Tweak SmiAdd for X64

src/x64/macro-assembler-x64.h

 // 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 560 matching lines @@
                       Label::Distance near_jump = Label::kFar);
 
   // Negating a smi can give a negative zero or too large positive value.
   // NOTICE: This operation jumps on success, not failure!
   void SmiNeg(Register dst,
               Register src,
               Label* on_smi_result,
               Label::Distance near_jump = Label::kFar);
 
   // Adds smi values and return the result as a smi.
-  // If dst is src1, then src1 will be destroyed, even if
-  // the operation is unsuccessful.
+  // If dst is src1, then src1 will be destroyed if the operation is
+  // successful, otherwise kept intact.
   void SmiAdd(Register dst,
               Register src1,
               Register src2,
               Label* on_not_smi_result,
               Label::Distance near_jump = Label::kFar);
   void SmiAdd(Register dst,
               Register src1,
               const Operand& src2,
               Label* on_not_smi_result,
               Label::Distance near_jump = Label::kFar);
@@ skipping 989 matching lines @@
     masm->popfq();                                                           \
   }                                                                          \
   masm->
 #else
 #define ACCESS_MASM(masm) masm->
 #endif
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_MACRO_ASSEMBLER_X64_H_