[Bug target/99591] New: Improving __builtin_add_overflow performance on x86-64

[eggert at gnu dot org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99591

            Bug ID: 99591
           Summary: Improving __builtin_add_overflow performance on x86-64
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eggert at gnu dot org
  Target Milestone: ---

This is with gcc (GCC) 10.2.1 20201125 (Red Hat 10.2.1-9) on x86-64. For the
function:

_Bool signed1_overflow (signed char a, signed char b)
{
  signed char r;
  return __builtin_add_overflow (a, b, &r);
}

gcc generates the code:

signed1_overflow:
        movsbl  %sil, %esi
        movsbl  %dil, %edi
        addb    %sil, %dil
        seto    %al
        ret

The movsbl instructions are unnecessary and can be omitted.


For the function:

_Bool signed2_overflow (short a, short b)
{
  short r;
  return __builtin_add_overflow (a, b, &r);
}

gcc generates:

signed2_overflow:
        movswl  %di, %edi
        movswl  %si, %esi
        xorl    %eax, %eax
        addw    %si, %di
        jo      .L8
.L6:
        andl    $1, %eax
        ret
.L8:
        movl    $1, %eax
        jmp     .L6

Better would be this:

signed2_overflow:
        addw    %si, %di
        seto    %al
        retq

There are similar opportunities for improvement in __builtin_sub_overflow and
__builtin_mul_overflow.

This bug report follows up on this discussion about Gnulib:

https://lists.gnu.org/r/bug-gnulib/2021-03/msg00078.html
https://lists.gnu.org/r/bug-gnulib/2021-03/msg00079.html
https://lists.gnu.org/r/bug-gnulib/2021-03/msg00080.html