it.
3945
%Todo: forward ref about tagging?
3948
The \emph{slow entry point} might be called with insufficient arguments
3949
and so it must test whether there are enough arguments on the stack.
3950
This \emph{argument satisfaction check} consists of checking that
3951
@Su-Sp@ is big enough to hold all the arguments (including any tags).
3955
\item If the argument satisfaction check fails, it is because there is
3956
one or more update frames on the stack before the rest of the
3957
arguments that the function needs. In this case, we construct a PAP
3958
(partial application, \secref{PAP}) containing the arguments
3959
which are on the stack. The PAP construction code will return to the
3960
update frame with the address of the PAP in \Arg{1}.
3962
\item If the argument satisfaction check succeeds, we jump to the fast
3963
entry point with the arguments in \Arg{1} \ldots \Arg{arity}.
3965
If the fast entry point expects to receive some of \Arg{i} on the
3966
stack, we can reduce the amount of movement required by making the
3967
stack layout for the fast entry point look like the stack layout for
3968
the slow entry point. Since the slow entry point is entered with the
3969
first argument on the top of the stack and with tags in front of any
3970
unpointed arguments, this means that if \Arg{i} is unpointed, there
3971
should be space below it for a tag and that the highest numbered
3972
argument should be passed on the top of the stack.
3974
We usually arrange that the fast entry point is placed immediately
3975
after the slow entry point --- so we can just ``fall through'' to the
3976
fast entry point without performing a jump.
3981
\item \emph{Known function closure (function with free variables) and
3984
A fast call can be made: push excess arguments onto stack and jump to
3985
function's \emph{fast entry point} passing a pointer to closure in
3986
\Arg{1} and arguments in \Arg{2} \ldots \Arg{m+1}.
3988
Like the fast entry point for a combinator, the fast entry point for a
3989
closure is only called with appropriate values in \Arg{1} \ldots
3990
\Arg{m+1} so we can start work straight away. The pointer to the
3991
closure is used to access the free variables of the closure.
3994
\item \emph{Known function closure and insufficient arguments.}
3996
A slow call can be made: push all arguments onto stack and jump to the
3997
closure's slow entry point passing a pointer to the closure in \Arg{1}.
3999
Again, the slow entry point performs an argument satisfaction check
4000
and either builds a PAP or pops the arguments off the stack into
4001
\Arg{2} \ldots \Arg{m+1} and jumps to the fast entry point.
4004
\item \emph{Unknown function closure, thunk or constructor.}
4006
Sometimes, the function being called is not statically identifiable.
4007
Consider, for example, the @compose@ function:
4009
compose f g x = f (g x)
4011
Since @f@ and @g@ are passed as arguments to @compose@, the latter has
4012
to make a heap call. In a heap call the arguments are pushed onto the
4013
stack, and the closure bound to the function is entered. In the
4014
example, a thunk for @(g x)@ will be allocated, (a pointer to it)
4015
pushed on the stack, and the closure bound to @f@ will be
4016
entered. That is, we will jump to @f@s entry point passing @f@ in
4017
\Arg{1}. If \Arg{1} is passed on the stack, it is pushed on top of
4018
the thunk for @(g x)@.
4020
The \emph{entry code} for an updateable thunk (which must have arity 0)
4021
pushes an update frame on the stack and starts executing the body of
4022
the closure --- using \Arg{1} to access any free variables. This is
4023
described in more detail in \secref{data-updates}.
4025
The \emph{entry code} for a non-updateable closure is just the
4026
closure's slow entry point.
4030
In addition to the above considerations, if there are \emph{too many}
4031
arguments then the extra arguments are simply pushed on the stack with
4034
To summarise, a closure's standard (slow) entry point performs the
4038
\item[Argument satisfaction check.] (function closure only)
4039
\item[Stack overflow check.]
4040
\item[Heap overflow check.]
4041
\item[Copy free variables out of closure.] %Todo: why?
4042
\item[Eager black holing.] (updateable thunk only) %Todo: forward ref.
4043
\item[Push update frame.]
4044
\item[Evaluate body of closure.]
