image/jpeg: don't call ensureNBits unless we have to.

benchmark                     old ns/op    new ns/op    delta
BenchmarkDecodeBaseline         3155638      2783998  -11.78%
BenchmarkDecodeProgressive      4008088      3660310   -8.68%

R=r, bradfitz
CC=golang-dev
https://golang.org/cl/6775072

diff --git a/src/pkg/image/jpeg/huffman.go b/src/pkg/image/jpeg/huffman.go
index 9393932aafb11dc748ba8509adb2319c84161b8c..2fc64ade546dd82db9a4a0eb7c5557391de76533 100644 (file)
--- a/src/pkg/image/jpeg/huffman.go
+++ b/src/pkg/image/jpeg/huffman.go
@@ -62,9 +62,10 @@ func (d *decoder) ensureNBits(n int) error {
 
 // The composition of RECEIVE and EXTEND, specified in section F.2.2.1.
 func (d *decoder) receiveExtend(t uint8) (int32, error) {
-       err := d.ensureNBits(int(t))
-       if err != nil {
-               return 0, err
+       if d.b.n < int(t) {
+               if err := d.ensureNBits(int(t)); err != nil {
+                       return 0, err
+               }
        }
        d.b.n -= int(t)
        d.b.m >>= t
@@ -168,9 +169,10 @@ func (d *decoder) decodeHuffman(h *huffman) (uint8, error) {
                return 0, FormatError("uninitialized Huffman table")
        }
        for i, code := 0, 0; i < maxCodeLength; i++ {
-               err := d.ensureNBits(1)
-               if err != nil {
-                       return 0, err
+               if d.b.n == 0 {
+                       if err := d.ensureNBits(1); err != nil {
+                               return 0, err
+                       }
                }
                if d.b.a&d.b.m != 0 {
                        code |= 1
@@ -187,8 +189,7 @@ func (d *decoder) decodeHuffman(h *huffman) (uint8, error) {
 
 func (d *decoder) decodeBit() (bool, error) {
        if d.b.n == 0 {
-               err := d.ensureNBits(1)
-               if err != nil {
+               if err := d.ensureNBits(1); err != nil {
                        return false, err
                }
        }
@@ -199,9 +200,10 @@ func (d *decoder) decodeBit() (bool, error) {
 }
 
 func (d *decoder) decodeBits(n int) (uint32, error) {
-       err := d.ensureNBits(n)
-       if err != nil {
-               return 0, err
+       if d.b.n < n {
+               if err := d.ensureNBits(n); err != nil {
+                       return 0, err
+               }
        }
        ret := d.b.a >> uint(d.b.n-n)
        ret &= (1 << uint(n)) - 1