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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
This commit is contained in:
Nigel Tao 2012-10-31 10:02:11 +11:00
parent d12a7d39d1
commit ad487dad75
1 changed files with 13 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

18
src/pkg/image/jpeg/huffman.go
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@ -62,10 +62,11 @@ func (d *decoder) ensureNBits(n int) error {
// The composition of RECEIVE and EXTEND, specified in section F.2.2.1. // The composition of RECEIVE and EXTEND, specified in section F.2.2.1.
func (d *decoder) receiveExtend(t uint8) (int32, error) { func (d *decoder) receiveExtend(t uint8) (int32, error) {
err := d.ensureNBits(int(t)) if d.b.n < int(t) {
if err != nil { if err := d.ensureNBits(int(t)); err != nil {
return 0, err return 0, err
} }
}
d.b.n -= int(t) d.b.n -= int(t)
d.b.m >>= t d.b.m >>= t
s := int32(1) << t s := int32(1) << t
@ -168,10 +169,11 @@ func (d *decoder) decodeHuffman(h *huffman) (uint8, error) {
return 0, FormatError("uninitialized Huffman table") return 0, FormatError("uninitialized Huffman table")
} }
for i, code := 0, 0; i < maxCodeLength; i++ { for i, code := 0, 0; i < maxCodeLength; i++ {
err := d.ensureNBits(1) if d.b.n == 0 {
if err != nil { if err := d.ensureNBits(1); err != nil {
return 0, err return 0, err
} }
}
if d.b.a&d.b.m != 0 { if d.b.a&d.b.m != 0 {
code |= 1 code |= 1
} }
@ -187,8 +189,7 @@ func (d *decoder) decodeHuffman(h *huffman) (uint8, error) {
func (d *decoder) decodeBit() (bool, error) { func (d *decoder) decodeBit() (bool, error) {
if d.b.n == 0 { if d.b.n == 0 {
err := d.ensureNBits(1) if err := d.ensureNBits(1); err != nil {
if err != nil {
return false, err return false, err
} }
} }
@ -199,10 +200,11 @@ func (d *decoder) decodeBit() (bool, error) {
} }
func (d *decoder) decodeBits(n int) (uint32, error) { func (d *decoder) decodeBits(n int) (uint32, error) {
err := d.ensureNBits(n) if d.b.n < n {
if err != nil { if err := d.ensureNBits(n); err != nil {
return 0, err return 0, err
} }
}
ret := d.b.a >> uint(d.b.n-n) ret := d.b.a >> uint(d.b.n-n)
ret &= (1 << uint(n)) - 1 ret &= (1 << uint(n)) - 1
d.b.n -= n d.b.n -= n