4048
\Subsection{Case expressions and return conventions}{return-conventions}
4050
The \emph{evaluation} of a thunk is always initiated by
4051
a @case@ expression. For example:
4053
case x of (a,b) -> E
4056
The code for a @case@ expression looks like this:
4059
\item Push the free variables of the branches on the stack (fv(@E@) in
4061
\item Push a \emph{return address} on the stack.
4062
\item Evaluate the scrutinee (@x@ in this case).
4065
Once evaluation of the scrutinee is complete, execution resumes at the
4066
return address, which points to the code for the expression @E@.
4068
When execution resumes at the return point, there must be some {\em
4069
return convention} that defines where the components of the pair, @a@
4070
and @b@, can be found. The return convention varies according to the
4071
type of the scrutinee @x@:
4077
(A space for) the return address is left on the top of the stack.
4078
Leaving the return address on the stack ensures that the top of the
4079
stack contains a valid activation record
4080
(\secref{activation-records}) --- should a garbage
4081
collection be required.
4083
\item If @x@ has a boxed type (e.g.~a data constructor or a function),
4084
a pointer to @x@ is returned in \Arg{1}.
4086
\ToDo{Warn that components of E should be extracted as soon as
4087
possible to avoid a space leak.}
4089
\item If @x@ is an unboxed type (e.g.~@Int#@ or @Float#@), @x@ is
4092
\item If @x@ is an unboxed tuple constructor, the components of @x@
4093
are returned in \Arg{1} \ldots \Arg{n} but no object is constructed in
4096
When passing an unboxed tuple to a function, the components are
4097
flattened out and passed in \Arg{1} \ldots \Arg{n} as usual.
4101
\Subsection{Vectored Returns}{vectored-returns}
4103
Many algebraic data types have more than one constructor. For
4104
example, the @Maybe@ type is defined like this:
4106
data Maybe a = Nothing | Just a
4108
How does the return convention encode which of the two constructors is
4109
being returned? A @case@ expression scrutinising a value of @Maybe@
4110
type would look like this:
4116
Rather than pushing a return address before evaluating the scrutinee,
4117
@E@, the @case@ expression pushes (a pointer to) a \emph{return
4118
vector}, a static table consisting of two code pointers: one for the
4119
@Just@ alternative, and one for the @Nothing@ alternative.
4125
The constructor @Nothing@ returns by jumping to the first item in the
4126
return vector with a pointer to a (statically built) Nothing closure
4129
It might seem that we could avoid loading \Arg{1} in this case since the
4130
first item in the return vector will know that @Nothing@ was returned
4131
(and can easily access the Nothing closure in the (unlikely) event
4132
that it needs it. The only reason we load \Arg{1} is in case we have to
4133
perform an update (\secref{data-updates}).
4137
The constructor @Just@ returns by jumping to the second element of the
4138
return vector with a pointer to the closure in \Arg{1}.
4142
In this way no test need be made to see which constructor returns;
4143
instead, execution resumes immediately in the appropriate branch of
4146
\Subsection{Direct Returns}{direct-returns}
4148
When a datatype has a large number of constructors, it may be
4149
inappropriate to use vectored returns. The vector tables may be
4150
large and sparse, and it may be better to identify the constructor
4151
using a test-and-branch sequence on the tag. For this reason, we
4152
provide an alternative return convention, called a \emph{direct
4155
In a direct return, the return address pushed on the stack really is a
4156
code pointer. The returning code loads a pointer to the closure being
4157
returned in \Arg{1} as usual, and also loads the tag into \Arg{2}.
4158
The code at the return address will test the tag and jump to the
4159
appropriate code for the case branch. If \Arg{2} isn't mapped to a
4160
real machine register on this architecture, then we don't load it on a
4161
return, instead using the tag directly from the info table.
4163
The choice of whether to use a vectored return or a direct return is
4164
made on a type-by-type basis --- up to a certain maximum number of
4165
constructors imposed by the update mechanism
4166
(\secref{data-updates}).
4168
Single-constructor data types also use direct returns, although in
4169
that case there is no need to return a tag in \Arg{2}.
