Always 64-bit align floating point values in heap numbers....

src/objects.h

 // Copyright 2006-2009 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 1046 matching lines @@
   static inline HeapNumber* cast(Object* obj);
 
   // Dispatched behavior.
   Object* HeapNumberToBoolean();
   void HeapNumberPrint();
   void HeapNumberPrint(StringStream* accumulator);
 #ifdef DEBUG
   void HeapNumberVerify();
 #endif
 
-  // Layout description.
-  static const int kValueOffset = HeapObject::kHeaderSize;
+  // Layout description.  The value offset has a strange name to remind users
+  // that the offset varies depending on the alignment of the HeapNumber.
+  static const int kValueMinimumOffset = HeapObject::kHeaderSize;
   // IEEE doubles are two 32 bit words.  The first is just mantissa, the second
   // is a mixture of sign, exponent and mantissa.  Our current platforms are all
   // little endian apart from non-EABI arm which is little endian with big
   // endian floating point word ordering!
 #if !defined(V8_HOST_ARCH_ARM) || defined(USE_ARM_EABI)
-  static const int kMantissaOffset = kValueOffset;
-  static const int kExponentOffset = kValueOffset + 4;
+  static const int kMantissaRelativeOffset = 0;
+  static const int kExponentRelativeOffset = 4;
 #else
-  static const int kMantissaOffset = kValueOffset + 4;
-  static const int kExponentOffset = kValueOffset;
+  static const int kMantissaRelativeOffset = 4;
+  static const int kExponentRelativeOffset = 0;
 # define BIG_ENDIAN_FLOATING_POINT 1
 #endif
-  static const int kSize = kValueOffset + kDoubleSize;
+  // The double precision floating point number takes 8 bytes, but if the object
+  // alignment is only 4 then the size is increased by 4 so the double precision
+  // value can always be stored at an aligned address.
+  static const int kSize =
+      kValueMinimumOffset + ((kObjectAlignment == 4) ? 12 : 8);
+
+  // The double inside the heap number has to be adjusted so it is always
+  // aligned when the object moves.
+  static inline bool NeedsSpecialCopying(int size,
+                                         Address new_address,
+                                         Address old_address) {
+    return kObjectAlignment == 4 &&
+           size == kSize &&
+           ((reinterpret_cast<intptr_t>(new_address) & 7) !=
+            (reinterpret_cast<intptr_t>(old_address) & 7)) &&
+           HeapObject::FromAddress(old_address)->IsHeapNumber();
+  }
 
   static const uint32_t kSignMask = 0x80000000u;
   static const uint32_t kExponentMask = 0x7ff00000u;
   static const uint32_t kMantissaMask = 0xfffffu;
   static const int kExponentBias = 1023;
   static const int kExponentShift = 20;
   static const int kMantissaBitsInTopWord = 20;
   static const int kNonMantissaBitsInTopWord = 12;
 
+  void CopyBodyTo(HeapObject* destination) {
+    double v = value();
+    reinterpret_cast<HeapNumber*>(destination)->set_value(v);
+  }
+
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(HeapNumber);
 };
 
 
 // The JSObject describes real heap allocated JavaScript objects with
 // properties.
 // Note that the map of JSObject changes during execution to enable inline
 // caching.
 class JSObject: public HeapObject {
@@ skipping 3762 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_