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From: "yongxiangng at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug ipa/110057] Missed devirtualization opportunities
Date: Thu, 01 Jun 2023 09:35:00 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110057

--- Comment #3 from Ng YongXiang <yongxiangng at gmail dot com> ---
I'm giving the example of an array for now, because gcc treatment of the
destructor is inconsistent and depends on the length of the array. Clang on=
 the
other hand is able to devirtualize the destructor in the array no matter the
length of the array.

Virtual call

https://godbolt.org/z/f33Gh5EGM

Devirtualized call

https://godbolt.org/z/jPz3Ka1cd

We know it is devirtualized because the destructor of the derived object is=
 not
called and the compiler assumes that the items in the array are all Base
objects.

So currently, gcc does perform this kind of "devirtualization" (I wouldn't
really call it a devirtualization for an array, because it is similar to
declaring Derived d; on the stack), but it is dependent on array length, an=
d I
think we should make it devirtualized for all length of array.

I changed my testing to dump tree optimized like below (let me know if ther=
e's
issues with the example because I am not familiar with hacking gcc).

/* { dg-do run } */
/* Virtual calls should be devirtualized because we know dynamic type of ob=
ject
in vector at compile time */
/* { dg-options "-O3 -fdump-tree-optimized -fno-inline"  } */

#include <vector>
#include <iostream>
#include <memory>

using std::vector;

class A
{
public:
    virtual ~A()
    {
    }
};

class B : public A
{
public:
    virtual ~B()
    {
    }
};

int main()
{
    B b[3];
    return 0;
}

/* { dg-final { scan-tree-dump-times "OBJ_TYPE_REF" 0 "optimized"} } */=