4171
\ToDo{for a nullary constructor we needn't return a pointer to the
4172
constructor in \Arg{1}.}
4174
\Subsection{Updates}{data-updates}
4176
The entry code for an updatable thunk (which must be of arity 0):
4179
\item copies the free variables out of the thunk into registers or
4181
\item pushes an \emph{update frame} onto the stack.
4183
An update frame is a small activation record consisting of
4185
\begin{tabular}{|l|l|l|}
4187
\emph{Fixed header} & \emph{Update Frame link} & \emph{Updatee} \\
4192
\note{In the semantics part of the STG paper (section 5.6), an update
4193
frame consists of everything down to the last update frame on the
4194
stack. This would make sense too --- and would fit in nicely with
4195
what we're going to do when we add support for speculative
4197
\ToDo{I think update frames contain cost centres sometimes}
4199
\item If we are doing ``eager blackholing,'' we then overwrite the
4200
thunk with a black hole (\secref{BLACKHOLE}). Otherwise, we leave it
4201
to the garbage collector to black hole the thunk.
4204
Start evaluating the body of the expression.
4208
When the expression finishes evaluation, it will enter the update
4209
frame on the top of the stack. Since the returner doesn't know
4210
whether it is entering a normal return address/vector or an update
4211
frame, we follow exactly the same conventions as return addresses and
4212
return vectors. That is, on entering the update frame:
4215
\item The value of the thunk is in \Arg{1}. (Recall that only thunks
4216
are updateable and that thunks return just one value.)
4218
\item If the data type is a direct-return type rather than a
4219
vectored-return type, then the tag is in \Arg{2}.
4221
\item The update frame is still on the stack.
4224
We can safely share a single statically-compiled update function
4225
between all types. However, the code must be able to handle both
4226
vectored and direct-return datatypes. This is done by arranging that
4227
the update code looks like this:
4235
|---------------| <- update code pointer
4240
Each entry in the return vector (which is large enough to cover the
4241
largest vectored-return type) points to the update code.
4245
\item overwrites the \emph{updatee} with an indirection to \Arg{1};
4246
\item loads @Su@ from the Update Frame link;
4247
\item removes the update frame from the stack; and
4248
\item enters \Arg{1}.
4251
We enter \Arg{1} again, having probably just come from there, because
4252
it knows whether to perform a direct or vectored return. This could
4253
be optimised by compiling special update code for each slot in the
4254
return vector, which performs the correct return.
4256
\Subsection{Semi-tagging}{semi-tagging}
4258
When a @case@ expression evaluates a variable that might be bound
4259
to a thunk it is often the case that the scrutinee is already evaluated.
4260
In this case we have paid the penalty of (a) pushing the return address (or
4261
return vector address) on the stack, (b) jumping through the info pointer
4262
of the scrutinee, and (c) returning by an indirect jump through the
4263
return address on the stack.
4265
If we knew that the scrutinee was already evaluated we could generate
4266
(better) code which simply jumps to the appropriate branch of the
4267
@case@ with a pointer to the scrutinee in \Arg{1}. (For direct
4268
returns to multiconstructor datatypes, we might also load the tag into
4271
An obvious idea, therefore, is to test dynamically whether the heap
4272
closure is a value (using the tag in the info table). If not, we
4273
enter the closure as usual; if so, we jump straight to the appropriate
4274
alternative. Here, for example, is pseudo-code for the expression
4275
@(case x of { (a,_,c) -> E }@:
4277
\Arg{1} = <pointer to x>;
4278
tag = \Arg{1}->entry->tag;
4280
Sp--; \\ insert space for return address
4284
goto \Arg{1}->entry;
4286
<info table for return address goes here>
4287
ret: a = \Arg{1}->data1; \\ suck out a and c to avoid space leak
4291
and here is the code for the expression @(case x of { [] -> E1; x:xs -> E2 }@:
4293
\Arg{1} = <pointer to x>;
4294
tag = \Arg{1}->entry->tag;
4296
Sp--; \\ insert space for return address
4300
goto \Arg{1}->entry;
4304
retvec: \\ reversed return vector
4305
<return info table for case goes here>
4307
panic("Direct return into vectored case");
4311
ret2: x = \Arg{1}->head;
4315
There is an obvious cost in compiled code size (but none in the size
4316
of the bytecodes). There is also a cost in execution time if we enter
4317
more thunks than data constructors.
