encoding/hex: pre-allocate Dump buffer

name          old time/op    new time/op    delta
Dump/256-4      7.76µs ± 2%    5.82µs ± 2%  -24.91%  (p=0.008 n=5+5)
Dump/1024-4     28.4µs ± 2%    22.6µs ± 3%  -20.58%  (p=0.008 n=5+5)
Dump/4096-4      112µs ± 2%      88µs ± 0%  -20.80%  (p=0.016 n=5+4)
Dump/16384-4     444µs ± 3%     361µs ± 7%  -18.73%  (p=0.008 n=5+5)

name          old alloc/op   new alloc/op   delta
Dump/256-4      4.00kB ± 0%    1.39kB ± 0%  -65.20%  (p=0.008 n=5+5)
Dump/1024-4     16.2kB ± 0%     5.5kB ± 0%  -66.04%  (p=0.008 n=5+5)
Dump/4096-4     63.9kB ± 0%    20.6kB ± 0%  -67.78%  (p=0.008 n=5+5)
Dump/16384-4     265kB ± 0%      82kB ± 0%  -69.00%  (p=0.008 n=5+5)

name          old allocs/op  new allocs/op  delta
Dump/256-4        7.00 ± 0%      3.00 ± 0%  -57.14%  (p=0.008 n=5+5)
Dump/1024-4       9.00 ± 0%      3.00 ± 0%  -66.67%  (p=0.008 n=5+5)
Dump/4096-4       11.0 ± 0%       3.0 ± 0%  -72.73%  (p=0.008 n=5+5)
Dump/16384-4      13.0 ± 0%       3.0 ± 0%  -76.92%  (p=0.008 n=5+5)

Change-Id: I0a0d6de315b979142b05e333880da8a5e52b12ef
Reviewed-on: https://go-review.googlesource.com/116495
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/hex/hex.go

@@ -6,10 +6,10 @@
 package hex
 
 import (
-	"bytes"
 	"errors"
 	"fmt"
 	"io"
+	"strings"
 )
 
 const hextable = "0123456789abcdef"
@@ -116,7 +116,16 @@ func DecodeString(s string) ([]byte, error) {
 // Dump returns a string that contains a hex dump of the given data. The format
 // of the hex dump matches the output of `hexdump -C` on the command line.
 func Dump(data []byte) string {
-	var buf bytes.Buffer
+	if len(data) == 0 {
+		return ""
+	}
+
+	var buf strings.Builder
+	// Dumper will write 79 bytes per complete 16 byte chunk, and at least
+	// 64 bytes for whatever remains. Round the allocation up, since only a
+	// maximum of 15 bytes will be wasted.
+	buf.Grow((1 + ((len(data) - 1) / 16)) * 79)
+
 	dumper := Dumper(&buf)
 	dumper.Write(data)
 	dumper.Close()

src/encoding/hex/hex_test.go

@@ -248,3 +248,16 @@ func BenchmarkEncode(b *testing.B) {
 		})
 	}
 }
+
+func BenchmarkDump(b *testing.B) {
+	for _, size := range []int{256, 1024, 4096, 16384} {
+		src := bytes.Repeat([]byte{2, 3, 5, 7, 9, 11, 13, 17}, size/8)
+		sink = make([]byte, 2*size)
+
+		b.Run(fmt.Sprintf("%v", size), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				Dump(src)
+			}
+		})
+	}
+}