[Bug tree-optimization/104376] New: Failure to optimize clz equivalent to clz

[gabravier at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104376

            Bug ID: 104376
           Summary: Failure to optimize clz equivalent to clz
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

#include <stdint.h>

uint32_t countLeadingZeros32(uint32_t x)
{
    if (x == 0)
        return 32;
    return (31 - __builtin_clz(x)) ^ 31;
}

On x86, with `-mlzcnt`, GCC outputs this:

countLeadingZeros32(unsigned int):
  mov eax, 32
  test edi, edi
  je .L1
  mov eax, 31
  lzcnt edi, edi
  sub eax, edi
  xor eax, 31
.L1:
  ret

LLVM instead outputs this:

countLeadingZeros32(unsigned int):
  lzcnt eax, edi
  ret