go.image/vp8: use branch-free abs
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
diff --git a/vp8/filter.go b/vp8/filter.go
index ea6ed56..e34a811 100644
--- a/vp8/filter.go
+++ b/vp8/filter.go
@@ -227,11 +227,19 @@
}
}
+// 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 {
