Michael Ilseman 973b5aed54 [stdlib] Introduce Thread Local Storage ...

Introduces a _ThreadLocalStorage struct to hold thread-local, but
global resources. Set it up to host a UBreakIterator and a cache key
for resetting text.

UBreakIterators are extremely expensive to create on the fly, so we
store one for each thread. UBreakIterators are also expensive to bind
to new text, so we cache the text it's currently bound to in order to
help avoid it.

The struct can be expanded with more functionality in the future, but
the standard library should only ever use a single key, and thus
everything should go on this struct. The _ThreadLocalStorage struct is
not meant to be copyable, creatable (by anyone else except the
once-per-thread initialize routine), and should accessed through the
pointers it provides.

Future immediate directions could include cashing multiple
UBreakIterators (e.g. avoid a text reset for mutual character
iteration patterns, etc).

Test added in test/stdlib/ThreadLocalStorage.swift.

2017-05-16 20:29:21 -07:00

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