runtime.GC()
		b.StopTimer()
	}
	close(teardown)
}

func BenchmarkMSpanCountAlloc(b *testing.B) {
	// Allocate one dummy mspan for the whole benchmark.
	s := runtime.AllocMSpan()
	defer runtime.FreeMSpan(s)

	// n is the number of bytes to benchmark against.
	// n must always be a multiple of 8, since gcBits is
	// always rounded up 8 bytes.
	for _, n := range []int{8, 16, 32, 64, 128} {

	var s *mspan
	systemstack(func() {
		lock(&mheap_.lock)
		s = (*mspan)(mheap_.spanalloc.alloc())
		unlock(&mheap_.lock)
	})
	return (*MSpan)(s)
}

// Free an allocated mspan.
func FreeMSpan(s *MSpan) {
	systemstack(func() {
		lock(&mheap_.lock)
		mheap_.spanalloc.free(unsafe.Pointer(s))
		unlock(&mheap_.lock)
	})
}

func MSpanCountAlloc(ms *MSpan, bits []byte) int {