[rfc6962] optimise SHA-256 tree hashing for interior nodes

rfc6962/rfc6962.go

@@ -17,7 +17,7 @@ package rfc6962
 
 import (
 	"crypto"
-	_ "crypto/sha256" // SHA256 is the default algorithm.
+	"crypto/sha256"
 )
 
 // Domain separation prefixes
@@ -41,12 +41,18 @@ func New(h crypto.Hash) *Hasher {
 
 // EmptyRoot returns a special case for an empty tree.
 func (t *Hasher) EmptyRoot() []byte {
+	if t.Hash == crypto.SHA256 {
+		h := sha256.Sum256(nil)
+		return h[:]
+	}
 	return t.New().Sum(nil)
 }
 
 // HashLeaf returns the Merkle tree leaf hash of the data passed in through leaf.
 // The data in leaf is prefixed by the LeafHashPrefix.
 func (t *Hasher) HashLeaf(leaf []byte) []byte {
+	// Note: A SHA-256 fast path using sha256.Sum256 is possible here if leaf data is
+	// staged into an array on the stack, but requires an arbitrary buffer size threshold.
 	h := t.New()
 	h.Write([]byte{RFC6962LeafHashPrefix})
 	h.Write(leaf)
@@ -56,6 +62,10 @@ func (t *Hasher) HashLeaf(leaf []byte) []byte {
 // HashChildren returns the inner Merkle tree node hash of the two child nodes l and r.
 // The hashed structure is NodeHashPrefix||l||r.
 func (t *Hasher) HashChildren(l, r []byte) []byte {
+	if t.Hash == crypto.SHA256 && len(l) == sha256.Size && len(r) == sha256.Size {
+		// Fast path for sha256 common case
+		return hashChildren256(l, r)
+	}
 	h := t.New()
 	b := append(append(append(
 		make([]byte, 0, 1+len(l)+len(r)),
@@ -66,3 +76,19 @@ func (t *Hasher) HashChildren(l, r []byte) []byte {
 	h.Write(b)
 	return h.Sum(nil)
 }
+
+// hashChildren256 hashes the fixed 65-byte RFC6962 interior node preimage on the stack.
+//
+// This optimization relies on using the concrete sha256.Sum256 function directly.
+// Because the compiler has full visibility into Sum256, it can prove that b does
+// not outlive the call, keeping b on the stack (1 allocation total for the returned slice).
+// Passing b through an interface (e.g. t.New().Write(b)) would force b to escape to the
+// heap because the compiler cannot prove an interface method won't retain it.
+func hashChildren256(l, r []byte) []byte {
+	var b [1 + 2*sha256.Size]byte
+	b[0] = RFC6962NodeHashPrefix
+	copy(b[1:], l)
+	copy(b[1+sha256.Size:], r)
+	h := sha256.Sum256(b[:])
+	return h[:]
+}

rfc6962/rfc6962_test.go

@@ -105,3 +105,12 @@ func BenchmarkHashChildren(b *testing.B) {
 		_ = h.HashChildren(l, r)
 	}
 }
+
+func BenchmarkHashLeaf(b *testing.B) {
+	h := DefaultHasher
+	data := []byte("benchmark leaf payload")
+	b.ResetTimer()
+	for range b.N {
+		_ = h.HashLeaf(data)
+	}
+}