Strict mode arguments do not share binding with formal parameters.

src/x64/full-codegen-x64.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 182 matching lines @@
     __ lea(rdx,
            Operand(rbp, StandardFrameConstants::kCallerSPOffset + offset));
     __ push(rdx);
     __ Push(Smi::FromInt(scope()->num_parameters()));
     // Arguments to ArgumentsAccessStub:
     //   function, receiver address, parameter count.
     // The stub will rewrite receiver and parameter count if the previous
     // stack frame was an arguments adapter frame.
     ArgumentsAccessStub stub(ArgumentsAccessStub::NEW_OBJECT);
     __ CallStub(&stub);
-    // Store new arguments object in both "arguments" and ".arguments" slots.
-    __ movq(rcx, rax);
-    Move(arguments->AsSlot(), rax, rbx, rdx);
-    Slot* dot_arguments_slot = scope()->arguments_shadow()->AsSlot();
-    Move(dot_arguments_slot, rcx, rbx, rdx);
+
+    Variable* arguments_shadow = scope()->arguments_shadow();
+    if (arguments_shadow != NULL) {
+      // Store new arguments object in both "arguments" and ".arguments" slots.
+      __ movq(rcx, rax);
+      Move(arguments->AsSlot(), rax, rbx, rdx);
+      Move(arguments_shadow->AsSlot(), rcx, rbx, rdx);
+    } else {
+      // arguments are allocated without shadow only in strict mode.
+      ASSERT(scope()->is_strict_mode());
+      Move(arguments->AsSlot(), rax, rbx, rdx);
+    }
   }
 
   if (FLAG_trace) {
     __ CallRuntime(Runtime::kTraceEnter, 0);
   }
 
   // Visit the declarations and body unless there is an illegal
   // redeclaration.
   if (scope()->HasIllegalRedeclaration()) {
     Comment cmnt(masm_, "[ Declarations");
@@ skipping 3744 matching lines @@
   __ ret(0);
 }
 
 
 #undef __
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64