strconv: simplify and optimize Itoa(small)

Use substring of digits for values < 10.

name                 old time/op  new time/op  delta
FormatIntSmall/7-4   4.54ns ± 1%  3.70ns ± 1%  -18.41%  (p=0.000 n=18+17)
FormatIntSmall/42-4  4.54ns ± 1%  4.13ns ± 1%   -9.02%  (p=0.000 n=16+18)

Change-Id: I0b521b563c13ef88aa2701049fa4a43760e884af
Reviewed-on: https://go-review.googlesource.com/111285Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/strconv/itoa.go

@@ -57,11 +57,10 @@ func AppendUint(dst []byte, i uint64, base int) []byte {
 
 // small returns the string for an i with 0 <= i < nSmalls.
 func small(i int) string {
-	off := 0
 	if i < 10 {
-		off = 1
+		return digits[i : i+1]
 	}
-	return smallsString[i*2+off : i*2+2]
+	return smallsString[i*2 : i*2+2]
 }
 
 const nSmalls = 100

src/strconv/itoa_test.go

@@ -200,11 +200,15 @@ func BenchmarkAppendUint(b *testing.B) {
 }
 
 func BenchmarkFormatIntSmall(b *testing.B) {
-	const smallInt = 42
+	smallInts := []int64{7, 42}
+	for _, smallInt := range smallInts {
+		b.Run(Itoa(int(smallInt)), func(b *testing.B) {
 			for i := 0; i < b.N; i++ {
 				s := FormatInt(smallInt, 10)
 				BenchSink += len(s)
 			}
+		})
+	}
 }
 
 func BenchmarkAppendIntSmall(b *testing.B) {