[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

["jason at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105838

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 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.

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 array 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 slot. 
This also saves stack space.