4319
Both the direct and vectored returns are easily modified to chase chains
4320
of indirections too. In the vectored case, this is most easily done by
4321
making sure that @IND = TAG_1 - 1@, and adding an extra field to every
4322
return vector. In the above example, the indirection code would be
4324
ind: \Arg{1} = \Arg{1}->next;
4327
where @ind_loop@ is the second line of code.
4329
Note that we have to leave space for a return address since the return
4330
address expects to find one. If the body of the expression requires a
4331
heap check, we will actually have to write the return address before
4332
entering the garbage collector.
4335
\Subsection{Heap and Stack Checks}{heap-and-stack-checks}
4337
The storage manager detects that it needs to garbage collect the old
4338
generation when the evaluator requests a garbage collection without
4339
having moved the heap pointer since the last garbage collection. It
4340
is therefore important that the GC routines \emph{not} move the heap
4341
pointer unless the heap check fails. This is different from what
4342
happens in the current STG implementation.
4344
Assuming that the stack can never shrink, we perform a stack check
4345
when we enter a closure but not when we return to a return
4346
continuation. This doesn't work for heap checks because we cannot
4347
predict what will happen to the heap if we call a function.
4349
If we wish to allow the stack to shrink, we need to perform a stack
4350
check whenever we enter a return continuation. Most of these checks
4351
could be eliminated if the storage manager guaranteed that a stack
4352
would always have 1000 words (say) of space after it was shrunk. Then
4353
we can omit stack checks for less than 1000 words in return
4356
When an argument satisfaction check fails, we need to push the closure
4357
(in R1) onto the stack - so we need to perform a stack check. The
4358
problem is that the argument satisfaction check occurs \emph{before}
4359
the stack check. The solution is that the caller of a slow entry
4360
point or closure will guarantee that there is at least one word free
4361
on the stack for the callee to use.
4363
Similarily, if a heap or stack check fails, we need to push the arguments
4364
and closure onto the stack. If we just came from the slow entry point,
4365
there's certainly enough space and it is the responsibility of anyone
4366
using the fast entry point to guarantee that there is enough space.
4368
\ToDo{Be more precise about how much space is required - document it
4369
in the calling convention section.}
4371
\Subsection{Handling interrupts/signals}{signals}
4374
May have to keep C stack pointer in register to placate OS?
4375
May have to revert black holes - ouch!
4380
\section{The Loader}
4381
\section{The Compilers}
4384
\part{Old stuff - needs to be mined for useful info}
4386
\section{The Scheduler}
4388
The Scheduler is the heart of the run-time system. A running program
4389
consists of a single running thread, and a list of runnable and
4390
blocked threads. The running thread returns to the scheduler when any
4391
of the following conditions arises:
4394
\item A heap check fails, and a garbage collection is required
4395
\item Compiled code needs to switch to interpreted code, and vice
4397
\item The thread becomes blocked.
4398
\item The thread is preempted.
4401
A running system has a global state, consisting of
4404
\item @Hp@, the current heap pointer, which points to the next
4405
available address in the Heap.
4406
\item @HpLim@, the heap limit pointer, which points to the end of the
4408
\item The Thread Preemption Flag, which is set whenever the currently
4409
running thread should be preempted at the next opportunity.
4410
\item A list of runnable threads.
4411
\item A list of blocked threads.
4414
Each thread is represented by a Thread State Object (TSO), which is
4415
described in detail in \secref{TSO}.
4417
The following is pseudo-code for the inner loop of the scheduler
4421
while (threads_exist) {
4422
// handle global problems: GC, parallelism, etc
4424
if (external_message) service_message();
4425
// deal with other urgent stuff
4427
pick a runnable thread;
4429
// enter object on top of stack
4430
// if the top object is a BCO, we must enter it
4431
// otherwise appply any heuristic we wish.
