[llvm-bugs] [Bug 45701] New: Failure to optimize out module before rotate

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=45701

            Bug ID: 45701
           Summary: Failure to optimize out module before rotate
           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

uint64_t ror64(uint64_t x, uint32_t y)
{
    y %= 64;
    if (y == 0)
        return x;
    return (x >> y) | (x << (64 - y));
}

GCC optimizes this to this :

ror64(unsigned long, unsigned int):
  mov rax, rdi
  mov ecx, esi
  ror rax, cl
  ret

LLVM does this :

ror64(unsigned long, unsigned int):
  mov ecx, esi
  mov rax, rdi
  ror rax, cl
  test cl, 63
  cmove rax, rdi
  ret

The test is redundant, since x86-64 `ror` will have the exact same behaviour
that the `%= 64` and the check emulate

(see also https://godbolt.org/z/zcYUbK)

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