// just prefetches the mark bits.
		mbits := span.markBitsForIndex(objIndex)
		if mbits.isMarked() {
			continue
		}
		mbits.setMarked()

		// Mark span.
		arena, pageIdx, pageMask := pageIndexOf(span.base())
		if arena.pageMarks[pageIdx]&pageMask == 0 {
			atomic.Or8(&arena.pageMarks[pageIdx], pageMask)
		}

		if span.spanclass.noscan() {
			gcw.bytesMarked += uint64(span.elemsize)
			continue

	if s == nil || s.state.get() != mSpanInUse || p < s.base() || p >= s.limit {
		return nil
	}
	return s
}

// pageIndexOf returns the arena, page index, and page mask for pointer p.
// The caller must ensure p is in the heap.
func pageIndexOf(p uintptr) (arena *heapArena, pageIdx uintptr, pageMask uint8) {
	ai := arenaIndex(p)
	arena = mheap_.arenas[ai.l1()][ai.l2()]

			throw("marking free object")
		}

		// If marked we have nothing to do.
		if mbits.isMarked() {
			return
		}
		mbits.setMarked()

		// Mark span.
		arena, pageIdx, pageMask := pageIndexOf(span.base())
		if arena.pageMarks[pageIdx]&pageMask == 0 {
			atomic.Or8(&arena.pageMarks[pageIdx], pageMask)
		}

		// If this is a noscan object, fast-track it to black
		// instead of greying it.