rlp: improve SplitListValues allocation efficiency

[mask-pp]

Improve the SplitListValues method by setting the capacity.

Benchmark code:

// splitListValues_origin extracts the raw elements from the list RLP-encoding blob.
func splitListValues_origin(b []byte) ([][]byte, error) {
	b, _, err := SplitList(b)
	if err != nil {
		return nil, err
	}
	var elements [][]byte
	for len(b) > 0 {
		_, tagsize, size, err := readKind(b)
		if err != nil {
			return nil, err
		}
		elements = append(elements, b[:tagsize+size])
		b = b[tagsize+size:]
	}
	return elements, nil
}

func BenchmarkSplitListValues(b *testing.B) {
	input := "F877A12000BF49F440A1CD0527E4D06E2765654C0F56452257516D793A9B8D604DCFDF2AB853F851808D10000000000000000000000000A056E81F171BCC55A6FF8345E692C0F86E5B48E01B996CADC001622FB5E363B421A0C5D2460186F7233C927E7DB2DCC703C0E500B653CA82273B7BFAD8045D85A470"
	data := unhex(input)

	b.Run("SplitListValues(origin)", func(b *testing.B) {
		for i := 0; i < b.N; i++ {
			splitListValues_origin(data) // warm up
		}
	})

	b.Run("SplitListValues(improved)", func(b *testing.B) {
		for i := 0; i < b.N; i++ {
			SplitListValues(data)
		}
	})
}

Result:

goos: darwin
goarch: arm64
pkg: github.com/ethereum/go-ethereum/rlp
cpu: Apple M3 Pro
BenchmarkSplitListValues
BenchmarkSplitListValues/SplitListValues(origin)
BenchmarkSplitListValues/SplitListValues(origin)-12         	12957044	        82.61 ns/op
BenchmarkSplitListValues/SplitListValues(improved)
BenchmarkSplitListValues/SplitListValues(improved)-12       	22989550	        51.51 ns/op

[Copilot]

Pull request overview

This PR optimizes the SplitListValues function by pre-allocating the result slice with the correct capacity. The optimization uses CountValues to determine the number of elements before allocation, avoiding dynamic slice growth and reallocations during append operations.

Key changes:

• Pre-calculate element count using CountValues before allocating the slice
• Initialize slice with capacity n to avoid reallocations
• Remove error checking in the parsing loop (relying on prior validation)

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[rjl493456442]

Thank you.