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

[Philip Reames]

On 07/09/2015 04:36 PM, Andrew Trick wrote:
>
>> On Jul 9, 2015, at 3:33 PM, Swaroop Sridhar 
>> <Swaroop.Sridhar at microsoft.com 
>> <mailto:Swaroop.Sridhar at microsoft.com>> wrote:
>>
>> Regarding Call-site size specification:
>>
>> CoreCLR (https://github.com/dotnet/coreclr) requires the size of the 
>> Call-instruction to be reported in the GCInfo encoding.
>>
>> The runtime performs querries for StackMap records using instruction 
>> offsets as follows:
>>
>> 1)Offset at the end of the call instruction (offset of next 
>> instruction-1) if the call instruction occurs in code where GC can 
>> only take control at safe-points.
>>
>
> As part of this change it would be great if LLVM could now guarantee 
> that the call will be emitted at the end of the patchable space. It 
> currently happens to emit at the beginning, but makes no guarantee. 
> Emitting at the end works better for tracking the return address.
This sounds reasonable.  I can't see an immediate reason why this would 
restrict future implementation flexibility.
>
>> 2)Offset of the start of the call instruction if the call instruction 
>> occurs within a code-range that allows full interruption (that is, 
>> all instructions in a range are considered safe-points)
>>
>
> If the JIT requires knowledge of call's encoding, it should probably 
> be emitting the call instruction itself within the patchable space 
> reserved by LLVM.
>
> Note that the patchpoint may include a mov in addition to call, so the 
> patchpoint address is not the same as the call address.
Just to be clear, the problem with (2) has nothing to do with 
StackMaps.  It has to do with the fact our entire backend confuses 
pointers and non-pointers.  To have option (2), I believe we'd need to 
effectively have a stackmap for every instruction.  That's a lot more 
work than anyone has serious proposed to date.  :)
>
>> LLVM/statepoint GC does not support option (2), but the CoreCLR’s 
>> GC-table Encoding has an interface designed to suite both modes.
>>
>> void DefineCallSites(UINT32* pCallSites, BYTE* pCallSiteSizes, 
>> UINT32numCallSites)
>>
>> Therefore, it is helpful to have Call-Site size specified in 
>> StackMapRecord.
>>
>> I agree with Andy, that the call-site size should include all bytes 
>> between the start of the call instruction and the start of the next 
>> instruction.
>>
>
> I suggested this because we want to support a dynamic callback that 
> determines the patchpoint size (given the set of register arguments), 
> and it would be nice to record that decision within the stack map. 
> This is important information for any code that is responsible for 
> patching because it must patch over all reserved bytes.
>
> The alternative would be for LLVM to emit the call at the end and 
> record just the size of the call instruction encoding. That seems like 
> a silly, x86-specific waste of stackmap space though. The JIT can 
> either do the encoding and keep track of the info, or a small 
> nop+move_immediate decoder can figure it out for all reasonable cases.
I would argue in favor of recording the number of bytes recorded for 
patching.  I think that gives us everything we need in practice. Any 
non-destructive or concurrent patch mechanism is going to need extremely 
fine grained control anyways.  Trying to do that partly within LLVM and 
partly without just seems like a mistake.
>
> Andy
>
>> Thanks,
>>
>> Swaroop.
>>
>> *From:*Juergen Ributzka [mailto:juergen at apple.com]
>> *Sent:* Thursday, July 9, 2015 2:04 PM
>> *To:* LLVM Dev
>> *Cc:* Lang Hames; Andrew Trick; Phil Pizlo; Philip Reames; Sanjoy 
>> Das; Swaroop Sridhar; Russell Hadley
>> *Subject:* [RFC] New StackMap format proposal (StackMap v2)
>>
>> 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.
>>
>> 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?
>>
>> 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. 
>> 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.
>>
>> 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
>>
>> 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)
>>
>> }
>>
>> 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
>>     bool : HasVariableSizeAlloca
>>     bool : HasStackRealignment
>>     bool : HasLiveOutInfo
>>     bool : Reserved [12]
>>   }
>>   uint16 : Frame Base Register Dwarf RegNum
>>   uint16 : Num Frame Registers
>>   uint16 : Frame Register Index
>>   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]
>>   }
>> }
>>
>> align to 8 bytes
>> StackMapRecord[] {
>>   uint64 : PatchPoint ID
>>   uint32 : Instruction Offset
>>   uint8  : Call size (bytes)
>>   uint8  : Flags {
>>     bool : HasLiveOutInfo
>>     bool : Reserved [7]
>>   }
>>   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
>> }
>>
>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150714/42c68a13/attachment.html>