log: optimize itoa

Reduce buffer to maximally needed size for conversion of 64bit integers.
Reduce number of used integer divisions.

benchmark            old ns/op     new ns/op     delta
BenchmarkItoa        144           119           -17.36%
BenchmarkPrintln     783           752           -3.96%

Change-Id: I6d57a7feebf90f303be5952767107302eccf4631
Reviewed-on: https://go-review.googlesource.com/2215Reviewed-by: Rob Pike <r@golang.org>

src/log/log.go

@@ -63,22 +63,19 @@ func New(out io.Writer, prefix string, flag int) *Logger {
 var std = New(os.Stderr, "", LstdFlags)
 
 // Cheap integer to fixed-width decimal ASCII.  Give a negative width to avoid zero-padding.
-// Knows the buffer has capacity.
 func itoa(buf *[]byte, i int, wid int) {
-	var u uint = uint(i)
-	if u == 0 && wid <= 1 {
-		*buf = append(*buf, '0')
-		return
-	}
-
 	// Assemble decimal in reverse order.
-	var b [32]byte
-	bp := len(b)
-	for ; u > 0 || wid > 0; u /= 10 {
-		bp--
+	var b [20]byte
+	bp := len(b) - 1
+	for i >= 10 || wid > 1 {
 		wid--
-		b[bp] = byte(u%10) + '0'
+		q := i / 10
+		b[bp] = byte('0' + i - q*10)
+		bp--
+		i = q
 	}
+	// i < 10
+	b[bp] = byte('0' + i)
 	*buf = append(*buf, b[bp:]...)
 }
 

src/log/log_test.go

@@ -117,3 +117,27 @@ func TestFlagAndPrefixSetting(t *testing.T) {
 		t.Error("message did not match pattern")
 	}
 }
+
+func BenchmarkItoa(b *testing.B) {
+	dst := make([]byte, 0, 64)
+	for i := 0; i < b.N; i++ {
+		dst = dst[0:0]
+		itoa(&dst, 2015, 4)   // year
+		itoa(&dst, 1, 2)      // month
+		itoa(&dst, 30, 2)     // day
+		itoa(&dst, 12, 2)     // hour
+		itoa(&dst, 56, 2)     // minute
+		itoa(&dst, 0, 2)      // second
+		itoa(&dst, 987654, 6) // microsecond
+	}
+}
+
+func BenchmarkPrintln(b *testing.B) {
+	const testString = "test"
+	var buf bytes.Buffer
+	l := New(&buf, "", LstdFlags)
+	for i := 0; i < b.N; i++ {
+		buf.Reset()
+		l.Println(testString)
+	}
+}