Make sure that top bits are zero when storing untagged 32 bit values

src/x64/macro-assembler-x64.cc

 // 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 2852 matching lines @@
   Assert(is_smi, "Operand is not a smi");
 }
 
 
 void MacroAssembler::AbortIfNotSmi(const Operand& object) {
   Condition is_smi = CheckSmi(object);
   Assert(is_smi, "Operand is not a smi");
 }
 
 
+void MacroAssembler::AbortIfNotZeroExtended(Register int32_register) {
+  ASSERT(!int32_register.is(kScratchRegister));
+  movq(kScratchRegister, 0x100000000l, RelocInfo::NONE);
+  cmpq(kScratchRegister, int32_register);
+  Assert(above_equal, "32 bit value in register is not zero-extended");
+}
+
+
 void MacroAssembler::AbortIfNotString(Register object) {
   testb(object, Immediate(kSmiTagMask));
   Assert(not_equal, "Operand is not a string");
   push(object);
   movq(object, FieldOperand(object, HeapObject::kMapOffset));
   CmpInstanceType(object, FIRST_NONSTRING_TYPE);
   pop(object);
   Assert(below, "Operand is not a string");
 }
 
@@ skipping 1495 matching lines @@
 
   and_(bitmap_scratch, Immediate(~Page::kPageAlignmentMask));
   addl(Operand(bitmap_scratch, MemoryChunk::kLiveBytesOffset), length);
 
   bind(&done);
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64