encoding/binary: add a non-reflect fast path for Read

before/after:
binary.BenchmarkRead  200000     10860 ns/op
binary.BenchmarkRead  500000      2846 ns/op

R=rsc
CC=golang-dev
https://golang.org/cl/4547062

src/pkg/encoding/binary/binary.go

@@ -125,6 +125,54 @@ func (bigEndian) GoString() string { return "binary.BigEndian" }
 // Bytes read from r are decoded using the specified byte order
 // and written to successive fields of the data.
 func Read(r io.Reader, order ByteOrder, data interface{}) os.Error {
+	// Fast path for basic types
+	var n int
+	switch data.(type) {
+	case *int8:
+		n = 1
+	case *uint8:
+		n = 1
+	case *int16:
+		n = 2
+	case *uint16:
+		n = 2
+	case *int32:
+		n = 4
+	case *uint32:
+		n = 4
+	case *int64:
+		n = 8
+	case *uint64:
+		n = 8
+	}
+	if n != 0 {
+		var buf [8]byte
+		bs := buf[:n]
+		if _, err := io.ReadFull(r, bs); err != nil {
+			return err
+		}
+		switch v := data.(type) {
+		case *int8:
+			*v = int8(buf[0])
+		case *uint8:
+			*v = buf[0]
+		case *int16:
+			*v = int16(order.Uint16(bs))
+		case *uint16:
+			*v = order.Uint16(bs)
+		case *int32:
+			*v = int32(order.Uint32(bs))
+		case *uint32:
+			*v = order.Uint32(bs)
+		case *int64:
+			*v = int64(order.Uint64(bs))
+		case *uint64:
+			*v = order.Uint64(bs)
+		}
+		return nil
+	}
+
+	// Fallback to reflect-based.
 	var v reflect.Value
 	switch d := reflect.ValueOf(data); d.Kind() {
 	case reflect.Ptr:

src/pkg/encoding/binary/binary_test.go

@@ -5,6 +5,7 @@
 package binary
 
 import (
+	"io"
 	"os"
 	"bytes"
 	"math"
@@ -160,3 +161,43 @@ func TestWriteT(t *testing.T) {
 		}
 	}
 }
+
+type byteSliceReader struct {
+	remain []byte
+}
+
+func (br *byteSliceReader) Read(p []byte) (int, os.Error) {
+	n := copy(p, br.remain)
+	br.remain = br.remain[n:]
+	return n, nil
+}
+
+func BenchmarkRead(b *testing.B) {
+	var ls Struct
+	bsr := &byteSliceReader{}
+	var r io.Reader = bsr
+
+	for i := 0; i < b.N; i++ {
+		bsr.remain = big
+		Read(r, BigEndian, &ls.Int8)
+		Read(r, BigEndian, &ls.Int16)
+		Read(r, BigEndian, &ls.Int32)
+		Read(r, BigEndian, &ls.Int64)
+		Read(r, BigEndian, &ls.Uint8)
+		Read(r, BigEndian, &ls.Uint16)
+		Read(r, BigEndian, &ls.Uint32)
+		Read(r, BigEndian, &ls.Uint64)
+	}
+
+	want := s
+	want.Float32 = 0
+	want.Float64 = 0
+	want.Complex64 = 0
+	want.Complex128 = 0
+	for i := range want.Array {
+		want.Array[i] = 0
+	}
+	if !reflect.DeepEqual(ls, want) {
+		panic("no match")
+	}
+}