[Bug target/96176] New: Failure to omit extraneous movzx in atomic compare exchange with unsigned char

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96176

            Bug ID: 96176
           Summary: Failure to omit extraneous movzx in atomic compare
                    exchange with unsigned char
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

void f(unsigned char *addr, unsigned char old_val, unsigned char new_val)
{
    __atomic_compare_exchange_n(addr, &old_val, new_val, 0, 0, 0);
}

On x86 with -O3, LLVM generates this :

f(unsigned char*, unsigned char, unsigned char): # @f(unsigned char*, unsigned
char, unsigned char)
  mov eax, esi
  lock cmpxchg byte ptr [rdi], dl
  ret

GCC generates this :

f(unsigned char*, unsigned char, unsigned char):
  mov eax, esi
  movzx edx, dl
  lock cmpxchg BYTE PTR [rdi], dl
  ret