flambda-backend: Advise against new bytecode instructions (#2694)

* Advise against new bytecode instrs

* Work around the script that parses instruct.h

Author: ncik-roberts

runtime/caml/instruct.h

@@ -65,6 +65,15 @@ enum instructions {
   MAKE_FAUX_MIXEDBLOCK,
 FIRST_UNIMPLEMENTED_OP};
 
+// Think carefully before adding a new bytecode instruction. In general,
+// this makes the flambda-backend compiler less compatible with the
+// OCaml ecosystem. Projects may need to be patched to build with
+// flambda-backend.
+//
+// We added the [MAKE_FAUX_MIXEDBLOCK] instruction without making this
+// consideration and it has turned out OK, but we might want to reverse
+// this decision if we run into difficulty in the future.
+
 #endif /* CAML_INTERNALS */
 
 #endif /* CAML_INSTRUCT_H */

runtime4/caml/instruct.h

@@ -64,6 +64,15 @@ enum instructions {
   MAKE_FAUX_MIXEDBLOCK,
 FIRST_UNIMPLEMENTED_OP};
 
+// Think carefully before adding a new bytecode instruction. In general,
+// this makes the flambda-backend compiler less compatible with the
+// OCaml ecosystem. Projects may need to be patched to build with
+// flambda-backend.
+//
+// We added the [MAKE_FAUX_MIXEDBLOCK] instruction without making this
+// consideration and it has turned out OK, but we might want to reverse
+// this decision if we run into difficulty in the future.
+
 #endif /* CAML_INTERNALS */
 
 #endif /* CAML_INSTRUCT_H */