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From: "federico at kircheis dot it" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/112335] missed optimization on reset and assign unique_ptr
Date: Fri, 03 Nov 2023 08:06:37 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112335

--- Comment #7 from Federico Kircheis <federico at kircheis dot it> ---
Thank you Jonathan, I've then misunderstood the example of Andrew, I though=
t he
was trying to create a scenario where after reset the pointer is not nullpt=
r.

> Demo: https://godbolt.org/z/PWd96j4fz

Thank you for the example, but honestly, I think I am still missing somethi=
ng.

If you modify the main this way

----
int main()
{
    global =3D nullptr;
    std::unique_ptr<s> local1(new s);
    bar1(global, local1);
    global =3D nullptr;
    std::unique_ptr<s> local2(new s);
    bar1(global, local2);
}
----


or this way


----
int main()
{
    global =3D nullptr;
    std::unique_ptr<s> local1(new s);
    bar2(global, local1);
    global =3D nullptr;
    std::unique_ptr<s> local2(new s);
    bar2(global, local2);
}
----

the output is the same.
First time it prints 0, the second time a non-0 address.

> But a much simpler example where the difference is observable is when the=
 two arguments to bar1 and bar2 are the same object:
> https://godbolt.org/z/z8fsTan8K

And this is absolutely correct, did not think of it.

> I'm not sure what you mean here. The postcondition holds unless the delet=
er *also* destroys the unique_ptr itself.

As written, I was not sure if this plays a role; this is what the standard =
says
(and I#ve misunderstood the example of Andrew).
A similar effect can be observed in the following example

----
#include <memory>

using s =3D int; // for simplicity

void bar1(std::unique_ptr<s>& ps1, std::unique_ptr<s>& ps2){
    ps1.reset();
    ps1 =3D std::move(ps2);
}
void bar2(std::unique_ptr<s>& ps1, std::unique_ptr<s>& ps2){
    ps1 =3D std::move(ps2);
}

void bar3(std::unique_ptr<s>& ps1, std::unique_ptr<s>& ps2){
    ps1.reset();
    ps1.reset();
    ps1 =3D std::move(ps2);
}
----

While you are right that bar1 and bar2 have different behavior if they poin=
t to
the same object, bar1 and bar3 are fundamentally the same function, even in
case of aliasing.
Unless the post-condition of reset is not hold; otherwise the second
"ps1.reset()" could be completely removed, as ps1 is nullptr.=