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

src/objects.h

Index: src/objects.h
===================================================================
--- src/objects.h	(revision 3405)
+++ src/objects.h	(working copy)
@@ -1064,22 +1064,39 @@
   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;
@@ -1088,6 +1105,11 @@
   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);
 };