encoding/gob: make integers encoding faster

name                old time/op  new time/op  delta
EncodeInt32Slice-4  14.6µs ± 2%  12.2µs ± 1%  -16.65%  (p=0.000 n=19+18)

Change-Id: I078a171f1633ff81d7e3f981dc9a398309ecb2c0
Reviewed-on: https://go-review.googlesource.com/38269Reviewed-by: Rob Pike <r@golang.org>

src/encoding/gob/encode.go

@@ -8,7 +8,9 @@ package gob
 
 import (
 	"encoding"
+	"encoding/binary"
 	"math"
+	"math/bits"
 	"reflect"
 	"sync"
 )
@@ -107,14 +109,12 @@ func (state *encoderState) encodeUint(x uint64) {
 		state.b.WriteByte(uint8(x))
 		return
 	}
-	i := uint64Size
-	for x > 0 {
-		state.buf[i] = uint8(x)
-		x >>= 8
-		i--
-	}
-	state.buf[i] = uint8(i - uint64Size) // = loop count, negated
-	state.b.Write(state.buf[i : uint64Size+1])
+
+	binary.BigEndian.PutUint64(state.buf[1:], x)
+	bc := bits.LeadingZeros64(x) >> 3      // 8 - bytelen(x)
+	state.buf[bc] = uint8(bc - uint64Size) // and then we subtract 8 to get -bytelen(x)
+
+	state.b.Write(state.buf[bc : uint64Size+1])
 }
 
 // encodeInt writes an encoded signed integer to state.w.

src/encoding/gob/timing_test.go

@@ -171,7 +171,7 @@ func BenchmarkEncodeInt32Slice(b *testing.B) {
 	enc := NewEncoder(&buf)
 	a := make([]int32, 1000)
 	for i := range a {
-		a[i] = 1234
+		a[i] = int32(i * 100)
 	}
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {