// the result length is large (roughly, over L2 cache size).
	const chunkLimit = 8 * 1024
	chunkMax := n
	if chunkMax > chunkLimit {
		chunkMax = chunkLimit / len(b) * len(b)
		if chunkMax == 0 {
			chunkMax = len(b)
		}
	}
	nb := bytealg.MakeNoZero(n)[:n:n]
	bp := copy(nb, b)
	for bp < n {
		chunk := min(bp, chunkMax)
		bp += copy(nb[bp:], nb[:chunk])
	}
	return nb
}