Fix -Wstrict-aliasing warnings.

src/heap.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1275 matching lines @@
     roots_[kStoreBufferTopRootIndex] = reinterpret_cast<Smi*>(top);
   }
 
   // Update the next script id.
   inline void SetLastScriptId(Object* last_script_id);
 
   // Generated code can embed this address to get access to the roots.
   Object** roots_array_start() { return roots_; }
 
   Address* store_buffer_top_address() {
-    return reinterpret_cast<Address*>(&roots_[kStoreBufferTopRootIndex]);
+    // Avoid type-punning compiler warnings.
+    union {
+      Object** in;
+      Address* out;
+    } u;
+    u.in = &roots_[kStoreBufferTopRootIndex];
+    return u.out;
   }
 
   // Get address of native contexts list for serialization support.
   Object** native_contexts_list_address() {
     return &native_contexts_list_;
   }
 
 #ifdef VERIFY_HEAP
   // Verify the heap is in its normal state before or after a GC.
   void Verify();
@@ skipping 1572 matching lines @@
   AssertNoAllocation no_alloc;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG || LIVE_OBJECT_LIST
 
 } }  // namespace v8::internal
 
 #endif  // V8_HEAP_H_