[InstSimplify] FP-related miscompile

[nikic]

From rust-lang/rust#102402:

define i1 @src(i64 %0) {
  %i2 = uitofp i64 %0 to float
  %i3 = fpext float %i2 to double
  %i4 = fmul double -0.000000e+00, %i3
  %i5 = fdiv double 1.000000e+00, %i4
  %i6 = fcmp oeq double %i5, 0xFFF0000000000000
  ret i1 %i6
}
; RUN: opt -S -instsimplify
define i1 @src(i64 %0) {
  ret i1 false
}

Counter-proof: https://alive2.llvm.org/ce/z/6Rx_Cb

[nikic]

I believe this is a bug in CannotBeOrderedLessThanZero. For fdiv, this probably needs to do a SignBitOnly check, because it can turn a negative zero into a negative non-zero.

[nikic]

Candidate patch: https://reviews.llvm.org/D134876

[nikic]

/cherry-pick ea32658 aa25c92

[llvmbot]

Failed to cherry-pick: aa25c92

https://github.com/llvm/llvm-project/actions/runs/3152450212

Please manually backport the fix and push it to your github fork. Once this is done, please add a comment like this:

/branch <user>/<repo>/<branch>