trie: avoid rlp-encoding when trie hash is already available

trie/hasher.go

@@ -165,20 +165,20 @@ func (h *hasher) store(n node, db *Database, force bool) (node, error) {
 	if _, isHash := n.(hashNode); n == nil || isHash {
 		return n, nil
 	}
-	// Generate the RLP encoding of the node
-	h.tmp.Reset()
-	if err := rlp.Encode(&h.tmp, n); err != nil {
-		panic("encode error: " + err.Error())
-	}
-	if len(h.tmp) < 32 && !force {
-		return n, nil // Nodes smaller than 32 bytes are stored inside their parent
-	}
-	// Larger nodes are replaced by their hash and stored in the database.
+	// We might already have the hash
 	hash, _ := n.cache()
 	if hash == nil {
+		// Generate the RLP encoding of the node
+		h.tmp.Reset()
+		if err := rlp.Encode(&h.tmp, n); err != nil {
+			panic("encode error: " + err.Error())
+		}
+		if len(h.tmp) < 32 && !force {
+			return n, nil // Nodes smaller than 32 bytes are stored inside their parent
+		}
+		// Larger nodes are replaced by their hash and stored in the database.
 		hash = h.makeHashNode(h.tmp)
 	}
-
 	if db != nil {
 		// We are pooling the trie nodes into an intermediate memory cache
 		hash := common.BytesToHash(hash)

trie/trie_test.go

@@ -512,6 +512,42 @@ func BenchmarkHash(b *testing.B) {
 	trie.Hash()
 }
 
+// Benchmarks the trie Commit following a Hash. Since the trie caches the result of any operation,
+// we cannot use b.N as the number of hashing rouns, since all rounds apart from
+// the first one will be NOOP. As such, we'll use b.N as the number of account to
+// insert into the trie before measuring the hashing.
+func BenchmarkCommitAfterHash(b *testing.B) {
+	// Make the random benchmark deterministic
+	random := rand.New(rand.NewSource(0))
+
+	// Create a realistic account trie to hash
+	addresses := make([][20]byte, b.N)
+	for i := 0; i < len(addresses); i++ {
+		for j := 0; j < len(addresses[i]); j++ {
+			addresses[i][j] = byte(random.Intn(256))
+		}
+	}
+	accounts := make([][]byte, len(addresses))
+	for i := 0; i < len(accounts); i++ {
+		var (
+			nonce   = uint64(random.Int63())
+			balance = new(big.Int).Rand(random, new(big.Int).Exp(common.Big2, common.Big256, nil))
+			root    = emptyRoot
+			code    = crypto.Keccak256(nil)
+		)
+		accounts[i], _ = rlp.EncodeToBytes([]interface{}{nonce, balance, root, code})
+	}
+	// Insert the accounts into the trie and hash it
+	trie := newEmpty()
+	for i := 0; i < len(addresses); i++ {
+		trie.Update(crypto.Keccak256(addresses[i][:]), accounts[i])
+	}
+	trie.Hash()
+	b.ResetTimer()
+	b.ReportAllocs()
+	trie.Commit(nil)
+}
+
 func tempDB() (string, *Database) {
 	dir, err := ioutil.TempDir("", "trie-bench")
 	if err != nil {