i := int(searchIdx / 64)
	// Start by quickly skipping over blocks of non-free or scavenged pages.
	for ; i >= 0; i-- {
		// 1s are scavenged OR non-free => 0s are unscavenged AND free
		x := fillAligned(m.scavenged[i]|m.pallocBits[i], uint(minimum))
		if x != ^uint64(0) {
			break
		}
	}
	if i < 0 {
		// Failed to find any free/unscavenged pages.
		return 0, 0
	}
	// We have something in the 64-bit chunk at i, but it could

	// as allocBits for newly allocated spans.
	//
	// The pointer arithmetic is done "by hand" instead of using
	// arrays to avoid bounds checks along critical performance
	// paths.
	// The sweep will free the old allocBits and set allocBits to the
	// gcmarkBits. The gcmarkBits are replaced with a fresh zeroed
	// out memory.
	allocBits  *gcBits
	gcmarkBits *gcBits

	mask  uint8
	index uintptr
}

//go:nosplit
func (s *mspan) allocBitsForIndex(allocBitIndex uintptr) markBits {
	bytep, mask := s.allocBits.bitp(allocBitIndex)
	return markBits{bytep, mask, allocBitIndex}
}

// refillAllocCache takes 8 bytes s.allocBits starting at whichByte
// and negates them so that ctz (count trailing zeros) instructions
// can be used. It then places these 8 bytes into the cached 64 bit