[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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