On Windows libc exports __cxa_init_primary_exception, std::uncaught_exception
and std::uncaught_exceptions only as _nid_postfix symbols (GCC_GLOBAL_ALIAS
is empty there), so host modules bind libstdc++-6.dll's versions:
- std::make_exception_ptr, which std::promise's destructor uses for
broken_promise, allocated the exception with libc and initialized it with
libstdc++'s __cxa_init_primary_exception, whose header layout differs.
A test that makes one and rethrows it dies with an access violation.
- std::uncaught_exceptions read libstdc++'s thread state, which libc's
throws never touch, so it returned 0 while an exception unwound. The
AGC driver's deferred-label guard relies on it to tell a failed record
from a finished one.
The three are now exported natively, like __cxa_throw and the rest.
__cxa_init_primary_exception also sets the reference count to 0, as the
Itanium ABI does: the exception_ptr created next takes the only
reference, so a made exception is destroyed once instead of leaking.
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