container/intsets: use fast popcount with gccgo

Use the compiler intrinsic __builtin_popcount.

Change-Id: I58286fbcf66d1068390ea9caff2f98b8fe244c2d
Reviewed-on: https://go-review.googlesource.com/16831
Reviewed-by: Andrew Gerrand <adg@golang.org>

container/intsets/popcnt_gccgo.go

@@ -0,0 +1,9 @@
+// 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 gccgo
+
+package intsets
+
+func popcount(x word) int

container/intsets/popcnt_gccgo_c.c

@@ -0,0 +1,19 @@
+// 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 gccgo
+
+#include <errno.h>
+#include <stdint.h>
+#include <unistd.h>
+
+#define _STRINGIFY2_(x) #x
+#define _STRINGIFY_(x) _STRINGIFY2_(x)
+#define GOSYM_PREFIX _STRINGIFY_(__USER_LABEL_PREFIX__)
+
+extern intptr_t popcount(uintptr_t x) __asm__(GOSYM_PREFIX GOPKGPATH ".popcount");
+
+intptr_t popcount(uintptr_t x) {
+	return __builtin_popcountl((unsigned long)(x));
+}

container/intsets/popcnt_generic.go

@@ -2,7 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !amd64 appengine gccgo
+// +build !amd64 appengine
+// +build !gccgo
 
 package intsets