Try aligning unboxed double array backing store in allocation or scavenge promotion.

src/v8globals.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 30 matching lines @@
 const intptr_t kSmiSignMask = kIntptrSignBit;
 
 const int kObjectAlignmentBits = kPointerSizeLog2;
 const intptr_t kObjectAlignment = 1 << kObjectAlignmentBits;
 const intptr_t kObjectAlignmentMask = kObjectAlignment - 1;
 
 // Desired alignment for pointers.
 const intptr_t kPointerAlignment = (1 << kPointerSizeLog2);
 const intptr_t kPointerAlignmentMask = kPointerAlignment - 1;
 
+#if V8_HOST_ARCH_64_BIT

[Erik Corry, 2012/04/16 14:35:19]

Instead of the ifdef you could just set kDoubleAlignment to 8.

+const intptr_t kDoubleAlignment = kPointerAlignment;
+#else
+const intptr_t kDoubleAlignment = 2 * kPointerAlignment;
+#endif
+const intptr_t kDoubleAlignmentMask = kDoubleAlignment - 1;
+
 // Desired alignment for maps.
 #if V8_HOST_ARCH_64_BIT
 const intptr_t kMapAlignmentBits = kObjectAlignmentBits;
 #else
 const intptr_t kMapAlignmentBits = kObjectAlignmentBits + 3;
 #endif
 const intptr_t kMapAlignment = (1 << kMapAlignmentBits);
 const intptr_t kMapAlignmentMask = kMapAlignment - 1;
 
 // Desired alignment for generated code is 32 bytes (to improve cache line
@@ skipping 488 matching lines @@
 
 enum ClearExceptionFlag {
   KEEP_EXCEPTION,
   CLEAR_EXCEPTION
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_V8GLOBALS_H_