Function swap_flush 

unsafe fn swap_flush(flush: u32, data: *mut ()) -> u32

Set the current thread’s flushing bits to flush & FLUSH, keeping every other bit, and return the flushing bits it had. A write, when needed, may read and write *data for the compiler.

Safety

The processor implements every bit of flush & FLUSH, as it does the flushing bits a thread had before; clearing (flush == 0) is always valid.