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From: "chfast at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/106786] New: Regression in cmp+sbb
Date: Wed, 31 Aug 2022 11:16:48 +0000
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X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 12.2.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106786

            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 =3D __builtin_ia32_sbb_u64(carry, x, y, &d);
    return {d, carryout !=3D 0};
}

bool bad(u64 x, u64 y) {
    const R z =3D subc(x, y, false);
    R a =3D 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=