p2p/enode: optimize LogDist (#32887)

This speeds up LogDist by 75% using 64-bit operations instead
of byte-wise XOR.

---------

Co-authored-by: Felix Lange <fjl@twurst.com>

p2p/enode/node.go

@@ -19,6 +19,7 @@ package enode
 import (
 	"crypto/ecdsa"
 	"encoding/base64"
+	"encoding/binary"
 	"encoding/hex"
 	"errors"
 	"fmt"
@@ -373,12 +374,14 @@ func DistCmp(target, a, b ID) int {
 // LogDist returns the logarithmic distance between a and b, log2(a ^ b).
 func LogDist(a, b ID) int {
 	lz := 0
-	for i := range a {
-		x := a[i] ^ b[i]
+	for i := 0; i < len(a); i += 8 {
+		ai := binary.BigEndian.Uint64(a[i : i+8])
+		bi := binary.BigEndian.Uint64(b[i : i+8])
+		x := ai ^ bi
 		if x == 0 {
-			lz += 8
+			lz += 64
 		} else {
-			lz += bits.LeadingZeros8(x)
+			lz += bits.LeadingZeros64(x)
 			break
 		}
 	}

p2p/enode/node_test.go

@@ -378,6 +378,28 @@ func TestID_logdist(t *testing.T) {
 	}
 }
 
+func makeIDs() (ID, ID) {
+	var a, b ID
+	size := len(a)
+	// last byte differs
+	for i := 0; i < size-1; i++ {
+		a[i] = 0xAA
+		b[i] = 0xAA
+	}
+	a[size-1] = 0xAA
+	b[size-1] = 0xAB
+	return a, b
+}
+
+// Benchmark LogDist
+func BenchmarkLogDist(b *testing.B) {
+	aID, bID := makeIDs() // 256-bit ID
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = LogDist(aID, bID)
+	}
+}
+
 // The random tests is likely to miss the case where a and b are equal,
 // this test checks it explicitly.
 func TestID_logdistEqual(t *testing.T) {