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

runtime/platform/utils_macos.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_MACOS_H_
 #define PLATFORM_UTILS_MACOS_H_
 
 #include <libkern/OSByteOrder.h>  // NOLINT
 
 namespace dart {
 
+inline int Utils::CountLeadingZeros(uword x) {
+#if defined(ARCH_IS_32_BIT)
+  return __builtin_clzl(x);
+#elif defined(ARCH_IS_64_BIT)
+  return __builtin_clzll(x);
+#else
+#error Architecture is not 32-bit or 64-bit.
+#endif
+}
+
+
 inline int Utils::CountTrailingZeros(uword x) {
 #if defined(ARCH_IS_32_BIT)
   return __builtin_ctzl(x);
 #elif defined(ARCH_IS_64_BIT)
   return __builtin_ctzll(x);
 #else
 #error Architecture is not 32-bit or 64-bit.
 #endif
-};
+}
 
 
 inline uint16_t Utils::HostToBigEndian16(uint16_t value) {
   return OSSwapHostToBigInt16(value);
 }
 
 
 inline uint32_t Utils::HostToBigEndian32(uint32_t value) {
   return OSSwapHostToBigInt32(value);
 }
@@ skipping 14 matching lines @@
 }
 
 
 inline uint64_t Utils::HostToLittleEndian64(uint64_t value) {
   return OSSwapHostToLittleInt64(value);
 }
 
 }  // namespace dart
 
 #endif  // PLATFORM_UTILS_MACOS_H_