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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/109442] Dead local copy of std::vector not removed
 from function
Date: Sat, 11 May 2024 16:44:12 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109442

--- Comment #20 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jan Hubicka from comment #19)
> Similar argument can IMO be used for eliding unused memory allocations. It
> is kind of up to std::vector implementation on how many
> allocations/deallocations it does, right?

It's up to std::allocator, which is not required to call operator new every
time memory is needed.=20


> So we need a way to annotate the new/delete calls in the standard library=
 as
> safe for such optimizations (i.e. implement clang's
> __bulitin_operator_new/delete?)

Yes, see PR 110137.

> How clang manages to optimize this out without additional hinting?

It supports __builtin_operator_{new,delete} and libstdc++ uses that when
compiled with clang.=