__fastfail() is used by the Visual C++ runtime and Windows system libraries to signal that the in-process state is corrupted and unrecoverable.
If __fastfail() is invoked, the NT kernel raises a second-chance non-continuable exception STATUS_STACK_BUFFER_OVERRUN. This quickly terminates the process, bypassing all in-process exception handlers (since they all rely on the potentially corrupted process state).
-- v2: ntdll: Properly parse UDF instruction in ARM. ntdll: Implement __fastfail(). ntdll/tests: Add tests for __fastfail(). include: Define fast fail codes and the __fastfail() intrinsic.