--- /dev/null
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !amd64 appengine
+// +build !gccgo
+
+package intsets
+
+import "runtime"
+
+// We compared three algorithms---Hacker's Delight, table lookup,
+// and AMD64's SSE4.1 hardware POPCNT---on a 2.67GHz Xeon X5550.
+//
+// % GOARCH=amd64 go test -run=NONE -bench=Popcount
+// POPCNT 5.12 ns/op
+// Table 8.53 ns/op
+// HackersDelight 9.96 ns/op
+//
+// % GOARCH=386 go test -run=NONE -bench=Popcount
+// Table 10.4 ns/op
+// HackersDelight 5.23 ns/op
+//
+// (AMD64's ABM1 hardware supports ntz and nlz too,
+// but they aren't critical.)
+
+// popcount returns the population count (number of set bits) of x.
+func popcount(x word) int {
+ if runtime.GOARCH == "386" {
+ return popcountHD(uint32(x))
+ }
+ return popcountTable(x)
+}