libstdc++/7445: poor performance of std::locale::classic() in multi-threaded applications

[Benjamin Kosnik]

> OK, I'd be happy to find a performance fix that doesn't make the
> library code subtly less portable when we have a test case.  A
> reference to code already in our test suite would be acceptable.

Yeah. This is kind of all hand-wavy at this point.

> Benjamin (as the "local locale" expert), is it *always* true that
> std::locale::classic() is called at least once during startup before
> control is transferred to main()?  If so (and will always be so), then
> all thread-related locking could be removed IMHO.  In practice, our
> implementation of the Standard C++ library (with optional threading
> support) already supposes that no thread which uses the library
> implementation may have ever been started before main() is invoked.
> If not true, then this change would make a very hard to detect
> threading bug when the assumption changes.

This is true. I would think the lock could then be removed? I'll let you
figure out what to do with current sources. Thanks for the help.

-benjamin