flambda-backend: Fix split_default_wrapper when default value could a…

…llocate in region (ocaml-flambda#2162)

lambda/lambda.ml

@@ -1716,3 +1716,51 @@ let is_check_enabled ~opt property =
     | Check_all -> true
     | Check_default -> not opt
     | Check_opt_only -> opt
+
+
+let may_allocate_in_region lam =
+  (* loop_region raises, if the lambda might allocate in parent region *)
+  let rec loop_region lam =
+    shallow_iter ~tail:(function
+      | Lexclave body -> loop body
+      | lam -> loop_region lam
+    ) ~non_tail:(fun lam -> loop_region lam) lam
+  and loop = function
+    | Lvar _ | Lmutvar _ | Lconst _ -> ()
+
+    | Lfunction {mode=Alloc_heap} -> ()
+    | Lfunction {mode=Alloc_local} -> raise Exit
+
+    | Lapply {ap_mode=Alloc_local}
+    | Lsend (_,_,_,_,_,Alloc_local,_,_) -> raise Exit
+
+    | Lprim (prim, args, _) ->
+       begin match primitive_may_allocate prim with
+       | Some Alloc_local -> raise Exit
+       | None | Some Alloc_heap ->
+          List.iter loop args
+       end
+    | Lregion (body, _layout) ->
+       (* [body] might allocate in the parent region because of exclave, and thus
+          [Lregion body] might allocate in the current region *)
+      loop_region body
+    | Lexclave _body ->
+      (* [_body] might do local allocations, but not in the current region;
+        rather, it's in the parent region *)
+      ()
+    | Lwhile {wh_cond; wh_body} -> loop wh_cond; loop wh_body
+    | Lfor {for_from; for_to; for_body} -> loop for_from; loop for_to; loop for_body
+    | ( Lapply _ | Llet _ | Lmutlet _ | Lletrec _ | Lswitch _ | Lstringswitch _
+      | Lstaticraise _ | Lstaticcatch _ | Ltrywith _
+      | Lifthenelse _ | Lsequence _ | Lassign _ | Lsend _
+      | Levent _ | Lifused _) as lam ->
+       iter_head_constructor loop lam
+  in
+  if not Config.stack_allocation then false
+  else begin
+    match loop lam with
+    | () -> false
+    | exception Exit -> true
+  end
+
+

lambda/lambda.mli

@@ -795,3 +795,6 @@ val array_ref_kind : alloc_mode -> array_kind -> array_ref_kind
 (** The mode will be discarded if unnecessary for the given [array_kind] *)
 val array_set_kind : modify_mode -> array_kind -> array_set_kind
 val is_check_enabled : opt:bool -> property -> bool
+
+(* Returns true if the given lambda can allocate on the local stack *)
+val may_allocate_in_region : lambda -> bool

lambda/simplif.ml

@@ -784,7 +784,11 @@ let split_default_wrapper ~id:fun_id ~kind ~params ~return ~body
       ->
         let wrapper_body, inner = aux ((optparam, id) :: map) add_region rest in
         Llet(Strict, k, id, def, wrapper_body), inner
-    | Lregion (rest, _) -> aux map true rest
+    | Lregion (rest, ret) ->
+        let wrapper_body, inner = aux map true rest in
+        if may_allocate_in_region wrapper_body then
+          Lregion (wrapper_body, ret), inner
+        else wrapper_body, inner
     | Lexclave rest -> aux map true rest
     | _ when map = [] -> raise Exit
     | body ->

lambda/translcore.ml

@@ -143,51 +143,6 @@ let transl_apply_position position =
     if Config.stack_allocation then Rc_close_at_apply
     else Rc_normal
 
-let may_allocate_in_region lam =
-  (* loop_region raises, if the lambda might allocate in parent region *)
-  let rec loop_region lam =
-    shallow_iter ~tail:(function
-      | Lexclave body -> loop body
-      | lam -> loop_region lam
-    ) ~non_tail:(fun lam -> loop_region lam) lam
-  and loop = function
-    | Lvar _ | Lmutvar _ | Lconst _ -> ()
-
-    | Lfunction {mode=Alloc_heap} -> ()
-    | Lfunction {mode=Alloc_local} -> raise Exit
-
-    | Lapply {ap_mode=Alloc_local}
-    | Lsend (_,_,_,_,_,Alloc_local,_,_) -> raise Exit
-
-    | Lprim (prim, args, _) ->
-       begin match Lambda.primitive_may_allocate prim with
-       | Some Alloc_local -> raise Exit
-       | None | Some Alloc_heap ->
-          List.iter loop args
-       end
-    | Lregion (body, _layout) ->
-       (* [body] might allocate in the parent region because of exclave, and thus
-          [Lregion body] might allocate in the current region *)
-      loop_region body
-    | Lexclave _body ->
-      (* [_body] might do local allocations, but not in the current region;
-        rather, it's in the parent region *)
-      ()
-    | Lwhile {wh_cond; wh_body} -> loop wh_cond; loop wh_body
-    | Lfor {for_from; for_to; for_body} -> loop for_from; loop for_to; loop for_body
-    | ( Lapply _ | Llet _ | Lmutlet _ | Lletrec _ | Lswitch _ | Lstringswitch _
-      | Lstaticraise _ | Lstaticcatch _ | Ltrywith _
-      | Lifthenelse _ | Lsequence _ | Lassign _ | Lsend _
-      | Levent _ | Lifused _) as lam ->
-       Lambda.iter_head_constructor loop lam
-  in
-  if not Config.stack_allocation then false
-  else begin
-    match loop lam with
-    | () -> false
-    | exception Exit -> true
-  end
-
 let maybe_region get_layout lam =
   let rec remove_tail_markers_and_exclave = function
     | Lapply ({ap_region_close = Rc_close_at_apply} as ap) ->