[Bug tree-optimization/106786] New: Regression in cmp+sbb

["chfast at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106786

            Bug ID: 106786
           Summary: Regression in cmp+sbb
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: chfast at gmail dot com
  Target Milestone: ---

I noticed a regression when using the builtin for sbb instruction
(__builtin_ia32_sbb_u64).

typedef unsigned long long u64;

struct R {
    u64 value;
    bool carry;
};

inline R subc(u64 x, u64 y, bool carry) noexcept {
    u64 d;
    const u64 carryout = __builtin_ia32_sbb_u64(carry, x, y, &d);
    return {d, carryout != 0};
}

bool bad(u64 x, u64 y) {
    const R z = subc(x, y, false);
    R a = subc(x, y, z.carry);
    return a.carry;
}

https://godbolt.org/z/f41KKe19q

The expected assembly is
        cmp     rdi, rsi
        sbb     rdi, rsi

But GCC 12.2.0 and trunk produces
        cmp     rdi, rsi
        setb    al
        movzx   eax, al
        add     al, -1
        sbb     rdi, rsi

The regression is in 12.2.0, the 11.3.0 optimizes properly.

There are simple changes which will bring back the expected optimization:
- change `const R z` to `R z`,
- change `bool carry` to `u64 carry`.

This may be related to calling convention / ABI because I noticed in one of the
tree optimization outputs for 12.2.0 that the `bool carry` is forced to be in
memory: `MEM <unsigned char> [(struct R *)&z + 8B]`.

https://godbolt.org/z/7zh7GxraK