Missed optimization: failure to use div-by-zero UB to delete redundant select

[mcy]

Given this function:

define i64 @incorrect_safe_div(i64 %n, i64 %d) {
  %1 = icmp eq i64 %d, 0
  %2 = udiv i64 %n, %d
  %3 = select i1 %1, i64 -1, i64 %2
  ret i64 %3
}

LLVM does not seem to notice that this select is unconditional, because %2 should imply that %d != 0 in the entire block, and therefore %1 can be replaced with false. Alive2 confirms this optimization is valid: https://alive2.llvm.org/ce/z/7nLDRe.

[nikic]

This is probably just a matter of adding udiv support to isKnownNonNullFromDominatingCondition().

[dc03]

@luxufan Are you still working on this? I have a patch almost ready to fix this.

[dc03]

Candidate PR: #67282