// free slots for this sweepgen.
func (c *mcentral) fullSwept(sweepgen uint32) *spanSet {
	return &c.full[sweepgen/2%2]
}

// Allocate a span to use in an mcache.
func (c *mcentral) cacheSpan() *mspan {
	// Deduct credit for this span allocation and sweep if necessary.
	spanBytes := uintptr(class_to_allocnpages[c.spanclass.sizeclass()]) * _PageSize
	deductSweepCredit(spanBytes, 0)

	traceDone := false