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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 10:23:00 +0000 [thread overview]
Message-ID: <bug-112335-4-D4RUZmpPXd@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-112335-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112335
--- Comment #9 from Federico Kircheis <federico at kircheis dot it> ---
Great, thank you for the clarifications, your redacted example makes now sense.
> https://godbolt.org/z/WGPTesEb3 shows that bar3 is not the same as bar1, because it runs an additional destructor if operator delete() plays silly games.
And this is what I meant before that such program is not supported by the
standard
----
void bar3(std::unique_ptr<s>& ps1, std::unique_ptr<s>& ps2){
ps1.reset();
ps1.reset();
ps1 = std::move(ps2);
}
---
is equivalent to
----
void bar3(std::unique_ptr<s>& ps1, std::unique_ptr<s>& ps2){
ps1.reset();
assert(ps1 == nullptr);
ps1.reset();
assert(ps1 == nullptr);
ps1 = std::move(ps2);
}
----
which is not.
And the same holds if "if (!global) global.reset(new s);" is moved into "~s()",
and removed from "operator delete".
If the users changes delete in such a way, at this point we have UB, as the
postcondition of unique_ptr::reset does not hold (the example given by Andrew)
Yes, the second ps1.reset(); is trivial to remove.
next prev parent reply other threads:[~2023-11-03 10:23 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-01 13:26 [Bug c++/112335] New: " federico at kircheis dot it
2023-11-01 15:51 ` [Bug c++/112335] " pinskia at gcc dot gnu.org
2023-11-02 7:17 ` federico at kircheis dot it
2023-11-02 7:19 ` federico at kircheis dot it
2023-11-02 7:30 ` pinskia at gcc dot gnu.org
2023-11-02 8:01 ` federico at kircheis dot it
2023-11-02 17:23 ` redi at gcc dot gnu.org
2023-11-03 8:06 ` federico at kircheis dot it
2023-11-03 9:10 ` redi at gcc dot gnu.org
2023-11-03 10:23 ` federico at kircheis dot it [this message]
2023-11-03 14:23 ` redi at gcc dot gnu.org
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