[llvm-bugs] [Bug 45759] New: Failure to elide useless mov in sign function

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=45759

            Bug ID: 45759
           Summary: Failure to elide useless mov in sign function
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: gabravier at gmail.com
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, spatel+llvm at rotateright.com

int sign(int x)
{
    return -((unsigned)x >> 31) | ((unsigned)-x >> 31);
}

With -O3, GCC outputs this :

sign(int):
  mov eax, edi
  sar edi, 31
  neg eax
  shr eax, 31
  or eax, edi
  ret

LLVM outputs this :

sign(int):
  mov eax, edi
  mov ecx, edi
  sar ecx, 31
  neg eax
  shr eax, 31
  or eax, ecx
  ret

Most likely this is specific to the X86 backend and its register allocator, so
I marked it as such

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