Improve the performance of bit set searches with a compiler intrinsic.

runtime/platform/utils_win.h

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#ifndef PLATFORM_UTILS_WIN_H_
+#define PLATFORM_UTILS_WIN_H_
+
+#include <intrin.h>
+
+namespace dart {
+
+inline int Utils::CountTrailingZeros(uint32_t x) {
+  unsigned long result;  // NOLINT
+  _BitScanReverse(&result, x);
+  return reinterpret_cast<int>(result);
+};
+
+inline int Utils::CountTrailingZeros(uint64_t x) {
+  unsigned long result;  // NOLINT
+  _BitScanReverse64(&result, x);
+  return reinterpret_cast<int>(result);
+};
+
+}  // namespace dart
+
+#endif  // PLATFORM_UTILS_WIN_H_