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From: "m.cencora at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/105260] Union with user-defined empty destructor leads to
 worse code-gen
Date: Wed, 13 Apr 2022 12:19:31 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105260

--- Comment #5 from m.cencora at gmail dot com ---
I've slighlty refactored the code, to remove the auto variables. This issue
remains

#include <new>

inline unsigned deserializeUInt(const unsigned char* &in)
{
    unsigned out;
    __builtin_memcpy(&out, in, sizeof(out));
    in +=3D sizeof(out);
    out =3D __builtin_bswap32(out);
    return out;
}

struct Foo
{
    unsigned a;
    unsigned b;

    static Foo deserialize(const unsigned char* &in)
    {
        return Foo{
            deserializeUInt(in),
            deserializeUInt(in)
        };
    }
};

struct Result
{
    unsigned idx;
    union
    {
        unsigned a;
        const void* ptr;
    };
};

Result dummyFunc(Foo);

void deserializeAndInvoke(const unsigned char* it)
{
#ifndef WORKAROUND
    union NoDestroy
    {
        ~NoDestroy() {}
        Foo value;
    };

    NoDestroy un{=20
        Foo::deserialize(it)
    };
    dummyFunc(un.value);

#elif WORKAROUND =3D=3D 1
    union NoDestroy
    {
        Foo value;
    };
    NoDestroy un{=20
        Foo::deserialize(it)
    };
    dummyFunc(un.value);

#elif WORKAROUND =3D=3D 2=20=20
    alignas(Foo) char rawStorage[sizeof(Foo)];

    Foo* foo =3D new (&rawStorage[0]) Foo{
        Foo::deserialize(it)
    };
    dummyFunc(*foo);
#endif

}=