[Bug middle-end/112600] New: Failed to optimize saturating addition using __builtin_add_overflow

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112600

            Bug ID: 112600
           Summary: Failed to optimize saturating addition using
                    __builtin_add_overflow
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

These two implementations of C++26 saturating addition (std::add_sat<unsigned>)
have equivalent behaviour:

unsigned
add_sat(unsigned x, unsigned y) noexcept
{
    unsigned z;
    if (!__builtin_add_overflow(x, y, &z))
            return z;
    return -1u;
}

unsigned
add_sat2(unsigned x, unsigned y) noexcept
{
    unsigned res;
    res = x + y;
    res |= -(res < x);
    return res;
}


For -O3 on x86_64 GCC uses a branch for the first one:

add_sat(unsigned int, unsigned int):
        add     edi, esi
        jc      .L3
        mov     eax, edi
        ret
.L3:
        or      eax, -1
        ret

For the second one we get better code:

add_sat2(unsigned int, unsigned int):
        add     edi, esi
        sbb     eax, eax
        or      eax, edi
        ret



Clang compiles them both to the same code:

add_sat(unsigned int, unsigned int):
        add     edi, esi
        mov     eax, -1
        cmovae  eax, edi
        ret