[llvm] r223143 - [Statepoints 4/4] Statepoint infrastructure for garbage collection: Documentation

Sean Silva chisophugis at gmail.com
Wed Feb 25 14:14:41 PST 2015


On Tue, Feb 24, 2015 at 5:31 PM, Philip Reames <listmail at philipreames.com>
wrote:

>  On the queue for tomorrow.
>
> Other things which need to happen:
> - Move intrinsic definitions into LangRef
>

Please don't do this until they are non-experimental (or have they become
non-experimental?). Actually, I don't think this would be the first time
we've promoted instrinsics to non-experimental, and I'm not sure how to
handle it best (what to move and what to copy, etc.).


> - Flesh out a description of the "statepoint-example" GC.
> - Document the fact there's no a form of statepoint sequence without
> explicitly relocations, update code with asserts & flags respectively
>
> I'm considering just removing the Statepoints page entirely and merging
> the content into GarbageCollection.  I probably wont actually go ahead with
> that just yet.
>

If you do this, please leave the Statepoints.rst page empty with a link to
GarbageCollection.rst; that way, links to Statepoints across the net don't
break. The real solution is for the server to serve an http redirect, but
we don't have a way of indicating that currently.


>
> I also need a place to transcribe my private TODO list somewhere public.
> The docs probably aren't the right place for this though.
>
>
> On 02/24/2015 04:56 PM, Sean Silva wrote:
>
> There are a couple todo/"put assembly here" in the file currently. It
> would be nice to flesh those out.
>
> On Tue, Feb 24, 2015 at 4:24 PM, Philip Reames <listmail at philipreames.com>
> wrote:
>
>>  Fixed.  Other comments welcome.
>>
>>
>> On 02/24/2015 02:44 PM, Philip Reames wrote:
>>
>> Your timing is good.  I'm working on docs today and should get to this by
>> end of day.  :)
>>
>> Philip
>>
>> On 02/24/2015 02:37 PM, Sean Silva wrote:
>>
>> Necro-nit (wasn't sure where to post this feedback; I realize that this
>> has been slightly updated in ToT): please update the prototypes here to
>> match their current definitions (e.g. `llvm.experimental.` prefix).
>>
>>  (sorry for the delay in getting to this)
>>
>>  -- Sean Silva
>>
>> On Tue, Dec 2, 2014 at 11:37 AM, Philip Reames <listmail at philipreames.com
>> > wrote:
>>
>>> Author: reames
>>> Date: Tue Dec  2 13:37:00 2014
>>> New Revision: 223143
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=223143&view=rev
>>> Log:
>>> [Statepoints 4/4] Statepoint infrastructure for garbage collection:
>>> Documentation
>>>
>>> This is the fourth and final patch in the statepoint series.  It
>>> contains the documentation for the statepoint intrinsics and their usage.
>>>
>>> There's definitely still room to improve the documentation here, but I
>>> wanted to get this landed so it was available for others.  There will
>>> likely be a series of small cleanup changes over the next few weeks as we
>>> work to clarify and revise the documentation.  If you have comments or
>>> questions, please feel free to discuss them either in this commit thread,
>>> the original review thread, or on llvmdev.  Comments are more than welcome.
>>>
>>> Reviewed by: atrick, ributzka
>>> Differential Revision: http://reviews.llvm.org/D5683
>>>
>>>
>>>
>>> Added:
>>>     llvm/trunk/docs/Statepoints.rst
>>>
>>> Added: llvm/trunk/docs/Statepoints.rst
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/Statepoints.rst?rev=223143&view=auto
>>>
>>> ==============================================================================
>>> --- llvm/trunk/docs/Statepoints.rst (added)
>>> +++ llvm/trunk/docs/Statepoints.rst Tue Dec  2 13:37:00 2014
>>> @@ -0,0 +1,209 @@
>>> +=====================================
>>> +Garbage Collection Safepoints in LLVM
>>> +=====================================
>>> +
>>> +.. contents::
>>> +   :local:
>>> +   :depth: 2
>>> +
>>> +Status
>>> +=======
>>> +
>>> +This document describes a set of experimental extensions to LLVM. Use
>>> with caution.  Because the intrinsics have experimental status,
>>> compatibility across LLVM releases is not guaranteed.
