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/2215
Reviewed-by: Rob Pike <r@golang.org>

diff --git a/src/log/log.go b/src/log/log.go
index cb5c76ba204571d0890a1969dde1f4870cadeb80..5ff2bc21e3a5b2a33cb2ed640b799f5bb8efe71a 100644 (file)
--- a/src/log/log.go
+++ b/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:]...)
 }
 

diff --git a/src/log/log_test.go b/src/log/log_test.go
index 158c3d93c7e2fb8262579d855767cabf381517f0..14e0b2926340564a1ed3964d96cfa65ef9807fa6 100644 (file)
--- a/src/log/log_test.go
+++ b/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)
+       }
+}