[llvm-bugs] [Bug 47305] New: Failure to optimize comparison of variable with same variable involved in bitwise or using andnot

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=47305

            Bug ID: 47305
           Summary: Failure to optimize comparison of variable with same
                    variable involved in bitwise or using andnot
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: gabravier at gmail.com
                CC: llvm-bugs at lists.llvm.org

bool f(int a, int b)
{
    return a == (a | b);
}

With -mbmi, GCC outputs this:

f(int, int):
  andn eax, edi, esi
  sete al
  ret

LLVM outputs this:

f(int, int):
  or esi, edi
  cmp esi, edi
  sete al
  ret

I have no idea whether there is an LLVM instruction that fits along with this
pattern (if not, I guess you could make a new one or else just change this to
an x86-specific issue) but this transformation seems useful.

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