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

Sean Silva chisophugis at gmail.com
Tue Feb 24 16:56:35 PST 2015


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.
>> +
>>
>>
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