[llvm-bugs] [Bug 33879] New: [X86] Missed optimizations for comparison of shift against zero

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=33879

            Bug ID: 33879
           Summary: [X86] Missed optimizations for comparison of shift
                    against zero
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: llvm-dev at redking.me.uk
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    peter at cordes.ca, spatel+llvm at rotateright.com

int test(unsigned i, unsigned j, int a, int b) {
  if (0 == (i >> j))
    return a;
  return b;
}

define i32 @test(i32, i32, i32, i32) {
  %5 = lshr i32 %0, %1
  %6 = icmp eq i32 %5, 0
  %7 = select i1 %6, i32 %2, i32 %3
  ret i32 %7
}

llc -mcpu=btver2

test(unsigned int, unsigned int, int, int):
        movl    %ecx, %eax
        movl    %esi, %ecx
        shrl    %cl, %edi
        testl   %edi, %edi
        cmovnel %eax, %edx
        movl    %edx, %eax
        retq

1 - we could remove the final move if the CMOVNE was changed/commuted to CMOVE
2 - why aren't we using the ZF flag result from SHR instead of the extra TEST?

gcc gets (1) right, but still doesn't do (2)

test(unsigned int, unsigned int, int, int):
        movl    %ecx, %eax
        movl    %esi, %ecx
        shrl    %cl, %edi
        testl   %edi, %edi
        cmove   %edx, %eax
        ret

Also note, that more recently we use the BMI2 shifts instead which don't update
the flags at all.

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