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From: "coillol at yandex dot ru" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/109039] New: Miscompilation with no_unique_address and
 bitfields
Date: Mon, 06 Mar 2023 10:46:30 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 12.1.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: coillol at yandex dot ru
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109039

            Bug ID: 109039
           Summary: Miscompilation with no_unique_address and bitfields
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: coillol at yandex dot ru
  Target Milestone: ---

g++ miscompiles the following code starting from version 12.1 (-std=3Dc++20=
, any
optimization mode).


```
#include <cassert>

struct X {
    short x0 : 7;
    short x1 : 8;

    X() { init(); assert(get() =3D=3D 3); }

    void init() { x0 =3D 1; x1 =3D 2; }

    int get() { return x0 + x1; }
};

struct S {
    [[no_unique_address]] X x;
    char c =3D 0;
};

int main() {
    S s;
    assert(s.x.get() =3D=3D 3);
    return 0;
}
```

Program hits assertion in main when executed. Links to godbolt:
https://godbolt.org/z/xG186673P (12.1), https://godbolt.org/z/MaP56rfhe
(trunk).=