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From: "jason 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 17:57:53 +0000
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X-Bugzilla-Keywords: compile-time-hog, memory-hog
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105838

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED

--- Comment #10 from Jason Merrill <jason at gcc dot gnu.org> ---
A lot of the problem here is that building a std::string involves building 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 t=
hem:
https://eel.is/c++draft/class.temporary#5 says we can only destroy temporar=
ies
early if there was no initializer for that array element.  So for each elem=
ent
of the initializer we have another EH region for its allocator temporary.

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 arra=
y of
strings.  This seems like a worthwhile general optimization.

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 slo=
t.=20
This also saves stack space.=