4432
if (thread->stack[thread->sp]->info.type == BCO) {
4433
status = runHugs(thread,&smInfo);
4435
status = runGHC(thread,&smInfo);
4437
switch (status) { // handle local problems
4438
case (StackOverflow): enlargeStack; break;
4439
case (Error e) : error(thread,e); break;
4440
case (ExitWith e) : exit(e); break;
4441
case (Yield) : break;
4443
} while (thread_runnable);
4447
\Subsection{Invoking the garbage collector}{ghc-invoking-gc}
4449
\Subsection{Putting the thread to sleep}{ghc-thread-sleeps}
4451
\Subsection{Calling C from Haskell}{ghc-ccall}
4453
We distinguish between "safe calls" where the programmer guarantees
4454
that the C function will not call a Haskell function or, in a
4455
multithreaded system, block for a long period of time and "unsafe
4456
calls" where the programmer cannot make that guarantee.
4458
Safe calls are performed without returning to the scheduler and are
4459
discussed elsewhere (\ToDo{discuss elsewhere}).
4461
Unsafe calls are performed by returning an array (outside the Haskell
4462
heap) of arguments and a C function pointer to the scheduler. The
4463
scheduler allocates a new thread from the operating system
4464
(multithreaded system only), spawns a call to the function and
4465
continues executing another thread. When the ccall completes, the
4466
thread informs the scheduler and the scheduler adds the thread to the
4467
runnable threads list.
4469
\ToDo{Describe this in more detail.}
4472
\Subsection{Calling Haskell from C}{ghc-c-calls-haskell}
4474
When C calls a Haskell closure, it sends a message to the scheduler
4475
thread. On receiving the message, the scheduler creates a new Haskell
4476
thread, pushes the arguments to the C function onto the thread's stack
4477
(with tags for unboxed arguments) pushes the Haskell closure and adds
4478
the thread to the runnable list so that it can be entered in the
4481
When the closure returns, the scheduler sends back a message which
4482
awakens the (C) thread.
4484
\ToDo{Do we need to worry about the garbage collector deallocating the
4485
thread if it gets blocked?}
4487
\Subsection{Switching Worlds}{switching-worlds}
4489
\ToDo{This has all changed: we always leave a closure on top of the
4490
stack if we mean to continue executing it. The scheduler examines the
4491
top of the stack and tries to guess which world we want to be in. If
4492
it finds a @BCO@, it certainly enters Hugs, if it finds a @GHC@
4493
closure, it certainly enters GHC and if it finds a standard closure,
4494
it is free to choose either one but it's probably best to enter GHC
4495
for everything except @BCO@s and perhaps @AP@s.}
4497
Because this is a combined compiled/interpreted system, the
4498
interpreter will sometimes encounter compiled code, and vice-versa.
4500
All world-switches go via the scheduler, ensuring that the world is in
4501
a known state ready to enter either compiled code or the interpreter.
4502
When a thread is run from the scheduler, the @whatNext@ field in the
4503
TSO (\secref{TSO}) is checked to find out how to execute the
4507
\item If @whatNext@ is set to @ReturnGHC@, we load up the required
4508
registers from the TSO and jump to the address at the top of the user
4510
\item If @whatNext@ is set to @EnterGHC@, we load up the required
4511
registers from the TSO and enter the closure pointed to by the top
4513
\item If @whatNext@ is set to @EnterHugs@, we enter the top thing on
4514
the stack, using the interpreter.
4517
There are four cases we need to consider:
4520
\item A GHC thread enters a Hugs-built closure.
4521
\item A GHC thread returns to a Hugs-compiled return address.
4522
\item A Hugs thread enters a GHC-built closure.
4523
\item A Hugs thread returns to a Hugs-compiled return address.
4526
GHC-compiled modules cannot call functions in a Hugs-compiled module
4527
directly, because the compiler has no information about arities in the
4528
external module. Therefore it must assume any top-level objects are
4529
CAFs, and enter their closures.
4531
\ToDo{Hugs-built constructors?}
4533
We now examine the various cases one by one and describe how the
4534
switch happens in each situation.
