Optimize writeElement.

This commit modifies the writeElement function to have a "fast path" which uses type
assertions for all of the types which btcwire write so the more expensive
reflection-based binary.Write can be avoided.

Also, this changes all cases that were writing raw ShaHash (32-byte) arrays (which
requires a stack copy) instead simply passing the pointer.

The following benchmark results show the results for serializing a block header
after these changes:

Before: BenchmarkWriteBlockHeader         500000              5566 ns/op
After:  BenchmarkWriteBlockHeader        1000000               991 ns/op

This is part of the ongoing effort to optimize serialization as noted in
conformal/btcd#27.

blockheader.go

@@ -107,7 +107,7 @@ func readBlockHeader(r io.Reader, pver uint32, bh *BlockHeader) error {
 // writeBlockHeader writes a bitcoin block header to w.
 func writeBlockHeader(w io.Writer, pver uint32, bh *BlockHeader) error {
 	sec := uint32(bh.Timestamp.Unix())
-	err := writeElements(w, bh.Version, bh.PrevBlock, bh.MerkleRoot,
+	err := writeElements(w, bh.Version, &bh.PrevBlock, &bh.MerkleRoot,
 		sec, bh.Bits, bh.Nonce)
 	if err != nil {
 		return err

common.go

@@ -17,9 +17,7 @@ import (
 const maxVarIntPayload = 9
 
 // readElement reads the next sequence of bytes from r using little endian
-// depending on the concrete type of element pointed to.  It also accepts a
+// depending on the concrete type of element pointed to.
-// scratch buffer that is used for the primitive values rather than creating
-// a new buffer on every call.
 func readElement(r io.Reader, element interface{}) error {
 	var scratch [8]byte
 
@@ -140,6 +138,106 @@ func readElements(r io.Reader, elements ...interface{}) error {
 
 // writeElement writes the little endian representation of element to w.
 func writeElement(w io.Writer, element interface{}) error {
+	var scratch [8]byte
+
+	// Attempt to read the element based on the concrete type via fast
+	// type assertions first.
+	switch e := element.(type) {
+	case int32:
+		b := scratch[0:4]
+		binary.LittleEndian.PutUint32(b, uint32(e))
+		_, err := w.Write(b)
+		if err != nil {
+			return err
+		}
+		return nil
+
+	case uint32:
+		b := scratch[0:4]
+		binary.LittleEndian.PutUint32(b, e)
+		_, err := w.Write(b)
+		if err != nil {
+			return err
+		}
+		return nil
+
+	case int64:
+		b := scratch[0:8]
+		binary.LittleEndian.PutUint64(b, uint64(e))
+		_, err := w.Write(b)
+		if err != nil {
+			return err
+		}
+		return nil
+
+	case uint64:
+		b := scratch[0:8]
+		binary.LittleEndian.PutUint64(b, e)
+		_, err := w.Write(b)
+		if err != nil {
+			return err
+		}
+		return nil
+
+	// Message header checksum.
+	case [4]byte:
+		_, err := w.Write(e[:])
+		if err != nil {
+			return err
+		}
+		return nil
+
+	// Message header command.
+	case [commandSize]uint8:
+		_, err := w.Write(e[:])
+		if err != nil {
+			return err
+		}
+		return nil
+
+	// IP address.
+	case [16]byte:
+		_, err := w.Write(e[:])
+		if err != nil {
+			return err
+		}
+		return nil
+
+	case *ShaHash:
+		_, err := w.Write(e[:])
+		if err != nil {
+			return err
+		}
+		return nil
+
+	case ServiceFlag:
+		b := scratch[0:8]
+		binary.LittleEndian.PutUint64(b, uint64(e))
+		_, err := w.Write(b)
+		if err != nil {
+			return err
+		}
+		return nil
+
+	case InvType:
+		b := scratch[0:4]
+		binary.LittleEndian.PutUint32(b, uint32(e))
+		_, err := w.Write(b)
+		if err != nil {
+			return err
+		}
+		return nil
+
+	case BitcoinNet:
+		b := scratch[0:4]
+		binary.LittleEndian.PutUint32(b, uint32(e))
+		_, err := w.Write(b)
+		if err != nil {
+			return err
+		}
+		return nil
+	}
+
 	return binary.Write(w, binary.LittleEndian, element)
 }
 

invvect.go

@@ -72,7 +72,7 @@ func readInvVect(r io.Reader, pver uint32, iv *InvVect) error {
 
 // writeInvVect serializes an InvVect to w depending on the protocol version.
 func writeInvVect(w io.Writer, pver uint32, iv *InvVect) error {
-	err := writeElements(w, iv.Type, iv.Hash)
+	err := writeElements(w, iv.Type, &iv.Hash)
 	if err != nil {
 		return err
 	}

msggetblocks.go

@@ -110,7 +110,7 @@ func (msg *MsgGetBlocks) BtcEncode(w io.Writer, pver uint32) error {
 		}
 	}
 
-	err = writeElement(w, msg.HashStop)
+	err = writeElement(w, &msg.HashStop)
 	if err != nil {
 		return err
 	}

msggetheaders.go

@@ -108,7 +108,7 @@ func (msg *MsgGetHeaders) BtcEncode(w io.Writer, pver uint32) error {
 		}
 	}
 
-	err = writeElement(w, msg.HashStop)
+	err = writeElement(w, &msg.HashStop)
 	if err != nil {
 		return err
 	}