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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/105838] [10/11/12/13 Regression] g++ 12.1.0 runs out of
 memory or time when building const std::vector of std::strings
Date: Mon, 05 Dec 2022 18:30:45 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105838

--- Comment #11 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Jason Merrill from comment #10)
> A lot of the problem here is that building a std::string involves buildin=
g a
> std::allocator<char> temporary to pass to the string constructor, and then
> we need to wait until the entire array is built before we can destroy any=
 of
> them: https://eel.is/c++draft/class.temporary#5 says we can only destroy
> temporaries early if there was no initializer for that array element.  So
> for each element of the initializer we have another EH region for its
> allocator temporary.
>=20
> We could do better for the general case by creating a parallel array of
> temporaries and using the same single cleanup region for it as for the ar=
ray
> of strings.  This seems like a worthwhile general optimization.
>=20
> We might be able to do better for the specific case by recognizing that
> std::allocator has no data and nothing cares about its address, so we can=
 go
> ahead and destroy it after initializing the string, and reuse the stack
> slot.  This also saves stack space.

Even if we don't emit a loop (which I still think is the way to go for larg=
er
initializers because anything else means just too large code), can't there =
for
the larger initializers simply be some variable holding a counter how many
initializers have been already initialized and a single EH region that will
perform all the cleanups based on that counter?=