code review 5236043: gob: avoid one allocation for every message written.

src/pkg/gob/encoder.go

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 package gob
 
 import (
         "bytes"
         "io"
         "os"
@@ skipping 53 matching lines @@
         }
 }
 
 // writeMessage sends the data item preceded by a unsigned count of its length.
 func (enc *Encoder) writeMessage(w io.Writer, b *bytes.Buffer) {
         // Space has been reserved for the length at the head of the message.
         // This is a little dirty: we grab the slice from the bytes.Buffer and massage
         // it by hand.
         message := b.Bytes()
         messageLen := len(message) - maxLength
-        // Encode the length at the beginning of the buffer.
+        // Encode the length.
         enc.countState.b.Reset()
         enc.countState.encodeUint(uint64(messageLen))
-        // Copy the message to the buffer.
-        countLen := copy(message, enc.countState.b.Bytes())
-        // Close the gap
-        if countLen < maxLength {
-                copy(message[countLen:], message[maxLength:])
-        }
+        // Copy the length to be a prefix of the message.
+        offset := maxLength - enc.countState.b.Len()
+        copy(message[offset:], enc.countState.b.Bytes())
         // Write the data.
-        _, err := w.Write(message[:countLen+len(message)-maxLength])
+        _, err := w.Write(message[offset:])
         // Drain the buffer and restore the space at the front for the count of the next message.
         b.Reset()
         b.Write(spaceForLength)
         if err != nil {
                 enc.setError(err)
         }
 }
 
 // sendActualType sends the requested type, without further investigation, unless
 // it's been sent before.
@@ skipping 150 matching lines @@
 
         // Encode the object.
         enc.encode(state.b, value, ut)
         if enc.err == nil {
                 enc.writeMessage(enc.writer(), state.b)
         }
 
         enc.freeEncoderState(state)
         return enc.err
 }