encoding/binary: remove TODO in Write and add benchmarks

Benchmarks:
goos: linux
goarch: amd64
BenchmarkReadSlice1000Uint8s-8           4097088               296 ns/op        3381.06 MB/s
BenchmarkWriteSlice1000Uint8s-8          4372588               271 ns/op        3694.96 MB/s

Change-Id: I5b6ef0da5052e3381ee9c714bbff541c11ed0259
Reviewed-on: https://go-review.googlesource.com/c/go/+/246837
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Alberto Donizetti <alb.donizetti@gmail.com>

src/encoding/binary/binary.go

@@ -306,7 +306,7 @@ func Write(w io.Writer, order ByteOrder, data interface{}) error {
 		case uint8:
 			bs[0] = v
 		case []uint8:
-			bs = v // TODO(josharian): avoid allocating bs in this case?
+			bs = v
 		case *int16:
 			order.PutUint16(bs, uint16(*v))
 		case int16:

src/encoding/binary/binary_test.go

@@ -706,3 +706,27 @@ func BenchmarkWriteSlice1000Float32s(b *testing.B) {
 	}
 	b.StopTimer()
 }
+
+func BenchmarkReadSlice1000Uint8s(b *testing.B) {
+	bsr := &byteSliceReader{}
+	slice := make([]uint8, 1000)
+	buf := make([]byte, len(slice))
+	b.SetBytes(int64(len(buf)))
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		bsr.remain = buf
+		Read(bsr, BigEndian, slice)
+	}
+}
+
+func BenchmarkWriteSlice1000Uint8s(b *testing.B) {
+	slice := make([]uint8, 1000)
+	buf := new(bytes.Buffer)
+	var w io.Writer = buf
+	b.SetBytes(1000)
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		buf.Reset()
+		Write(w, BigEndian, slice)
+	}
+}