Cleanups: Simplify thread and condition_variable, identify unused dllexports

[StephanTLavavej]

This PR contains fairly simple logic transformations - they aren't exactly trivial (as one must consult other code to verify their correctness), but they're far from complicated. There's no behavioral impact, no changes to dllexports, and no interaction with the "unlocked" state of the redist.

• First, _Xtime_diff_to_millis() is unused. We can drop its declaration in inc/xtimec.h and mark its definition in src/xtime.cpp as preserved for bincompat. When doing this, all we need to do is verify that the declaration and the definition contain all of the same annotations (in particular, _CRTIMP2_PURE controls dllexporting).
• Simplify thread::detach(), part 1.
  • inc/xthreads.h provided an inline helper _Check_C_return(). Originally, it may have been used widely, but now there's only one usage. We can manually fuse it into thread::detach().
• Simplify thread::detach(), part 2.
  • _Thrd_detach() returns either _Thrd_success or _Thrd_error:

    STL/stl/src/cthread.cpp

    Lines 71 to 73 in 16bb556

    	int _Thrd_detach(_Thrd_t thr) { // tell OS to release thread's resources when it terminates
    	return CloseHandle(thr._Hnd) ? _Thrd_success : _Thrd_error;
    	}

  • _Throw_C_error() translates that to _Throw_Cpp_error(_INVALID_ARGUMENT):

    STL/stl/src/thread0.cpp

    Lines 47 to 48 in 16bb556

    	case _Thrd_error:
    	_Throw_Cpp_error(_INVALID_ARGUMENT);

  • Thus, we can simplify the logic even more. Now it resembles thread::join() above, and more clearly shows what exception will be thrown.
• Simplify threads, part 3.
  • _Cnd_timedwait() returns only _Thrd_success or _Thrd_timedout. There's only one return, and only two values that res can take:

    STL/stl/src/cond.cpp

    Lines 63 to 83 in 16bb556

    	int _Cnd_timedwait(const _Cnd_t cond, const _Mtx_t mtx, const xtime* const target) { // wait until signaled or timeout
    	int res = _Thrd_success;
    	const auto cs = static_cast<Concurrency::details::stl_critical_section_interface*>(_Mtx_getconcrtcs(mtx));
    	if (target == nullptr) { // no target time specified, wait on mutex
    	_Mtx_clear_owner(mtx);
    	cond->_get_cv()->wait(cs);
    	_Mtx_reset_owner(mtx);
    	} else { // target time specified, wait for it
    	xtime now;
    	xtime_get(&now, TIME_UTC);
    	_Mtx_clear_owner(mtx);
    	if (!cond->_get_cv()->wait_for(cs, _Xtime_diff_to_millis2(target, &now))) { // report timeout
    	xtime_get(&now, TIME_UTC);
    	if (_Xtime_diff_to_millis2(target, &now) == 0) {
    	res = _Thrd_timedout;
    	}
    	}
    	_Mtx_reset_owner(mtx);
    	}
    	return res;
    	}

  • This allows us to simplify the calls to _Cnd_timedwait(), as we don't need to worry about unexpected return values.
  • After this, _Throw_C_error() is unused, so we can drop its declaration and mark its definition as preserved for bincompat.
  • There's one difference between the declaration and the definition, but we don't need the extern "C++" from the declaration in inc/xthreads.h - that's for Standard Library Modules, but src/thread0.cpp is always built classically.

[strega-nil-ms]

LGTM, thanks!

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.