encoding/hex: simplify encoder arithmetic

Two additions are faster than two multiplications and one addition. The
code seems simpler to me too, as it's more obvious that we advance two
destination bytes for each source byte.

name            old time/op  new time/op  delta
Encode/256-8     374ns ± 0%   331ns ± 0%  -11.44%  (p=0.008 n=5+5)
Encode/1024-8   1.47µs ± 0%  1.29µs ± 0%  -11.89%  (p=0.004 n=6+5)
Encode/4096-8   5.85µs ± 1%  5.15µs ± 0%  -11.89%  (p=0.004 n=6+5)
Encode/16384-8  23.3µs ± 0%  20.6µs ± 0%  -11.68%  (p=0.004 n=6+5)

Change-Id: Iabc63616c1d9fded55fa668ff41dd49efeaa2ea4
Reviewed-on: https://go-review.googlesource.com/c/go/+/151198
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: roger peppe <rogpeppe@gmail.com>

diff --git a/src/encoding/hex/hex.go b/src/encoding/hex/hex.go
index 7675de9bd9c90267b1aea62289b382c31d1ef6d4..fbba78ffd215a79cdfabea983d5e29b4bf9f7a54 100644 (file)
--- a/src/encoding/hex/hex.go
+++ b/src/encoding/hex/hex.go
@@ -23,11 +23,12 @@ func EncodedLen(n int) int { return n * 2 }
 // of bytes written to dst, but this value is always EncodedLen(len(src)).
 // Encode implements hexadecimal encoding.
 func Encode(dst, src []byte) int {
-       for i, v := range src {
-               dst[i*2] = hextable[v>>4]
-               dst[i*2+1] = hextable[v&0x0f]
+       j := 0
+       for _, v := range src {
+               dst[j] = hextable[v>>4]
+               dst[j+1] = hextable[v&0x0f]
+               j += 2
        }
-
        return len(src) * 2
 }
 

diff --git a/src/encoding/hex/hex_test.go b/src/encoding/hex/hex_test.go
index ba703cf1c1f063f3b697238f07981c93d241cb56..dbb00b94caca9d6b8d3b9e2914a9d3bdc39d4091 100644 (file)
--- a/src/encoding/hex/hex_test.go
+++ b/src/encoding/hex/hex_test.go
@@ -242,6 +242,7 @@ func BenchmarkEncode(b *testing.B) {
                sink = make([]byte, 2*size)
 
                b.Run(fmt.Sprintf("%v", size), func(b *testing.B) {
+                       b.SetBytes(int64(size))
                        for i := 0; i < b.N; i++ {
                                Encode(sink, src)
                        }
@@ -269,6 +270,7 @@ func BenchmarkDump(b *testing.B) {
                sink = make([]byte, 2*size)
 
                b.Run(fmt.Sprintf("%v", size), func(b *testing.B) {
+                       b.SetBytes(int64(size))
                        for i := 0; i < b.N; i++ {
                                Dump(src)
                        }