encoding/hex: simplify decoder arithmetic

mvdan · mvdan · commit aa5165d62cf6 · 2019-03-02T16:48:05.000Z
Remove all multiplications and divisions from the main decoding loop. name old time/op new time/op delta Decode/256-8 323ns ± 0% 293ns ± 0% -9.29% (p=0.000 n=5+4) Decode/1024-8 1.26µs ± 0% 1.14µs ± 0% -9.48% (p=0.000 n=6+5) Decode/4096-8 4.99µs ± 0% 4.51µs ± 0% -9.55% (p=0.002 n=6+6) Decode/16384-8 20.0µs ± 0% 18.1µs ± 0% -9.54% (p=0.002 n=6+6) name old speed new speed delta Decode/256-8 791MB/s ± 0% 872MB/s ± 0% +10.34% (p=0.002 n=6+6) Decode/1024-8 814MB/s ± 0% 899MB/s ± 0% +10.48% (p=0.004 n=6+5) Decode/4096-8 821MB/s ± 0% 908MB/s ± 0% +10.55% (p=0.002 n=6+6) Decode/16384-8 821MB/s ± 0% 908MB/s ± 0% +10.54% (p=0.002 n=6+6) Change-Id: Ie9f91242ce04c130a77c1184379e3b9de38fe713 Reviewed-on: https://go-review.googlesource.com/c/151199 Run-TryBot: Daniel Martí <mvdan@mvdan.cc> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/src/encoding/hex/hex.go b/src/encoding/hex/hex.go
@@ -55,23 +55,24 @@ func DecodedLen(x int) int { return x / 2 }
 // If the input is malformed, Decode returns the number
 // of bytes decoded before the error.
 func Decode(dst, src []byte) (int, error) {
-	var i int
-	for i = 0; i < len(src)/2; i++ {
-		a, ok := fromHexChar(src[i*2])
+	i, j := 0, 1
+	for ; j < len(src); j += 2 {
+		a, ok := fromHexChar(src[j-1])
 		if !ok {
-			return i, InvalidByteError(src[i*2])
+			return i, InvalidByteError(src[j-1])
 		}
-		b, ok := fromHexChar(src[i*2+1])
+		b, ok := fromHexChar(src[j])
 		if !ok {
-			return i, InvalidByteError(src[i*2+1])
+			return i, InvalidByteError(src[j])
 		}
 		dst[i] = (a << 4) | b
+		i++
 	}
 	if len(src)%2 == 1 {
 		// Check for invalid char before reporting bad length,
 		// since the invalid char (if present) is an earlier problem.
-		if _, ok := fromHexChar(src[i*2]); !ok {
-			return i, InvalidByteError(src[i*2])
+		if _, ok := fromHexChar(src[j-1]); !ok {
+			return i, InvalidByteError(src[j-1])
 		}
 		return i, ErrLength
 	}
diff --git a/src/encoding/hex/hex_test.go b/src/encoding/hex/hex_test.go
@@ -249,6 +249,20 @@ func BenchmarkEncode(b *testing.B) {
 	}
 }
 
+func BenchmarkDecode(b *testing.B) {
+	for _, size := range []int{256, 1024, 4096, 16384} {
+		src := bytes.Repeat([]byte{'2', 'b', '7', '4', '4', 'f', 'a', 'a'}, size/8)
+		sink = make([]byte, size/2)
+
+		b.Run(fmt.Sprintf("%v", size), func(b *testing.B) {
+			b.SetBytes(int64(size))
+			for i := 0; i < b.N; i++ {
+				Decode(sink, src)
+			}
+		})
+	}
+}
+
 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)
