Change the x64 optimizing compiler to stop using the fixed context register rsi.

src/ia32/lithium-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 185 matching lines @@
     for (int i = 0; i < num_parameters; i++) {
       Variable* var = scope()->parameter(i);
       if (var->IsContextSlot()) {
         int parameter_offset = StandardFrameConstants::kCallerSPOffset +
             (num_parameters - 1 - i) * kPointerSize;
         // Load parameter from stack.
         __ mov(eax, Operand(ebp, parameter_offset));
         // Store it in the context.
         int context_offset = Context::SlotOffset(var->index());
         __ mov(Operand(esi, context_offset), eax);
-        // Update the write barrier. This clobbers all involved
-        // registers, so we have to use a third register to avoid
-        // clobbering esi.
-        __ mov(ecx, esi);
-        __ RecordWrite(ecx, context_offset, eax, ebx);
+        // Update the write barrier.
+        __ RecordWrite(esi, context_offset, eax, ebx);
       }
     }
     Comment(";;; End allocate local context");
   }
 
   // Trace the call.
   if (FLAG_trace) {
     // We have not executed any compiled code yet, so esi still holds the
     // incoming context.
     __ CallRuntime(Runtime::kTraceEnter, 0);
@@ skipping 4156 matching lines @@
                                          env->deoptimization_index());
   __ InvokeBuiltin(Builtins::IN, CALL_FUNCTION, safepoint_generator);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32