Incremental mode now works for x64. The only difference

src/ia32/full-codegen-ia32.cc

 // Copyright 2011 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 3185 matching lines @@
   __ mov(Operand(index_2, 0), object);
   __ mov(Operand(index_1, 0), temp);
 
   Label no_remembered_set;
   __ CheckPageFlag(elements,
                    temp,
                    MemoryChunk::SCAN_ON_SCAVENGE,
                    not_zero,
                    &no_remembered_set,
                    Label::kNear);
+  // Possible optimization: do a check that both values are Smis
+  // (or them and test against Smi mask.)
 
-  __ mov(object, elements);
   // Since we are swapping two objects, the incremental marker is not disturbed,
   // so we don't call the stub that handles this.
   __ RememberedSetHelper(
       index_1, temp, kDontSaveFPRegs, MacroAssembler::kFallThroughAtEnd);
   __ RememberedSetHelper(
       index_2, temp, kDontSaveFPRegs, MacroAssembler::kFallThroughAtEnd);
 
   __ bind(&no_remembered_set);
 
   // We are done. Drop elements from the stack, and return undefined.
@@ skipping 1131 matching lines @@
   // And return.
   __ ret(0);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32