4536
\subsection{A GHC thread enters a Hugs-built closure}
4537
\label{sec:ghc-to-hugs-switch}
4539
There is three possibilities: GHC has entered a @PAP@, or it has
4540
entered a @AP@, or it has entered the BCO directly (for a top-level
4541
function closure). @AP@s and @PAP@s are ``standard closures'' and
4542
so do not require us to enter the bytecode interpreter.
4544
The entry code for a BCO does the following:
4547
\item Push the address of the object entered on the stack.
4548
\item Save the current state of the thread in its TSO.
4549
\item Return to the scheduler, setting @whatNext@ to @EnterHugs@.
4552
BCO's for thunks and functions have the same entry conventions as
4553
slow entry points: they expect to find their arguments on the stac
4554
with unboxed arguments preceded by appropriate tags.
4556
\subsection{A GHC thread returns to a Hugs-compiled return address}
4557
\label{sec:ghc-to-hugs-switch}
4559
Hugs return addresses are laid out as in \figref{hugs-return-stack}.
4560
If GHC is returning, it will return to the address at the top of the
4561
stack, namely @HUGS_RET@. The code at @HUGS_RET@ performs the
4565
\item pushes \Arg{1} (the return value) on the stack.
4566
\item saves the thread state in the TSO
4567
\item returns to the scheduler with @whatNext@ set to @EnterHugs@.
4570
\noindent When Hugs runs, it will enter the return value, which will
4571
return using the correct Hugs convention
4572
(\secref{hugs-return-convention}) to the return address underneath it
4575
\subsection{A Hugs thread enters a GHC-compiled closure}
4576
\label{sec:hugs-to-ghc-switch}
4578
Hugs can recognise a GHC-built closure as not being one of the
4579
following types of object:
4585
\item An indirection, or
4586
\item A constructor.
4589
When Hugs is called on to enter a GHC closure, it executes the
4590
following sequence of instructions:
4593
\item Push the address of the closure on the stack.
4594
\item Save the current state of the thread in the TSO.
4595
\item Return to the scheduler, with the @whatNext@ field set to
4599
\subsection{A Hugs thread returns to a GHC-compiled return address}
4600
\label{sec:hugs-to-ghc-switch}
4602
When Hugs encounters a return address on the stack that is not
4603
@HUGS_RET@, it knows that a world-switch is required. At this point
4604
the stack contains a pointer to the return value, followed by the GHC
4605
return address. The following sequence is then performed:
4608
\item save the state of the thread in the TSO.
4609
\item return to the scheduler, setting @whatNext@ to @EnterGHC@.
4612
The first thing that GHC will do is enter the object on the top of the
4613
stack, which is a pointer to the return value. This value will then
4614
return itself to the return address using the GHC return convention.
4622
We're nuking the following:
4629
Return in registers.
4630
This lets us remove update code pointers from info tables,
4631
removes the need for phantom info tables, simplifies
4636
Careful analysis suggests that it doesn't buy us very much
4637
and it is hard to work with.
4639
Eliminating threaded GCs eliminates the desire to share SMReps
4640
so they are (once more) part of the Info table.
4644
Doesn't buy us anything on a register-poor architecture and
4645
isn't so important if we have semi-tagging.
4648
- Probably bad on register poor architecture
4649
- Can avoid need to write return address to stack on reg rich arch.
4650
- when a function does a small amount of work, doesn't
4651
enter any other thunks and then returns.
4652
eg entering a known constructor (but semitagging will catch this)
4653
- Adds complications
4659
This lets us drop CONST closures and CHARLIKE closures (assuming we
4660
don't support Unicode). The only point of these closures was to
4661
avoid updating with an indirection.
4663
We also drop @MIN_UPD_SIZE@ --- all we need is space to insert an
4664
indirection or a black hole.
4667
STATIC SMReps are now called CONST
4672
\item The profiling ``kind'' field is now encoded in the @INFO_TYPE@ field.
4673
This identifies the general sort of the closure for profiling purposes.
4675
\item Various papers describe deleting update frames for unreachable objects.
4676
This has never been implemented and we don't plan to anytime soon.
added
removed
docs/rts/rts.tex