>>> +
>>> +LLVM currently supports an alternate mechanism for conservative garbage
>>> collection support using the gc_root intrinsic.  The mechanism described
>>> here shares little in common with the alternate implementation and it is
>>> hoped that this mechanism will eventually replace the gc_root mechanism.
>>> +
>>> +Overview
>>> +========
>>> +
>>> +To collect dead objects, garbage collectors must be able to identify
>>> any references to objects contained within executing code, and, depending
>>> on the collector, potentially update them.  The collector does not need
>>> this information at all points in code - that would make the problem much
>>> harder - but only at well defined points in the execution known as
>>> 'safepoints'  For a most collectors, it is sufficient to track at least one
>>> copy of each unique pointer value.  However, for a collector which wishes
>>> to relocate objects directly reachable from running code, a higher standard
>>> is required.
>>> +
>>> +One additional challenge is that the compiler may compute intermediate
>>> results ("derived pointers") which point outside of the allocation or even
>>> into the middle of another allocation.  The eventual use of this
>>> intermediate value must yield an address within the bounds of the
>>> allocation, but such "exterior derived pointers" may be visible to the
>>> collector.  Given this, a garbage collector can not safely rely on the
>>> runtime value of an address to indicate the object it is associated with.
>>> If the garbage collector wishes to move any object, the compiler must
>>> provide a mapping for each pointer to an indication of its allocation.
>>> +
>>> +To simplify the interaction between a collector and the compiled code,
>>> most garbage collectors are organized in terms of two three abstractions:
>>> load barriers, store barriers, and safepoints.
>>> +
>>> +#. A load barrier is a bit of code executed immediately after the
>>> machine load instruction, but before any use of the value loaded.
>>> Depending on the collector, such a barrier may be needed for all loads,
>>> merely loads of a particular type (in the original source language), or
>>> none at all.
>>> +#. Analogously, a store barrier is a code fragement that runs
>>> immediately before the machine store instruction, but after the computation
>>> of the value stored.  The most common use of a store barrier is to update a
>>> 'card table' in a generational garbage collector.
>>> +
>>> +#. A safepoint is a location at which pointers visible to the compiled
>>> code (i.e. currently in registers or on the stack) are allowed to change.
>>> After the safepoint completes, the actual pointer value may differ, but the
>>> 'object' (as seen by the source language) pointed to will not.
>>> +
>>> +  Note that the term 'safepoint' is somewhat overloaded.  It refers to
>>> both the location at which the machine state is parsable and the
>>> coordination protocol involved in bring application threads to a point at
>>> which the collector can safely use that information.  The term "statepoint"
>>> as used in this document refers exclusively to the former.
>>> +
>>> +This document focuses on the last item - compiler support for
>>> safepoints in generated code.  We will assume that an outside mechanism has
>>> decided where to place safepoints.  From our perspective, all safepoints
>>> will be function calls.  To support relocation of objects directly
>>> reachable from values in compiled code, the collector must be able to:
>>> +
>>> +#. identify every copy of a pointer (including copies introduced by the
>>> compiler itself) at the safepoint,
>>> +#. identify which object each pointer relates to, and
>>> +#. potentially update each of those copies.
>>> +
>>> +This document describes the mechanism by which an LLVM based compiler
>>> can provide this information to a language runtime/collector and ensure
>>> that all pointers can be read and updated if desired.  The heart of the
>>> approach is to construct (or rewrite) the IR in a manner where the possible
>>> updates performed by the garbage collector are explicitly visible in the
>>> IR.  Doing so requires that we:
>>> +
>>> +#. create a new SSA value for each potentially relocated pointer, and
>>> ensure that no uses of the original (non relocated) value is reachable
>>> after the safepoint,
>>> +#. specify the relocation in a way which is opaque to the compiler to
>>> ensure that the optimizer can not introduce new uses of an unrelocated
>>> value after a statepoint. This prevents the optimizer from performing
>>> unsound optimizations.
>>> +#. recording a mapping of live pointers (and the allocation they're
>>> associated with) for each statepoint.
