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

runtime/platform/utils.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_H_
 #define PLATFORM_UTILS_H_
 
 #include "platform/assert.h"
 #include "platform/globals.h"
 
@@ skipping 63 matching lines @@
   template<typename T>
   static inline T* RoundUp(T* x, intptr_t n) {
     return reinterpret_cast<T*>(RoundUp(reinterpret_cast<uword>(x), n));
   }
 
   static uintptr_t RoundUpToPowerOfTwo(uintptr_t x);
   static int CountOneBits(uint32_t x);
 
   static int HighestBit(int64_t v);
 
+  static int CountLeadingZeros(uword x);
   static int CountTrailingZeros(uword x);
 
   // Computes a hash value for the given string.
   static uint32_t StringHash(const char* data, int length);
 
   // Computes a hash value for the given word.
   static uint32_t WordHash(intptr_t key);
 
   // Check whether an N-bit two's-complement representation can hold value.
   template<typename T>
@@ skipping 110 matching lines @@
 #include "platform/utils_linux.h"
 #elif defined(TARGET_OS_MACOS)
 #include "platform/utils_macos.h"
 #elif defined(TARGET_OS_WINDOWS)
 #include "platform/utils_win.h"
 #else
 #error Unknown target os.
 #endif
 
 #endif  // PLATFORM_UTILS_H_