freeIndex := s.nextFreeIndex()
	if freeIndex == s.nelems {
		// The span is full.
		if s.allocCount != s.nelems {
			println("runtime: s.allocCount=", s.allocCount, "s.nelems=", s.nelems)
			throw("s.allocCount != s.nelems && freeIndex == s.nelems")
		}
		c.refill(spc)
		shouldhelpgc = true
		s = c.alloc[spc]

		freeIndex = s.nextFreeIndex()
	}

	if freeIndex >= s.nelems {
		throw("freeIndex is not valid")
	}

	// undefined and should never be referenced.
	//
	// Object n starts at address n*elemsize + (start << pageShift).
	freeindex uint16
	// TODO: Look up nelems from sizeclass and remove this field if it
	// helps performance.
	nelems uint16 // number of object in the span.
	// freeIndexForScan is like freeindex, except that freeindex is
	// used by the allocator whereas freeIndexForScan is used by the

			if err != nil {
				return nil, err
			}

			newItems, err := meta.ExtractList(listObj)
			if err != nil {
				return nil, err
			}
			items = append(items, newItems...)

			for i := 0; i < len(newItems); i++ {
				select {
				case toProcess <- newItems[i]:
				case <-workersExited:

// faster if profiling warrants it.
func (s *mspan) nextFreeIndex() uint16 {
	sfreeindex := s.freeindex
	snelems := s.nelems
	if sfreeindex == snelems {
		return sfreeindex
	}
	if sfreeindex > snelems {
		throw("s.freeindex > s.nelems")
	}

	aCache := s.allocCache

	bitIndex := sys.TrailingZeros64(aCache)
	for bitIndex == 64 {