Speed up freelist by using intrinsics to find last set bit.

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>
 #include <stdlib.h>
 
 namespace dart {
 
+inline int Utils::CountLeadingZeros(uword x) {
+  unsigned long result;  // NOLINT
+#if defined(ARCH_IS_32_BIT)
+  _BitScanForward(&result, x);
+#elif defined(ARCH_IS_64_BIT)
+  _BitScanForward64(&result, x);
+#else
+#error Architecture is not 32-bit or 64-bit.
+#endif
+  return static_cast<int>(result);
+}
+
+
 inline int Utils::CountTrailingZeros(uword x) {
   unsigned long result;  // NOLINT
 #if defined(ARCH_IS_32_BIT)
   _BitScanReverse(&result, x);
 #elif defined(ARCH_IS_64_BIT)
   _BitScanReverse64(&result, x);
 #else
 #error Architecture is not 32-bit or 64-bit.
 #endif
   return static_cast<int>(result);
-};
+}
 
 
 inline uint16_t Utils::HostToBigEndian16(uint16_t value) {
   return _byteswap_ushort(value);
 }
 
 
 inline uint32_t Utils::HostToBigEndian32(uint32_t value) {
   return _byteswap_ulong(value);
 }
@@ skipping 14 matching lines @@
 }
 
 
 inline uint64_t Utils::HostToLittleEndian64(uint64_t value) {
   return value;
 }
 
 }  // namespace dart
 
 #endif  // PLATFORM_UTILS_WIN_H_