[LLVMdev] Attaching range metadata to IntrinsicInst

[Jingyue Wu]

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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