>>> +
>>> +At the most abstract level, inserting a safepoint can be thought of as
>>> replacing a call instruction with a call to a multiple return value
>>> function which both calls the original target of the call, returns it's
>>> result, and returns updated values for any live pointers to garbage
>>> collected objects.
>>> +
>>> +  Note that the task of identifying all live pointers to garbage
>>> collected values, transforming the IR to expose a pointer giving the base
>>> object for every such live pointer, and inserting all the intrinsics
>>> correctly is explicitly out of scope for this document.  The recommended
>>> approach is described in the section of Late Safepoint Placement below.
>>> +
>>> +This abstract function call is concretely represented by a sequence of
>>> intrinsic calls known as a 'statepoint sequence'.
>>> +
>>> +
>>> +Let's consider a simple call in LLVM IR:
>>> +  todo
>>> +
>>> +Depending on our language we may need to allow a safepoint during the
>>> execution of the function called from this site.  If so, we need to let the
>>> collector update local values in the current frame.
>>> +
>>> +Let's say we need to relocate SSA values 'a', 'b', and 'c' at this
>>> safepoint.  To represent this, we would generate the statepoint sequence::
>>> +  put an example sequence here
>>> +
>>> +Ideally, this sequence would have been represented as a M argument, N
>>> return value function (where M is the number of values being relocated +
>>> the original call arguments and N is the original return value + each
>>> relocated value), but LLVM does not easily support such a representation.
>>> +
>>> +Instead, the statepoint intrinsic marks the actual site of the
>>> safepoint or statepoint.  The statepoint returns a token value (which
>>> exists only at compile time).  To get back the original return value of the
>>> call, we use the 'gc_result' intrinsic.  To get the relocation of each
>>> pointer in turn, we use the 'gc_relocate' intrinsic with the appropriate
>>> index.  Note that both the gc_relocate and gc_result are tied to the
>>> statepoint.  The combination forms a "statepoint sequence" and represents
>>> the entitety of a parseable call or 'statepoint'.
>>> +
>>> +When lowered, this example would generate the following x86 assembly::
>>> +  put assembly here
>>> +
>>> +Each of the potentially relocated values has been spilled to the stack,
>>> and a record of that location has been recorded to the StackMap section.
>>> If the garbage collector needs to update any of these pointers during the
>>> call, it knows exactly what to change.
>>> +
>>> +Intrinsics
>>> +===========
>>> +
>>> +'''gc_statepoint''' Intrinsic
>>> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>> +
>>> +Syntax:
>>> +"""""""
>>> +
>>> +::
>>> +
>>> +      declare i32
>>> +        @gc_statepoint(func_type <target>, i64 <#call args>.
>>> +                       i64 <unused>, ... (call parameters),
>>> +                       i64 <# deopt args>, ... (deopt parameters),
>>> +                       ... (gc parameters))
>>> +
>>> +Overview:
>>> +"""""""""
>>> +
>>> +The statepoint intrinsic represents a call which is parse-able by the
>>> runtime.
>>> +
>>> +Operands:
>>> +"""""""""
>>> +
>>> +The 'target' operand is the function actually being called.  The target
>>> can be specified as either a symbolic LLVM funciton, or as an arbitrary
>>> Value of appropriate function type.  Note that the function type must match
>>> the signature of the callee and the types of the 'call parameters'
>>> arguments.
>>> +
>>> +The '#call args' operand is the number of arguments to the actual
>>> call.  It must exactly match the number of arguments passed in the 'call
>>> parameters' variable length section.
>>> +
>>> +The 'unused' operand is unused and likely to be removed.  Please do not
>>> use.
>>> +
>>> +The 'call parameters' arguments are simply the arguments which need to
>>> be passed to the call target.  They will be lowered according to the
>>> specified calling convention and otherwise handled like a normal call
>>> instruction.  The number of arguments must exactly match what is specified
>>> in '# call args'.  The types must match the signature of 'target'.
>>> +
>>> +The 'deopt parameters' arguments contain an arbitrary list of Values
>>> which is meaningful to the runtime.  The runtime may read any of these
>>> values, but is assumed not to modify them.  If the garbage collector might
>>> need to modify one of these values, it must also be listed in the 'gc
>>> pointer' argument list.  The '# deopt args' field indicates how many
>>> operands are to be interpreted as 'deopt parameters'.
