How the MCF thread model works

The now committed patch implements compiler, linker and standard library support for the MCF thread model. The CRT part has not been committed to mingw-w64.

Changes to GCC

If GCC is configured with --enable-threads=mcf:

1. It has __USING_MCFGTHREAD__ as a pre-defined macro, which enables special code paths in libstdc++ and mingw-w64 CRT.
2. It has -lmcfgthread -lntdll as default libraries. (NTDLL is only needed for static linking.)
3. It installs 'gthr-mcf.h' as 'bits/gthr-default.h', which is just a wrapper for 'mcfgthread/gthr.h'.

In libgcc, most use of threading infrastructure is internal. In libstdc++, there are some public functions, which, when __USING_MCFGTHREAD__ is defined, should be redirected to MCF implementations:

1. __cxa_thread_atexit(dtor, obj, dso): On Linux it is provided in libstdc++ and jumps to __cxa_thread_atexit_impl(dtor, obj, dso) in libc. Recent mingw-w64 provides this function, so it should be opted out to prefer the mingw-w64 one.
2. __cxa_guard_acquire(guard): On Linux it is implemented with futex. On most other platforms it is implemented with a single static mutex. It should be opted to call __MCF_cxa_guard_acquire(guard).
3. __cxa_guard_release(guard): It is opted to call __MCF_cxa_guard_release(guard).
4. __cxa_guard_abort(guard): It is opted to call __MCF_cxa_guard_abort(guard).

Changes to mingw-w64

The mingw-w64 CRT needs these changes:

1. Some process-termination routines in the Microsoft CRT effect non-standard behavior and have to be replaced (https://github.com/msys2/MINGW-packages/issues/2519). Because the Microsoft exit() should not be called any more, it is necessary to flush standard files after static destructors. This is done by calling __MCF_cxa_atexit(fflush, NULL, &__dso_handle) before __main().
2. If a DLL entry-point function receives a DLL_PROCESS_DETACH notification, it shall call __MCF_cxa_finalize(&__dso_handle), as specified by the Itanium C++ ABI.
3. _Exit(status): It is opted to call __MCF__Exit(status).
4. _exit(status): It is equivalent to _Exit(status), and is opted to call __MCF__Exit(status).
5. at_quick_exit(func): This function is linked statically by each module, and is opted to call __MCF_cxa_at_quick_exit(func, NULL, &__dso_handle).
6. __cxa_at_quick_exit(dtor, obj, dso): It is opted to call __MCF_cxa_at_quick_exit(dtor, obj, dso).
7. quick_exit(status): It is opted to call __MCF_quick_exit(status).
8. atexit(func): This function is linked statically by each module, and is opted to call __MCF_cxa_atexit(func, NULL, &__dso_handle).
9. __cxa_atexit(dtor, obj, dso): It is opted to call __MCF_cxa_atexit(dtor, obj, dso).
10. exit(status): It is opted to call __MCF_exit(status).
11. __cxa_finalize(dso): It is opted to call __MCF_cxa_finalize(dso).
12. __cxa_at_thread_exit(dtor, obj, dso): It is opted to call __MCF_cxa_at_thread_exit(dtor, obj, dso).