Newer version of bitCount()

sdboyer · sdboyer · commit 1cc1cec4bcc6 · 2015-09-01T11:42:57.000-04:00
This updates to "version 3" of Jörg Arndt's 32-bit bitcount function, according to http://www.jjj.de/bitwizardry/files/bitcount.h
diff --git a/trie/ctrie/ctrie.go b/trie/ctrie/ctrie.go
@@ -759,11 +759,11 @@ func flagPos(hashcode uint32, lev uint, bmp uint32) (uint32, uint32) {
 }
 
 func bitCount(x uint32) uint32 {
-	x = ((x >> 1) & 0x55555555) + (x & 0x55555555)
+	x -= (x >> 1) & 0x55555555
 	x = ((x >> 2) & 0x33333333) + (x & 0x33333333)
-	x = ((x >> 4) & 0x0f0f0f0f) + (x & 0x0f0f0f0f)
-	x = ((x >> 8) & 0x00ff00ff) + (x & 0x00ff00ff)
-	return ((x >> 16) & 0x0000ffff) + (x & 0x0000ffff)
+	x = ((x >> 4) + x) & 0x0f0f0f0f
+	x *= 0x01010101
+	return x >> 24
 }
 
 // gcas is a generation-compare-and-swap which has semantics similar to RDCSS,

Original file line number	Diff line number	Diff line change
`@@ -759,11 +759,11 @@ func flagPos(hashcode uint32, lev uint, bmp uint32) (uint32, uint32) {`
`759`	`759`	`}`
`760`	`760`
`761`	`761`	`func bitCount(x uint32) uint32 {`
`762`		`- x = ((x >> 1) & 0x55555555) + (x & 0x55555555)`
	`762`	`+ x -= (x >> 1) & 0x55555555`
`763`	`763`	`x = ((x >> 2) & 0x33333333) + (x & 0x33333333)`
`764`		`- x = ((x >> 4) & 0x0f0f0f0f) + (x & 0x0f0f0f0f)`
`765`		`- x = ((x >> 8) & 0x00ff00ff) + (x & 0x00ff00ff)`
`766`		`- return ((x >> 16) & 0x0000ffff) + (x & 0x0000ffff)`
	`764`	`+ x = ((x >> 4) + x) & 0x0f0f0f0f`
	`765`	`+ x *= 0x01010101`
	`766`	`+ return x >> 24`
`767`	`767`	`}`
`768`	`768`
`769`	`769`	`// gcas is a generation-compare-and-swap which has semantics similar to RDCSS,`