[Bug c++/82361] New: Useless "mov eax, eax" in generated code

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82361

            Bug ID: 82361
           Summary: Useless "mov eax, eax" in generated code
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

Following function

struct ret_t {
    unsigned quot, rem;
};

ret_t my_div(unsigned a, unsigned b) {
    return {a / b, a % b};
}


Produces suboptimal assembly:

my_div(unsigned int, unsigned int):
  mov eax, edi
  xor edx, edx
  div esi
  sal rdx, 32
  mov eax, eax  # <======
  or rax, rdx
  ret

Clang produces an assembly without "mov eax, eax" instead:

my_div(unsigned int, unsigned int): # @my_div(unsigned int, unsigned int)
  xor edx, edx
  mov eax, edi
  div esi
  shl rdx, 32
  or rax, rdx
  ret