| Issue |
179613
|
| Summary |
[InstCombine] Dropping `fabs` around `fmul (fabs nnan x)`, `+inf` changes `NaN` sign for `x=+0.0`
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
ariskeirio
|
**Description**
I observed a transformation involving `fabs` and `fmul` with `+inf` that cannot verify under Alive2 due to a **NaN payload mismatch**, even though both sides evaluate to NaN.https://alive2.llvm.org/ce/z/7yBXf4
**Input Program**
**Source**
```llvm
define float @src(float %x) {
%fabs.x = call nnan float @llvm.fabs.f32(float %x)
%mul.fabs.x = fmul float %fabs.x, 0x7FF0000000000000
%x.is.zero = fcmp oeq float %x, 0.000000e+00
%2 = call float @llvm.fabs.f32(float %mul.fabs.x)
%select = select i1 %x.is.zero, float %2, float %fabs.x
ret float %select
}
```
This is transformed to:
**Target**
```llvm
define float @tgt(float %x) {
%fabs.x = call nnan float @llvm.fabs.f32(float %x)
%mul.fabs.x = fmul float %fabs.x, 0x7FF0000000000000
%x.is.zero = fcmp oeq float %x, 0.000000e+00
%select = select i1 %x.is.zero, float %mul.fabs.x, float %fabs.x
ret float %select
}
```
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