>>> +
>>> +The 'gc parameters' arguments contain every pointer to a garbage
>>> collector object which potentially needs to be updated by the garbage
>>> collector.  Note that the argument list must explicitly contain a base
>>> pointer for every derived pointer listed.  The order of arguments is
>>> unimportant.  Unlike the other variable length parameter sets, this list is
>>> not length prefixed.
>>> +
>>> +Semantics:
>>> +""""""""""
>>> +
>>> +A statepoint is assumed to read and write all memory.  As a result,
>>> memory operations can not be reordered past a statepoint.  It is illegal to
>>> mark a statepoint as being either 'readonly' or 'readnone'.
>>> +
>>> +Note that legal IR can not perform any memory operation on a 'gc
>>> pointer' argument of the statepoint in a location statically reachable from
>>> the statepoint.  Instead, the explicitly relocated value (from a
>>> ''gc_relocate'') must be used.
>>> +
>>> +'''gc_result''' Intrinsic
>>> +^^^^^^^^^^^^^^^^^^^^^^^^^^
>>> +
>>> +Syntax:
>>> +"""""""
>>> +
>>> +::
>>> +
>>> +      declare type*
>>> +        @gc_result_ptr(i32 %statepoint_token)
>>> +
>>> +      declare fX
>>> +        @gc_result_float(i32 %statepoint_token)
>>> +
>>> +      declare iX
>>> +        @gc_result_int(i32 %statepoint_token)
>>> +
>>> +Overview:
>>> +"""""""""
>>> +
>>> +'''gc_result''' extracts the result of the original call instruction
>>> which was replaced by the '''gc_statepoint'''.  The '''gc_result'''
>>> intrinsic is actually a family of three intrinsics due to an implementation
>>> limitation.  Other than the type of the return value, the semantics are the
>>> same.
>>> +
>>> +Operands:
>>> +"""""""""
>>> +
>>> +The first and only argument is the '''gc.statepoint''' which starts the
>>> safepoint sequence of which this '''gc_result'' is a part.  Despite the
>>> typing of this as a generic i32, *only* the value defined by a
>>> '''gc.statepoint''' is legal here.
>>> +
>>> +Semantics:
>>> +""""""""""
>>> +
>>> +The ''gc_result'' represents the return value of the call target of the
>>> ''statepoint''.  The type of the ''gc_result'' must exactly match the type
>>> of the target.  If the call target returns void, there will be no
>>> ''gc_result''.
>>> +
>>> +A ''gc_result'' is modeled as a 'readnone' pure function.  It has no
>>> side effects since it is just a projection of the return value of the
>>> previous call represented by the ''gc_statepoint''.
>>> +
>>> +'''gc_relocate''' Intrinsic
>>> +^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>> +
>>> +Syntax:
>>> +"""""""
>>> +
>>> +::
>>> +
>>> +      declare <type> addrspace(1)*
>>> +        @gc_relocate(i32 %token, i32 %base_offset, i32 %pointer_offset)
>>> +
>>> +Overview:
>>> +"""""""""
>>> +
>>> +A ''gc_relocate'' returns the potentially relocated value of a pointer
>>> at the safepoint.
>>> +
>>> +Operands:
>>> +"""""""""
>>> +
>>> +The first argument is the '''gc.statepoint''' which starts the
>>> safepoint sequence of which this '''gc_relocation'' is a part.  Despite the
>>> typing of this as a generic i32, *only* the value defined by a
>>> '''gc.statepoint''' is legal here.
>>> +
>>> +The second argument is an index into the statepoints list of arguments
>>> which specifies the base pointer for the pointer being relocated.  This
>>> index must land within the 'gc parameter' section of the statepoint's
>>> argument list.
>>> +
>>> +The third argument is an index into the statepoint's list of arguments
>>> which specify the (potentially) derived pointer being relocated.  It is
>>> legal for this index to be the same as the second argument if-and-only-if a
>>> base pointer is being relocated. This index must land within the 'gc
>>> parameter' section of the statepoint's argument list.
