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From: "gcc at jonathanmueller dot dev" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/109096] New: __has_unique_object_representations does not
 account for unnamed bitfield
Date: Fri, 10 Mar 2023 21:52:53 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 13.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109096

            Bug ID: 109096
           Summary: __has_unique_object_representations does not account
                    for unnamed bitfield
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc at jonathanmueller dot dev
  Target Milestone: ---

__has_unique_object_representations should return false for types that cont=
ain
an unnamed bit field of non-zero bit-width, since that introduces padding b=
its;
yet it doesn't:

```
#include <cassert>
#include <cstring>

struct foo
{
    int member : 8;
    // We explitly introduce 24 padding bits here.
    int : 24;

    bool operator=3D=3D(const foo&) const =3D default;
};

// Yet this assertion passes?!
static_assert(__has_unique_object_representations(foo));

// But we clearly don't have unique object representations:
int main()
{
    // Create two objects with different object representations.
    foo a, b;
    std::memset(&a, 0xFF, sizeof(foo));
    std::memset(&b, 0x99, sizeof(foo));

    // Make all members identical to give them the same value representatio=
n.
    a.member =3D 0;
    b.member =3D 0;
    // This passes; they have the same value representation.
    assert(a =3D=3D b);

    // But this fails; they don't have unique object representations!
    assert(std::memcmp(&a, &b, sizeof(foo)) =3D=3D 0);
}
```

https://godbolt.org/z/Y4GzqYE8s=