[all-commits] [llvm/llvm-project] 16a062: [ValueTracking] Compute known FPClass from signbit...

[Yingwei Zheng via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 16a0629e7c16cc1ec1a5066c57be3044a1e00395
      https://github.com/llvm/llvm-project/commit/16a0629e7c16cc1ec1a5066c57be3044a1e00395
  Author: Yingwei Zheng <dtcxzyw2333 at gmail.com>
  Date:   2024-02-14 (Wed, 14 Feb 2024)

  Changed paths:
    M llvm/lib/Analysis/DomConditionCache.cpp
    M llvm/lib/Analysis/ValueTracking.cpp
    M llvm/test/Transforms/InstCombine/fpclass-from-dom-cond.ll

  Log Message:
  -----------
  [ValueTracking] Compute known FPClass from signbit idiom (#80740)

This patch improves `computeKnownFPClass` by using context-sensitive
information from `DomConditionCache`.
The motivation of this patch is to optimize the following case found in
[fmt/format.h](https://github.com/fmtlib/fmt/blob/e17bc67547a66cdd378ca6a90c56b865d30d6168/include/fmt/format.h#L3555-L3566):
```
define float @test(float %x, i1 %cond) {
  %i32 = bitcast float %x to i32
  %cmp = icmp slt i32 %i32, 0
  br i1 %cmp, label %if.then1, label %if.else

if.then1:
  %fneg = fneg float %x
  br label %if.end

if.else:
  br i1 %cond, label %if.then2, label %if.end

if.then2:
  br label %if.end

if.end:
  %value = phi float [ %fneg, %if.then1 ], [ %x, %if.then2 ], [ %x, %if.else ]
  %ret = call float @llvm.fabs.f32(float %value)
  ret float %ret
}
```
We can prove the sign bit of %value is always zero. Then the fabs can be
eliminated.

This pattern also exists in cpython/duckdb/oiio/openexr.

Compile-time impact:
https://llvm-compile-time-tracker.com/compare.php?from=f82e0809ba12170e2f648f8a1ac01e78ef06c958&to=041218bf5491996edd828cc15b3aec5a59ddc636&stat=instructions:u


|stage1-O3|stage1-ReleaseThinLTO|stage1-ReleaseLTO-g|stage1-O0-g|stage2-O3|stage2-O0-g|stage2-clang|
|--|--|--|--|--|--|--|
|-0.00%|+0.01%|+0.00%|-0.03%|+0.00%|+0.00%|+0.02%|