[clang] unnecessary conditions marked with [[likely]] or [[unlikely]] are not removed by the optimizer

[jonathanpoelen]

https://godbolt.org/z/jEM7nrKhs

With the following code

struct A {
    int type;
};

struct B : A {
    int value;
};

struct C : A {
    int value;
    int value2;
};

int foo(A *p) {
    if (p->type == 0) [[likely]] return static_cast<B*>(p)->value;
    return static_cast<C*>(p)->value;
}

int bar(A *p) {
    if (p->type == 0) [[unlikely]] return static_cast<B*>(p)->value;
    return static_cast<C*>(p)->value;
}

int zeta(A *p) {
    if (p->type == 0) return static_cast<B*>(p)->value;
    return static_cast<C*>(p)->value;
}

The offset of B::value and C::value are identical, so the 2 branches should give the same code. This is the case with gcc and the zeta function:

mov     eax, DWORD PTR [rdi+4]
ret

On the other hand, the 2 branches using [[(un)likely]] retain the cmp instruction with clang:

cmp     dword ptr [rdi], 0
mov     eax, dword ptr [rdi + 4]
ret

Testes with clang-15, clang-16 and clang-trunk.

[nikic]

I believe #68882 will fix this.

[nikic]

Just checked, and it does fix it.