X64: Update allocation to work with no scratch registers at all.

src/x64/macro-assembler-x64.h

 // Copyright 2009 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 752 matching lines @@
                       Handle<Code> code_constant,
                       Register code_register,
                       Label* done,
                       InvokeFlag flag);
 
   // Activation support.
   void EnterFrame(StackFrame::Type type);
   void LeaveFrame(StackFrame::Type type);
 
   // Allocation support helpers.
+  // Loads the top of new-space into the result register.
+  // If flags contains RESULT_CONTAINS_TOP then result_end is valid and
+  // already contains the top of new-space, and scratch is invalid.
+  // Otherwise the address of the new-space top is loaded into scratch (if
+  // scratch is valid), and the new-space top is loaded into result.
   void LoadAllocationTopHelper(Register result,
                                Register result_end,
                                Register scratch,
                                AllocationFlags flags);
+  // Update allocation top with value in result_end register.
+  // If scratch is valid, it contains the address of the allocation top.
   void UpdateAllocationTopHelper(Register result_end, Register scratch);
 };
 
 
 // The code patcher is used to patch (typically) small parts of code e.g. for
 // debugging and other types of instrumentation. When using the code patcher
 // the exact number of bytes specified must be emitted. Is not legal to emit
 // relocation information. If any of these constraints are violated it causes
 // an assertion.
 class CodePatcher {
@@ skipping 47 matching lines @@
   }                                                                       \
   masm->
 #else
 #define ACCESS_MASM(masm) masm->
 #endif
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_MACRO_ASSEMBLER_X64_H_