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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: Wed, 12 Apr 2023 10:29:41 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109442

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #9)
> Yep, looking at that link this is exactly what would be needed.  Note
> that in the middle-end we already see the calls to
> DECL_IS_REPLACEABLE_OPERATOR_NEW_P and replaceable operator delete.  We
> just restrict all optimizations to calls emitted from new/delete expressi=
ons.
> I'm not sure the clang builtin is exactly providing the new/delete
> expression semantics resolving to replaceable operator new/delete or if
> there's other details.  That is, I'm curious whether __builtin_operator_n=
ew
> provides more guarantees than a new/delete expression - the builtin seman=
tics
> isn't very well defined unfortunately.  It might be safer to go our own
> way here.

I would prefer if we don't deviate, and get Clang to clarify things instead=
 of
reinventing something that looks similar but isn't.

> I suppose the standard library could as well use malloc/free?

No. std::allocator is required to use operator new and operator delete to
obtain memory, but it's unspecified when or how often it calls them. So it'=
s OK
to call them once at startup to obtain ALL THE MEMORY that any std::allocat=
or
will ever need for the duration of the program, or to merge allocations, or=
 to
elide them completely. But it's not OK to use malloc.

std::allocator::allocate says:
"The storage for the array is obtained by calling ::operator new (17.6.3), =
but
it is unspecified when or how often this function is called."=