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diff --git a/docs/Statepoints.rst b/docs/Statepoints.rst new file mode 100644 index 000000000000..53643b1c6d31 --- /dev/null +++ b/docs/Statepoints.rst @@ -0,0 +1,411 @@ +===================================== +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 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 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: + + todo + +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 +:ref:`Stack Map section <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 function, 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 %statepoint_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''. + + +Stack Map Format +================ + +Locations for each pointer value which may need read and/or updated by +the runtime or collector are provided via the :ref:`Stack Map format +<stackmap-format>` specified in the PatchPoint documentation. + +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. + +Bugs and Enhancements +===================== + +Currently known bugs and enhancements under consideration can be +tracked by performing a `bugzilla search +<http://llvm.org/bugs/buglist.cgi?cmdtype=runnamed&namedcmd=Statepoint%20Bugs&list_id=64342>`_ +for [Statepoint] in the summary field. When filing new bugs, please +use this tag so that interested parties see the newly filed bug. As +with most LLVM features, design discussions take place on `llvmdev +<http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>`_, and patches +should be sent to `llvm-commits +<http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits>`_ for review. + |