SWIFT_STDLIB_SINGLE_THREADED_RUNTIME is too much of a blunt instrument here.
It covers both the Concurrency runtime and the rest of the runtime, but we'd
like to be able to have e.g. a single-threaded Concurrency runtime while
the rest of the runtime is still thread safe (for instance).
So: rename it to SWIFT_STDLIB_SINGLE_THREADED_CONCURRENCY and make it just
control the Concurrency runtime, then add a SWIFT_STDLIB_THREADING_PACKAGE
setting at the CMake/build-script level, which defines
SWIFT_STDLIB_THREADING_xxx where xxx depends on the chosen threading package.
This is especially useful on systems where there may be a choice of threading
package that you could use.
rdar://90776105
Some notes:
1. Even though I refactored out AccessSet/Access from Exclusivity.cpp ->
ExclusivityPrivate.h, I left the actual implementations of insert/remove in
Exclusivity.cpp to allow for the most aggressive optimization for use in
Exclusivity.cpp without exposing a bunch of internal details to other parts of
the runtime. Smaller routines like getHead() and manipulating the linked list
directly I left as methods that can be used by other parts of the runtime. I am
going to use these methods to enable backwards deployment of exclusivity support
for concurrency.
2. I moved function replacements out of the Exclusivity header/cpp files since
it has nothing to do with Exclusivity beyond taking advantage of the TLS context
that we are already using.
