[LLVMdev] [RFC] New StackMap format proposal (StackMap v2) (original) (raw)

Hal Finkel hfinkel at anl.gov
Tue Jul 14 23:15:50 PDT 2015

----- Original Message -----

From: "Philip Reames" <listmail at philipreames.com> To: "Juergen Ributzka" <juergen at apple.com>, "LLVM Dev" <llvmdev at cs.uiuc.edu> Cc: "Lang Hames" <lhames at apple.com> Sent: Wednesday, July 15, 2015 1:02:21 AM Subject: Re: [LLVMdev] [RFC] New StackMap format proposal (StackMap v2)

Comments inline. I tried to reply to particular points of discussion downthread, so this is probably best read last. On 07/09/2015 02:04 PM, Juergen Ributzka wrote: Hi @ll, over the past year we gained more experience with the patchpoint/stackmap/statepoint intrinsics and it exposed limitations in the stackmap format. The following proposal includes feedback and request from several interested parties and I would like to hear your feedback. Missing correlation between functions and stackmap records: Originally the client had to keep track of the ID to know which stackmap record belongs to which function, but this would stop working once functions are inlined. The new format fixes that by adding a direct reference from the function to the stackmap records. +1

Call Size and Function Size: These are additional information that are of interest and have been added to the format. @Swaroop: Could you please provide a little more detailed explanation on the "Call Size" field and what exactly is there recorded. Is it just the call instruction or also the materialization code for the address? For what is this used for? +1 subject to details being settled Flat format: We think moving to a flat form will make parsing easier, because every record has a fixed size and offsets can be calculated easily. I'm still not convinced this is actually a good idea, but I have no strong objection either. The plan is to also provide parsers for both stackmap versions (there is already one for the first format in tree) and a corresponding C-API to make it easier for clients to adopt the new format. There is no plan to drop the original format and we will continue to support both formats. I will ask for feedback on the C API in a separate RFC. I strongly feel that trying to support multiple versions of the file format in tree is a bad idea. It's a distraction, support burden, and provides extremely little value. What's the value in supporting a parser for a format that ToT will no longer be able to emit? Anyone who is actually consuming the old format would have to be using the old version in process anyways.

I agree. There seems to be no reason to support the old format. We did label these 'experimental' after all.

-Hal

(Possible exception: Is this intended to simplify the object caching model? If so, have we ever given any guarantees about object caching in the JIT across versions? That seems risky at best..) I'm also opposed to the complexity of C API, but I'll defer that until the separate RFC is actually posted.

Another benefit we hope to achieve from this format is to optimize for size by uniquing entries - but that is independent optimization and not required. More detailed frame record: Clients require more information about the function frame, such as spilled registers, etc. The frame base register i.e. might change when dynamic stack realignment is performed on X86. If there is anything missing please let me know. Thanks Cheers, Juergen Is the format Little endian, bit endian? native endian? Header v2 { uint8 : Stack Map Version (2) uint8 : Reserved [3] (0) uint32 : Constants Offset (bytes) uint32 : Frame Records Offset (bytes) uint32 : Frame Registers Offset (bytes) uint32 : StackMap Records Offset (bytes) uint32 : Locations Offset (bytes) uint32 : LiveOuts Offset (bytes) Let's reserve at least two uint32s for future field groups so that we can extend in a backwards compatible way. Another option would be to steal one of the reserved bytes and require parsers to ignore sections they don't understand. Actually, so long as we document that there might be unknown bytes between Header.end and Constants.begin we should be fine. It just needs to be clearly documented. As an example, if we need to add the SymbolConstants section mentioned down thread, that shouldn't require an version change. } align to 8 bytes Constants[] { uint64 : LargeConstant } align to 8 bytes FrameRecord[] { uint64 : Function Address uint32 : Function Size uint32 : Stack Size uint16 : Flags { bool : HasFrame What does this mean? bool : HasVariableSizeAlloca bool : HasStackRealignment bool : HasLiveOutInfo bool : Reserved [12] } uint16 : Frame Base Register Dwarf RegNum I don't think this is actually sufficient for dynamic stack realignment. Don't you end up with some things indexed off RSP and others indexed off RBP? Actually, wait.. that's entirely handled within the record format. What does tracking the Frame Base Register give us? I think I'm missing a use case here. uint16 : Num Frame Registers uint16 : Frame Register Index Are these (begin, begin+length) offsets into the Frame Registers table? If so, should we swap them? uint16 : Num StackMap Records uint16 : StackMap Record Index } align to 4 bytes FrameRegister[] { uint16 : Dwarf RegNum int16 : Offset uint8 : Size in Bytes uint8 : Flags { bool : IsSpilled bool : Reserved [7] } } This seems tied specifically to the assumption that registers are spilled on entry. Given the recent work towards shrink wrapping, is that something we want to do? p.s. What's with the IsSpilled bit? Why would you have a record for an unspilled register? align to 8 bytes StackMapRecord[] { uint64 : PatchPoint ID "ID" (remove the patchpoint since statepoint now has one too) uint32 : Instruction Offset uint8 : Call size (bytes) "Patchable size" per down thread discussion uint8 : Flags { bool : HasLiveOutInfo bool : Reserved [7] } Why do we need these flags? Isn't this Num LiveOuts == 0? uint16 : Num Locations uint16 : Location Index uint16 : Num LiveOuts uint16 : LiveOut Index } align to 4 bytes Location[] { uint8 : Register | Direct | Indirect | Constant | ConstantIndex uint8 : Reserved (location flags) uint16 : Dwarf RegNum int32 : Offset or SmallConstant } align to 2 bytes LiveOuts[] { uint16 : Dwarf RegNum uint8 : Reserved uint8 : Size in Bytes }

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-- Hal Finkel Assistant Computational Scientist Leadership Computing Facility Argonne National Laboratory

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