// the caller can make x observable to the garbage
	// collector. Otherwise, on weakly ordered machines,
	// the garbage collector could follow a pointer to x,
	// but see uninitialized memory or stale heap bits.
	publicationBarrier()
	// As x and the heap bits are initialized, update
	// freeIndexForScan now so x is seen by the GC
	// (including conservative scan) as an allocated object.
	// While this pointer can't escape into user code as a

		// Update related page sweeper stats.
		h.pagesInUse.Add(npages)
	}

	// Make sure the newly allocated span will be observed
	// by the GC before pointers into the span are published.
	publicationBarrier()
}

// Try to add at least npage pages of memory to the heap,
// returning how much the heap grew by and whether it worked.
//
// h.lock must be held.