Added x86_64 System V ABI extern(D) TODO note.

gen/abi-x86-64.cpp

@@ -7,6 +7,16 @@
 //
 //===----------------------------------------------------------------------===//
 //
+// BIG RED TODO NOTE: On x86_64, the C ABI should also be used for extern(D)
+// functions, as mandated by the language standard and required for DMD
+// compatibility. The below description and implementation dates back to the
+// time where x86_64 was still an exotic target for D. Also, the frontend
+// toArgTypes() machinery should be used for doing the type classification to
+// reduce code duplication and make sure the va_arg implementation is always
+// up to date with the code we emit.
+//
+//===----------------------------------------------------------------------===//
+//
 // extern(C) implements the C calling convention for x86-64, as found in
 // http://www.x86-64.org/documentation/abi-0.99.pdf
 //