if !found {
		return nil
	}
	diff := make([]byte, len(snapshot))
	for i := range diff {
		diff[i] = snapshot[i] &^ base[i]
	}
	return diff
}

// countNewCoverageBits returns the number of bits set in snapshot that are not
// set in base.
func countNewCoverageBits(base, snapshot []byte) int {
	n := 0
	for i := range snapshot {
		n += bits.OnesCount8(snapshot[i] &^ base[i])
	}
	return n
}

		if err != nil {
			errMsg = err.Error()
			if errMsg == "" {
				errMsg = "fuzz function failed with no input"
			}
			return dur, nil, errMsg
		}
		if ws.coverageMask != nil && countNewCoverageBits(ws.coverageMask, coverageSnapshot) > 0 {
			return dur, coverageSnapshot, ""
		}
		return dur, nil, ""
	}

	if args.Warmup {
		dur, _, errMsg := fuzzOnce(CorpusEntry{Values: vals})