Lint pass should warn when fast math flags are used with noundef call return attribute

[arsenm]

This IR exhibits undefined behavior:

; RUN: opt -passes=lint %s

define float @noundef_with_fastmath_flags(float %x) {
  %result = call nnan noundef float @llvm.sqrt.f32(float %x)
  ret float %result
}

The sqrt will introduce poison if the input is negative or a nan due to the nnan flag. The lint utility pass should detect this anomaly and print a warning.

Probably need to introduce a cast to FPMathOperator, and check for poison generating flags around here

[llvmbot]

Hi!

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[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Matt Arsenault (arsenm)

Details

This IR exhibits undefined behavior: ``` ; RUN: opt -passes=lint %s

define float @noundef_with_fastmath_flags(float %x) {
%result = call nnan noundef float @llvm.sqrt.f32(float %x)
ret float %result
}

The sqrt will introduce poison if the input is negative or a nan due to the `nnan` flag. The lint utility pass should detect this anomaly and print a warning.

Probably need to introduce a cast to FPMathOperator, and check for poison generating flags around [here](https://github.com/llvm/llvm-project/blob/47fc1dd90ed4fa7c806795fada77e8c7cdfd6760/llvm/lib/Analysis/Lint.cpp#L214)
</details>

[guiolidc]

Hi! I would like to work on this issue.

[nikic]

I don't really understand why we want to lint this. noundef nnan encodes that NaN results in UB, which typically matches frontend semantics (frontend tend to specify in terms of IUB, not poison).