Simplify some code related to x64 calling convention.

src/x64/assembler-x64.h

Index: src/x64/assembler-x64.h
diff --git a/src/x64/assembler-x64.h b/src/x64/assembler-x64.h
index 2445e2335c12c147037156b505d47a0118596f7c..5bb1292f2fc6e294cc1b4eafbc610d0c84256fcf 100644
--- a/src/x64/assembler-x64.h
+++ b/src/x64/assembler-x64.h
@@ -200,6 +200,19 @@ const Register r14 = { kRegister_r14_Code };
 const Register r15 = { kRegister_r15_Code };
 const Register no_reg = { kRegister_no_reg_Code };
 
+#ifdef _WIN64
+  // Windows calling convention

[mvstanton, 2013/04/24 10:54:54]

Can you mention fastcall here?

+  const Register arg_reg_1 = rcx;
+  const Register arg_reg_2 = rdx;
+  const Register arg_reg_3 = r8;
+  const Register arg_reg_4 = r9;
+#else
+  // AMD64 calling convention
+  const Register arg_reg_1 = rdi;
+  const Register arg_reg_2 = rsi;
+  const Register arg_reg_3 = rdx;
+  const Register arg_reg_4 = rcx;
+#endif  // _WIN64
 
 struct XMMRegister {
   static const int kMaxNumRegisters = 16;