// than regular goroutine switches, so this path is heavily
// optimized to remove unnecessary work.
// The fast path here is three CAS: the one at the top on gp.atomicstatus,
// the one in the middle to choose the next g,
// and the one at the bottom on gnext.atomicstatus.
// It is important not to add more atomic operations or other
// expensive operations to the fast path.
func coroswitch_m(gp *g) {
	c := gp.coroarg