[Bug c++/113982] New: Poor codegen for 64-bit add with carry widening functions

["janschultke at googlemail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113982

            Bug ID: 113982
           Summary: Poor codegen for 64-bit add with carry widening
                    functions
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: janschultke at googlemail dot com
  Target Milestone: ---

I was trying to get optimal codegen for a 64-bit addition with a carry, but
it's tough to do with GCC:

> struct add_result {
>     unsigned long long sum;
>     bool carry;
> };
> 
> add_result add_wide_1(unsigned long long x, unsigned long long y) {
>     auto r = (unsigned __int128) x + y;
>     return add_result{static_cast<unsigned long long>(r), bool(r >> 64)};
> }
> 
> add_result add_wide_2(unsigned long long x, unsigned long long y) {
>     unsigned long long r;
>     bool carry = __builtin_add_overflow(x, y, &r);
>     return add_result{r, carry};
> }


## Expected output (clang -march=x86-64-v4 -O3)

add_wide_1(unsigned long long, unsigned long long):
        mov     rax, rdi
        add     rax, rsi
        setb    dl
        ret
add_wide_2(unsigned long long, unsigned long long):
        mov     rax, rdi
        add     rax, rsi
        setb    dl
        ret

## Actual output (GCC -march=x86-64-v4 -O3) (https://godbolt.org/z/qGc9WeEvK)

add_wide_1(unsigned long long, unsigned long long):
        mov     rcx, rdi
        lea     rax, [rdi+rsi]
        xor     edx, edx
        xor     edi, edi
        add     rsi, rcx
        adc     rdi, 0
        mov     dl, dil
        and     dl, 1
        ret
add_wide_2(unsigned long long, unsigned long long):
        add     rdi, rsi
        mov     edx, 0
        mov     rax, rdi
        setc    dl
        ret


The output for the 128-bit version looks pretty bad.
It looks like GCC isn't aware that we only access the carry bit, so it doesn't
need to do full 128-bit arithmetic so to speak.

The add_wide_2 output also isn't optimal. Why would it output "mov edx, 0"
instead of "xor edx, edx"?