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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: Thu, 02 Nov 2023 08:01:07 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112335

--- Comment #5 from Federico Kircheis <federico at kircheis dot it> ---
Ok, the described case would be something like

----
std::unique_ptr<s> t;
__thread bool tt;
inline s::~s()
{
  if (tt)
    return;
  tt =3D true;
  t.reset(new s);
  tt =3D false;
}

std::unique_ptr<s> t2;
bar(t, t2);
----


The postcondition of p.reset() is that p =3D=3D nullptr.

----
void reset(pointer p =3D pointer()) noexcept;
Preconditions: The expression get_deleter()(get()) is well-formed, has
well-defined behavior, and does not throw exceptions.
Effects: Assigns p to the stored pointer, and then if and only if the old v=
alue
of the stored pointer, old_p, was not equal to nullptr, calls
get_deleter()(old_p). [Note: The order of these operations is significant
because the call to get_deleter() may destroy *this. =E2=80=94 end note]
Postconditions: get() =3D=3D p. [Note: The postcondition does not hold if t=
he call
to get_deleter() destroys *this since this->get() is no longer a valid
expression. =E2=80=94 end note]
----

I do not think that the case you have in mind is supported by the standard,=
 but
the postcondition does not hold with some deleters, so not sure if that cou=
ld
be relevant here=