Make my_region optional

Functions that contain no escaping local allocations shouldn't need region parameters.
This PR enforces it by making these parameters optional.
All local allocations (and applications) still need to take a region,
so if there is no current region this will cause a fatal error at compile time.

middle_end/flambda2/bound_identifiers/bound_for_function.ml

@@ -19,8 +19,8 @@ type t =
     exn_continuation : Continuation.t;
     params : Bound_parameters.t;
     my_closure : Variable.t;
-    my_region : Variable.t;
-    my_ghost_region : Variable.t;
+    my_region : Variable.t option;
+    my_ghost_region : Variable.t option;
     my_depth : Variable.t
   }
 
@@ -39,8 +39,8 @@ let[@ocamlformat "disable"] print ppf
     Continuation.print exn_continuation
     Bound_parameters.print params
     Variable.print my_closure
-    Variable.print my_region
-    Variable.print my_ghost_region
+    (Format.pp_print_option Variable.print) my_region
+    (Format.pp_print_option Variable.print) my_ghost_region
     Variable.print my_depth
 
 let create ~return_continuation ~exn_continuation ~params ~my_closure ~my_region
@@ -49,10 +49,19 @@ let create ~return_continuation ~exn_continuation ~params ~my_closure ~my_region
   (if Flambda_features.check_invariants ()
   then
     let params_set = Bound_parameters.var_set params in
-    let my_set =
-      Variable.Set.of_list [my_closure; my_region; my_ghost_region; my_depth]
+    let my_set, expected_size =
+      let regions, num_regions =
+        match my_region, my_ghost_region with
+        | None, None -> [], 0
+        | Some region, Some ghost_region -> [region; ghost_region], 2
+        | None, Some _ | Some _, None ->
+          Misc.fatal_errorf
+            "[my_region] and [my_ghost_region] must be both present or both \
+             absent"
+      in
+      Variable.Set.of_list (my_closure :: my_depth :: regions), 2 + num_regions
     in
-    if Variable.Set.cardinal my_set <> 4
+    if Variable.Set.cardinal my_set <> expected_size
        || not (Variable.Set.is_empty (Variable.Set.inter my_set params_set))
     then
       Misc.fatal_errorf
@@ -106,10 +115,17 @@ let free_names
     Name_occurrences.add_variable free_names my_closure Name_mode.normal
   in
   let free_names =
-    Name_occurrences.add_variable free_names my_region Name_mode.normal
+    Option.fold ~none:free_names
+      ~some:(fun my_region ->
+        Name_occurrences.add_variable free_names my_region Name_mode.normal)
+      my_region
   in
   let free_names =
-    Name_occurrences.add_variable free_names my_ghost_region Name_mode.normal
+    Option.fold ~none:free_names
+      ~some:(fun my_ghost_region ->
+        Name_occurrences.add_variable free_names my_ghost_region
+          Name_mode.normal)
+      my_ghost_region
   in
   Name_occurrences.add_variable free_names my_depth Name_mode.normal
 
@@ -130,8 +146,10 @@ let apply_renaming
   in
   let params = Bound_parameters.apply_renaming params renaming in
   let my_closure = Renaming.apply_variable renaming my_closure in
-  let my_region = Renaming.apply_variable renaming my_region in
-  let my_ghost_region = Renaming.apply_variable renaming my_ghost_region in
+  let my_region = Option.map (Renaming.apply_variable renaming) my_region in
+  let my_ghost_region =
+    Option.map (Renaming.apply_variable renaming) my_ghost_region
+  in
   let my_depth = Renaming.apply_variable renaming my_depth in
   (* CR mshinwell: this should have a phys-equal check *)
   { return_continuation;
@@ -158,8 +176,17 @@ let ids_for_export
   let ids = Ids_for_export.add_continuation ids exn_continuation in
   let ids = Ids_for_export.union ids (Bound_parameters.ids_for_export params) in
   let ids = Ids_for_export.add_variable ids my_closure in
-  let ids = Ids_for_export.add_variable ids my_region in
-  let ids = Ids_for_export.add_variable ids my_ghost_region in
+  let ids =
+    Option.fold ~none:ids
+      ~some:(fun my_region -> Ids_for_export.add_variable ids my_region)
+      my_region
+  in
+  let ids =
+    Option.fold ~none:ids
+      ~some:(fun my_ghost_region ->
+        Ids_for_export.add_variable ids my_ghost_region)
+      my_ghost_region
+  in
   Ids_for_export.add_variable ids my_depth
 
 let rename
@@ -175,8 +202,8 @@ let rename
     exn_continuation = Continuation.rename exn_continuation;
     params = Bound_parameters.rename params;
     my_closure = Variable.rename my_closure;
-    my_region = Variable.rename my_region;
-    my_ghost_region = Variable.rename my_ghost_region;
+    my_region = Option.map (Variable.rename ?append:None) my_region;
+    my_ghost_region = Option.map (Variable.rename ?append:None) my_ghost_region;
     my_depth = Variable.rename my_depth
   }
 
@@ -216,10 +243,21 @@ let renaming
       ~guaranteed_fresh:my_closure2
   in
   let renaming =
-    Renaming.add_fresh_variable renaming my_region1 ~guaranteed_fresh:my_region2
+    match my_region1, my_region2 with
+    | None, None -> renaming
+    | Some my_region1, Some my_region2 ->
+      Renaming.add_fresh_variable renaming my_region1
+        ~guaranteed_fresh:my_region2
+    | None, Some _ | Some _, None ->
+      Misc.fatal_error "Mismatched [my_region] field in renaming"
   in
   let renaming =
-    Renaming.add_fresh_variable renaming my_ghost_region1
-      ~guaranteed_fresh:my_ghost_region2
+    match my_ghost_region1, my_ghost_region2 with
+    | None, None -> renaming
+    | Some my_ghost_region1, Some my_ghost_region2 ->
+      Renaming.add_fresh_variable renaming my_ghost_region1
+        ~guaranteed_fresh:my_ghost_region2
+    | None, Some _ | Some _, None ->
+      Misc.fatal_error "Mismatched [my_ghost_region] field in renaming"
   in
   Renaming.add_fresh_variable renaming my_depth1 ~guaranteed_fresh:my_depth2

middle_end/flambda2/bound_identifiers/bound_for_function.mli

@@ -24,8 +24,8 @@ val create :
   exn_continuation:Continuation.t ->
   params:Bound_parameters.t ->
   my_closure:Variable.t ->
-  my_region:Variable.t ->
-  my_ghost_region:Variable.t ->
+  my_region:Variable.t option ->
+  my_ghost_region:Variable.t option ->
   my_depth:Variable.t ->
   t
 
@@ -37,9 +37,9 @@ val params : t -> Bound_parameters.t
 
 val my_closure : t -> Variable.t
 
-val my_region : t -> Variable.t
+val my_region : t -> Variable.t option
 
-val my_ghost_region : t -> Variable.t
+val my_ghost_region : t -> Variable.t option
 
 val my_depth : t -> Variable.t