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image/jpeg: make decoder.receiveExtend branchless

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On linux/amd64:

name                 old speed      new speed      delta
DecodeBaseline-8     76.4MB/s ± 0%  84.3MB/s ± 0%  +10.38%  (p=0.008 n=5+5)
DecodeProgressive-8  51.0MB/s ± 1%  52.6MB/s ± 0%   +3.20%  (p=0.008 n=5+5)

Thanks to David Le Corfec for the suggestion.

Updates #24499

Change-Id: I749102ff0b50044dfd6a73172a1aa03f89ad97bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/750900
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Nigel Tao <nigeltao@google.com>
This commit is contained in:
Nigel Tao
2026-03-05 19:10:23 +11:00
parent 1a8f9d8141
commit 94a89c61e4
1 changed files with 22 additions and 3 deletions
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25
src/image/jpeg/huffman.go
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@@ -70,6 +70,16 @@ func (d *decoder) ensureNBits(n int32) error {
// receiveExtend is the composition of RECEIVE and EXTEND, specified in section
// F.2.2.1.
//
// It returns the signed integer that's encoded in t bits, where t < 16. The
// possible return values are:
//
// - t == 0: 0
// - t == 1: -1, +1
// - t == 2: -3, -2, +2, +3
// - t == 3: -7, -6, -5, -4, +4, +5, +6, +7
// - ...
// - t == 15: -32767, -32766, ..., -16384, +16384, ..., +32766, +32767
func (d *decoder) receiveExtend(t uint8) (int32, error) {
if d.bits.n < int32(t) {
if err := d.ensureNBits(int32(t)); err != nil {
@@ -80,9 +90,18 @@ func (d *decoder) receiveExtend(t uint8) (int32, error) {
d.bits.m >>= t
s := int32(1) << t
x := int32(d.bits.a>>uint8(d.bits.n)) & (s - 1)
if x < s>>1 {
x += ((-1) << t) + 1
}
// This adjustment, assuming two's complement, is a branchless equivalent of:
//
// if x < s>>1 {
// x += ((-1) << t) + 1
// }
//
// sign is either -1 or 0, depending on whether x is in the low or high
// half of the range 0 .. 1<<t.
sign := (x >> (t - 1)) - 1
x += sign & (((-1) << t) + 1)
return x, nil
}