reduce max_sensible_number_of_arguments

driver/optmain.ml

@@ -29,7 +29,7 @@ module Backend = struct
 
   (* CR mshinwell: this needs tying through to [Proc], although it may
      necessitate the introduction of a new field in that module. *)
-  let max_sensible_number_of_arguments = 15
+  let max_sensible_number_of_arguments = 9
 end
 let backend = (module Backend : Backend_intf.S)
 

middle_end/backend_intf.mli

@@ -45,7 +45,9 @@ module type S = sig
   (** [true] iff the target architecture is big endian. *)
   val big_endian : bool
 
-  (** Optimization passes should try not to add arguments to functions
-      that cause the total number of arguments to exceed this value. *)
+  (** The maximum number of arguments that is is reasonable for a function
+      to have.  This should be fewer than the threshold that causes non-self
+      tail call optimization to be inhibited (in particular, if it would
+      entail passing arguments on the stack; see [Selectgen]). *)
   val max_sensible_number_of_arguments : int
 end

middle_end/flambda.mli

@@ -409,6 +409,8 @@ val fold_lets_option
   -> init:'a
   -> for_defining_expr:('a -> Variable.t -> named -> 'a * Variable.t * named)
   -> for_last_body:('a -> t -> t * 'b)
+  (* CR-someday mshinwell: consider making [filter_defining_expr]
+     optional *)
   -> filter_defining_expr:('b -> Variable.t -> named -> Variable.Set.t ->
                            'b * Variable.t * named option)
   -> t * 'b

middle_end/unbox_closures.ml

@@ -86,6 +86,9 @@ let create_wrapper
     ~dbg:Debuginfo.none
     ~inline:Default_inline
 
+(* It is important to limit the number of arguments added by this pass:
+   if arguments end up being passed on the stack, tail call optimization
+   will be disabled (see asmcomp/selectgen.ml). *)
 let too_many_arguments ~backend
       ~(function_decl : Flambda.function_declaration) ~bound_by_closure =
   let num_existing_arguments = List.length function_decl.params in