encoding/hex: simplify decoder arithmetic

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>

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
 	}

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)