[LLVMdev] Attaching range metadata to IntrinsicInst
Nick Lewycky
nlewycky at google.com
Tue Jun 17 14:46:26 PDT 2014
On 17 June 2014 14:09, Jingyue Wu <jingyue at google.com> wrote:
> Hi Nick,
>
> That makes sense. I think a main issue here is that the ranges of these
> PTX special registers (e.g., threadIdx.x) depend on -target-cpu which is
> only visible to clang and llc. Would you mind we specify "target cpu" in
> the IR similar to what we did for "target triple"?
>
Aha, that's the salient point. I'd like to see llvm using what it knows
about intrinsics statically. Something like "popcount" is a great example.
Needing to know more than the data in the intrinsic, needing to know about
what subarch is being targeted is different. I didn't realize we had such
intrinsics.
If the ranges really can't be deduced from the intrinsics as written -- and
that's enough to make me wonder whether these intrinsics are properly
designed but I won't dart down that rabbit hole now ---- if the ranges
really can't be deduced from the intrinsics alone then you should fall back
to using range metadata as you initially suggested. Sorry for running in a
circle on the design.
As an alternative, I asked Eric in person and his suggestion was to query
TargetTransformInfo for information about the intrinsic. That's also
plausible, it depends on whether you feel like the authoritative
information should be coming from the frontend or from the backend. I could
see this going either way.
Nick
> On Tue, Jun 17, 2014 at 12:19 PM, Nick Lewycky <nlewycky at google.com>
> wrote:
>
>> On 17 June 2014 06:41, Eli Bendersky <eliben at google.com> wrote:
>>
>>> On Tue, Jun 17, 2014 at 1:38 AM, Nick Lewycky <nicholas at mxc.ca> wrote:
>>>
>>>> Chandler Carruth wrote:
>>>>
>>>>> This seems fine to me, but I'd like to make sure it looks OK to Nick as
>>>>> well.
>>>>>
>>>>
>>>> I strongly prefer baking in knowledge about the intrinsics themselves
>>>> into the passes if possible. Metadata will always be secondary.
>>>>
>>>
>>> So you're saying that in this particular case you'd prefer LLVM passes
>>> to know about the range of these PTX intrinsics, rather than Clang adding
>>> them as metadata?
>>>
>>
>> Yep.
>>
>> ValueTracking.cpp already has some iffy target knowledge (someone
>>> sneaked a direct Intrinsic::x86_sse42_crc32_64_64 check in there), but
>>> extending it to other intrinsics in other targets seems like too much...
>>>
>>
>> That's not iffy. That's exactly how it should work, and we should have
>> more of that. There is a major gotcha and that's dealing with the case
>> where the intrinsics don't exist because the backend wasn't compiled in. If
>> x86_sse42_crc32_64_64 is in there (and also in instcombine btw), presumably
>> that problem is solved somehow? Or does llvm actually not build if you
>> don't enable the x86 target? I feel like we would've heard about that.
>>
>> Nick
>>
>> So should target info be passed into it in some way? Any suggestions
>>> where to put it? TargetLibraryInfo? TargetTransformInfo? In any case this
>>> seems like the target interface will have to be augmented, and we'll have
>>> to carry an object around into ValueTracking's compute* functions. If this
>>> is the right way, then this is the way it will be done - design ideas are
>>> appreciated.
>>>
>>> Eli
>>>
>>>
>>>
>>>
>>>>
>>>> Separately, should value tracking look use range metadata when it's
>>>> available? Absolutely.
>>>>
>>>> I think it should apply to all CallInst not just IntrinsicInst (which
>>>> is derived from CallInst).
>>>>
>>>> Nick
>>>>
>>>> On Tue, Jun 17, 2014 at 12:37 AM, Jingyue Wu <jingyue at google.com
>>>>> <mailto:jingyue at google.com>> wrote:
>>>>>
>>>>> Hi,
>>>>>
>>>>> The range metadata can only be attached to LoadInst for now. I am
>>>>> considering extending its usage to IntrinsicInst so that the
>>>>> frontend can annotate the range of the return value of an intrinsic
>>>>> call. e.g.,
>>>>> %a = call i32 @llvm.xxx(), !range !0
>>>>> !0 = metadata !{ i32 0, i23 1024 }
>>>>>
>>>>> The motivation behind this extension is some optimizations we are
>>>>> working on for CUDA programs. Some special registers in CUDA (e.g.,
>>>>> threadIdx.x) are bounded per CUDA programming guide, and knowing
>>>>> their ranges can improve the precision of ValueTracking and benefit
>>>>> optimizations such as InstCombine.
>>>>>
>>>>> To implement this idea, we need ValueTracking to be aware of the
>>>>> ranges of these special variables. These special registers are so
>>>>> far read-only and accessed using intrinsics. e.g.,
>>>>> %threadIdx.x = call i32 @llvm.nvvm.read.ptx.sreg.tid.x().
>>>>>
>>>>> One possible approach is to have ValueTracking compute the known
>>>>> bits of these intrinsics as special cases. This approach is already
>>>>> taken for the x86_sse42_crc32_64_64 intrinsic. However, this
>>>>> approach may not be elegant because the ranges of these CUDA
>>>>> special
>>>>> registers depend on the GPU compute capability specified by
>>>>> -target-cpu. For instance, blockIdx.x is bounded by 65535 in sm_20
>>>>> but 2^31-1 in sm_30. Exposing -target-cpu to ValueTracking is
>>>>> probably discouraged.
>>>>>
>>>>> Therefore, the approach I am considering is to have clang annotate
>>>>> the ranges of these CUDA special registers according to the
>>>>> -target-cpu flag, and have ValueTracking pick the range metadata
>>>>> for
>>>>> optimization. By doing so, we hide the target-specific info from
>>>>> ValueTracking.
>>>>>
>>>>> The code change in llvm minus clang won't be large. The core change
>>>>> is only a few lines:
>>>>> http://reviews.llvm.org/differential/diff/10464/. If this
>>>>> extension
>>>>> sounds good to you, I'll definitely add more tests and revise the
>>>>> documents on range metadata.
>>>>>
>>>>> Best,
>>>>> Jingyue
>>>>>
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>>>>>
>>>>>
>>>>>
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>>>>
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