missed FP fold, (x >= 0) ? x : -x == 0 => x == 0

[zhengyang92]

https://alive2.llvm.org/ce/z/NFEKjp
https://godbolt.org/z/7Ef9Kf4s3

define i1 @src(float %0) {
entry:
%1 = fcmp oge float %0, 0.000000e+00
%2 = fneg float %0
%3 = select i1 %1, float %0, float %2
%4 = fcmp oeq float %3, 0.000000e+00
ret i1 %4
}

define i1 @tgt(float %0) {
entry:
%oeq = fcmp oeq float %0, 0.000000e+00
ret i1 %oeq
}

@jayfoad @regehr

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. In the comments of the issue, request for it to be assigned to you.
2. Fix the issue locally.
3. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
4. Create a Git commit.
5. Run git clang-format HEAD~1 to format your changes.
6. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

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

Author: Zhengyang Liu (zhengyang92)

https://alive2.llvm.org/ce/z/NFEKjp https://godbolt.org/z/7Ef9Kf4s3 ```llvm define i1 @src(float %0) { entry: %1 = fcmp oge float %0, 0.000000e+00 %2 = fneg float %0 %3 = select i1 %1, float %0, float %2 %4 = fcmp oeq float %3, 0.000000e+00 ret i1 %4 }

define i1 @tgt(float %0) {
entry:
%oeq = fcmp oeq float %0, 0.000000e+00
ret i1 %oeq
}

@<!-- -->jayfoad @<!-- -->regehr 


</details>

[SahilPatidar]

@dtcxzyw, I'd be happy to take a look at this issue.

[SahilPatidar]

@dtcxzyw, Does this whole expression need to be folded, or has some work already been done on it?

[jayfoad]

I think this generalizes to cond ? x : -x == 0 => x == 0 for any condition cond.

[regehr]

@jayfoad looks like it!

https://alive2.llvm.org/ce/z/VgXu2n