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From: "yongxiangng at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/110057] New: Missed devirtualization
 opportunities
Date: Wed, 31 May 2023 10:08:25 +0000
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X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110057

            Bug ID: 110057
           Summary: Missed devirtualization opportunities
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: yongxiangng at gmail dot com
  Target Milestone: ---

There are some missed devirtualization opportunities involving vectors. The
issue is discussed in the stackoverflow here.

https://stackoverflow.com/questions/73257739/missed-optimization-stdvectort=
pop-back-not-qualifying-destructor-call/

Because we know the type of the objects in the vector (they have to be _Tp),
there is no need to invoke the virtual methods and we can devirtualize them=
. I
believe this applies not just to the destructor, but also other member
functions.

This missed optimization does not only affect vector. When creating an arra=
y,
the virtual destructor is invoked even though it should be quite clear to t=
he
compiler that it can resolve the type of the object in the array at compile
time.

There is a possibility of using placement new and "spoofing" the type
https://godbolt.org/z/3MdMsdKha (thanks Jonathan Wakely), but I'm not sure =
if
this is legal c++ and if this applies to raw arrays

I think this might be a problem with the IPA component, I might have
mislabelled the issue because there isn't an option for IPA. Please let me =
know
if this makes sense or I have some misconception.

A sample program that shows this behavior is below.

/* { 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-ssa"  } */

#include <vector>

using std::vector;

class A {
public:
    virtual ~A()
    {

    }
};

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

    }
};

int main()
{
    vector<B> arr;
//    B b[40];
    return 0;
}

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