[Bug libstdc++/109442] Dead local copy of std::vector not removed from function

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109442

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #7)
> I think it's valid in theory. I don't know if it's possible for GCC to do it
> in practice. There doesn't seem to be anything the library can do to help,
> so WONTFIX.

I think it's valid in theory because the implementation of std::vector is
opaque. It's not required to actually call operator new and operator delete to
obtain its storage, and so if the library and compiler collaborate to provide
storage some other way (e.g. using a stack buffer) then that's allowed.

But in practice it's hard to do that.

Maybe it could be done with a new __builtin_elidable_operator_new and
__builtin_elidable_operator_delete pair that the library could use, or
attributes like you suggested in comment 4. That would allow the library to say
"I'm calling operator new and operator delete because I need to obtain memory,
but **these particular invocations** are not required to be visible to the
user, and therefore can be elided in the same way as new expressions and delete
expressions".

In fact Clang's __builtin_operator_new already has exactly those semantics:

https://clang.llvm.org/docs/LanguageExtensions.html#builtin-operator-new-and-builtin-operator-delete

And we already use those builtins if available, see PR 94295.

So maybe what we need is to implement those.