--- /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,!gccgo
+
+package intsets
+
+func popcnt(x word) int
+func havePOPCNT() bool
+
+var hasPOPCNT = havePOPCNT()
+
+// popcount returns the population count (number of set bits) of x.
+func popcount(x word) int {
+ if hasPOPCNT {
+ return popcnt(x)
+ }
+ return popcountTable(x) // faster than Hacker's Delight
+}