[llvm] r223143 - [Statepoints 4/4] Statepoint infrastructure for garbage collection: Documentation
Philip Reames
listmail at philipreames.com
Tue Feb 24 14:44:30 PST 2015
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 <mailto: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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