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From: "user202729 at protonmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug ipa/110057] Missed devirtualization opportunities
Date: Mon, 27 May 2024 09:29:27 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110057

user202729 <user202729 at protonmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |user202729 at protonmail d=
ot com

--- Comment #11 from user202729 <user202729 at protonmail dot com> ---
One way this could be implemented is the following.

* implement built-in `__builtin_vptr(object)` and `__builtin_vptr(Type)` th=
at
obtains the pointer to the vtable (or NULL if the object type has no virtual
method)
* in the implementation `vector.pop_back()` etc., add the the following:

    if (__builtin_vptr(this->_M_impl._M_finish) !=3D __builtin_vptr(_Tp))
      __builtin_unreachable();

* the optimizer should be able to figure out the rest.

What do you think about this approach?

--------

Alternatively, we can also use the existing `typeid()` --- although the
optimizer is not smart enough to optimize the following code yet.

```
#include<cstdio>
#include<typeinfo>
struct Base{
    Base() {}
    virtual void f(){ std::printf("Base\n"); }
};
struct Derived final: Base{
    Derived() {}
    virtual void f(){ std::printf("Derived\n"); }
};
void f(Base* b){
    if(!b or typeid(*b)!=3Dtypeid(Base)) __builtin_unreachable();
    b->f();
}
```=