base64: don't reallocate/copy []byte <-> string

Converting []byte to string and vice versa allocates and copies the
buffer, which is not really needed and has a significant performance
hit.

benchmark                        old MB/s     new MB/s     speedup
BenchmarkEncodeToString-8        608.04       678.24       1.12x
BenchmarkDecodeString/2-8        93.07        111.61       1.20x
BenchmarkDecodeString/4-8        160.39       183.22       1.14x
BenchmarkDecodeString/8-8        210.65       249.11       1.18x
BenchmarkDecodeString/64-8       476.13       653.02       1.37x
BenchmarkDecodeString/8192-8     837.51       950.17       1.13x

Author: kirillx

src/encoding/base64/base64.go

@@ -8,7 +8,9 @@ package base64
 import (
 	"encoding/binary"
 	"io"
+	"reflect"
 	"strconv"
+	"unsafe"
 )
 
 /*
@@ -177,7 +179,8 @@ func (enc *Encoding) Encode(dst, src []byte) {
 func (enc *Encoding) EncodeToString(src []byte) string {
 	buf := make([]byte, enc.EncodedLen(len(src)))
 	enc.Encode(buf, src)
-	return string(buf)
+
+	return *(*string)(unsafe.Pointer(&buf))
 }
 
 type encoder struct {
@@ -382,7 +385,12 @@ func (enc *Encoding) decodeQuantum(dst, src []byte, si int) (nsi, n int, err err
 // DecodeString returns the bytes represented by the base64 string s.
 func (enc *Encoding) DecodeString(s string) ([]byte, error) {
 	dbuf := make([]byte, enc.DecodedLen(len(s)))
-	n, err := enc.Decode(dbuf, []byte(s))
+
+	h := (*reflect.StringHeader)(unsafe.Pointer(&s))
+	shdr := reflect.SliceHeader{Data: h.Data, Len: h.Len, Cap: h.Len}
+	sbuf := *(*[]byte)(unsafe.Pointer(&shdr))
+
+	n, err := enc.Decode(dbuf, sbuf)
 	return dbuf[:n], err
 }