>>> +
>>> +Semantics:
>>> +""""""""""
>>> +The return value of ''gc_relocate'' is the potentially relocated value
>>> of the pointer specified by it's arguments.  It is unspecified how the
>>> value of the returned pointer relates to the argument to the
>>> ''gc_statepoint'' other than that a) it points to the same source language
>>> object with the same offset, and b) the 'based-on' relationship of the
>>> newly relocated pointers is a projection of the unrelocated pointers.  In
>>> particular, the integer value of the pointer returned is unspecified.
>>> +
>>> +A ''gc_relocate'' is modeled as a 'readnone' pure function.  It has no
>>> side effects since it is just a way to extract information about work done
>>> during the actual call modeled by the ''gc_statepoint''.
>>> +
>>> +
>>> +StackMap Format
>>> +================
>>> +
>>> +Locations for each pointer value which may need read and/or updated by
>>> the runtime or collector are provided via the StackMap format specified in
>>> the PatchPoint documentation.
>>> +
>>> +.. TODO: link
>>> +
>>> +Each statepoint generates the following Locations:
>>> +
>>> +* Constant which describes number of following deopt *Locations* (not
>>> operands)
>>> +* Variable number of Locations, one for each deopt parameter listed in
>>> the IR statepoint (same number as described by previous Constant)
>>> +* Variable number of Locations pairs, one pair for each unique pointer
>>> which needs relocated.  The first Location in each pair describes the base
>>> pointer for the object.  The second is the derived pointer actually being
>>> relocated.  It is guaranteed that the base pointer must also appear
>>> explicitly as a relocation pair if used after the statepoint. There may be
>>> fewer pairs then gc parameters in the IR statepoint. Each *unique* pair
>>> will occur at least once; duplicates are possible.
>>> +
>>> +Note that the Locations used in each section may describe the same
>>> physical location.  e.g. A stack slot may appear as a deopt location, a gc
>>> base pointer, and a gc derived pointer.
>>> +
>>> +The ID field of the 'StkMapRecord' for a statepoint is meaningless and
>>> it's value is explicitly unspecified.
>>> +
>>> +The LiveOut section of the StkMapRecord will be empty for a statepoint
>>> record.
>>> +
>>> +Safepoint Semantics & Verification
>>> +==================================
>>> +
>>> +The fundamental correctness property for the compiled code's
>>> correctness w.r.t. the garbage collector is a dynamic one.  It must be the
>>> case that there is no dynamic trace such that a operation involving a
>>> potentially relocated pointer is observably-after a safepoint which could
>>> relocate it.  'observably-after' is this usage means that an outside
>>> observer could observe this sequence of events in a way which precludes the
>>> operation being performed before the safepoint.
>>> +
>>> +To understand why this 'observable-after' property is required,
>>> consider a null comparison performed on the original copy of a relocated
>>> pointer.  Assuming that control flow follows the safepoint, there is no way
>>> to observe externally whether the null comparison is performed before or
>>> after the safepoint.  (Remember, the original Value is unmodified by the
>>> safepoint.)  The compiler is free to make either scheduling choice.
>>> +
>>> +The actual correctness property implemented is slightly stronger than
>>> this.  We require that there be no *static path* on which a potentially
>>> relocated pointer is 'observably-after' it may have been relocated.  This
>>> is slightly stronger than is strictly necessary (and thus may disallow some
>>> otherwise valid programs), but greatly simplifies reasoning about
>>> correctness of the compiled code.
>>> +
>>> +By construction, this property will be upheld by the optimizer if
>>> correctly established in the source IR.  This is a key invariant of the
>>> design.
>>> +
>>> +The existing IR Verifier pass has been extended to check most of the
>>> local restrictions on the intrinsics mentioned in their respective
>>> documentation.  The current implementation in LLVM does not check the key
>>> relocation invariant, but this is ongoing work on developing such a
>>> verifier.  Please ask on llvmdev if you're interested in experimenting with
>>> the current version.
>>> +
>>>
>>>
>>> _______________________________________________
>>> llvm-commits mailing list
>>> llvm-commits at cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>>
>>
>>
>>
>>
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>>
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>
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