encoding/hex: don't over allocate memory in DecodeString

Change-Id: Ie9137639e2b39690f2246948d7ff58f09fc84e68

Author: mateusz834

src/encoding/hex/hex.go

@@ -136,11 +136,9 @@ func EncodeToString(src []byte) string {
 // If the input is malformed, DecodeString returns
 // the bytes decoded before the error.
 func DecodeString(s string) ([]byte, error) {
-	src := []byte(s)
-	// We can use the source slice itself as the destination
-	// because the decode loop increments by one and then the 'seen' byte is not used anymore.
-	n, err := Decode(src, src)
-	return src[:n], err
+	dst := make([]byte, DecodedLen(len(s)))
+	n, err := Decode(dst, []byte(s))
+	return dst[:n], err
 }
 
 // Dump returns a string that contains a hex dump of the given data. The format

src/encoding/hex/hex_test.go

@@ -275,6 +275,20 @@ func BenchmarkDecode(b *testing.B) {
 	}
 }
 
+func BenchmarkDecodeString(b *testing.B) {
+	for _, size := range []int{256, 1024, 4096, 16384} {
+		src := strings.Repeat("2b744faa", 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++ {
+				sink, _ = DecodeString(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)