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go.image/vp8: use branch-free abs

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This change seems to restore performance to
the level before CL 108140045.

benchmark                         old ns/op    new ns/op    delta
BenchmarkDecodeVP8SimpleFilter       796416       784394   -1.51%
BenchmarkDecodeVP8NormalFilter      4931138      4693078   -4.83%
BenchmarkDecodeVP8L                 7820030      7821796   +0.02%

LGTM=nigeltao, r
R=nigeltao, r
CC=golang-codereviews
https://golang.org/cl/107340047
This commit is contained in:
Rui Ueyama 2014-06-30 10:33:37 +10:00 committed by Nigel Tao
parent f8a86727c2
commit dc115882f1
1 changed files with 12 additions and 4 deletions
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16
vp8/filter.go
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@ -227,11 +227,19 @@ func (d *Decoder) computeFilterParams() {
}
}
// intSize is either 32 or 64.
const intSize = 32 << (^uint(0) >> 63)
func abs(x int) int {
if x < 0 {
return -x
}
return x
// m := -1 if x < 0. m := 0 otherwise.
m := x >> (intSize - 1)
// In two's complement representation, the negative number
// of any number (except the smallest one) can be computed
// by flipping all the bits and add 1. This is faster than
// code with a branch.
// See Hacker's Delight, section 2-4.
return (x ^ m) - m
}
func clamp15(x int) int {