Make progress on propagating the changes

14 files changed, 259 insertions, 949 deletions

diff --git a/compiler/Config.ml b/compiler/Config.ml
index 6fd866e8..af0e62d1 100644
--- a/compiler/Config.ml
+++ b/compiler/Config.ml
@@ -263,50 +263,6 @@ let decompose_nested_let_patterns = ref false
  *)
 let unfold_monadic_let_bindings = ref false
 
-(** Controls whether we try to filter the calls to monadic functions
-    (which can fail) when their outputs are not used.
-
-    The useless calls are calls to backward functions which have no outputs.
-    This case happens if the original Rust function only takes *shared* borrows
-    as inputs, and is thus pretty common.
-
-    We are allowed to do this only because in this specific case,
-    the backward function fails *exactly* when the forward function fails
-    (they actually do exactly the same thing, the only difference being
-    that the forward function can potentially return a value), and upon
-    reaching the place where we should introduce a call to the backward
-    function, we know we have introduced a call to the forward function.
-
-    Also note that in general, backward functions "do more things" than
-    forward functions, and have more opportunities to fail (even though
-    in the generated code, calls to the backward functions should fail
-    exactly when the corresponding, previous call to the forward functions
-    failed).
-
-    This optimization is done in {!SymbolicToPure}.  We might want to move it to
-    the micro-passes subsequent to the translation from symbolic to pure, but it
-    is really super easy to do it when going from symbolic to pure. Note that
-    we later filter the useless *forward* calls in the micro-passes, where it is
-    more natural to do.
-
-    See the comments for {!PureMicroPasses.expression_contains_child_call_in_all_paths}
-    for additional explanations.
- *)
-let filter_useless_monadic_calls = ref true
-
-(** If {!filter_useless_monadic_calls} is activated, some functions
-    become useless: if this option is true, we don't extract them.
-
-    The calls to functions which always get filtered are:
-    - the forward functions with unit return value
-    - the backward functions which don't output anything (backward
-      functions coming from rust functions with no mutable borrows
-      as input values - note that if a function doesn't take mutable
-      borrows as inputs, it can't return mutable borrows; we actually
-      dynamically check for that).
- *)
-let filter_useless_functions = ref true
-
 (** Simplify the forward/backward functions, in case we merge them
     (i.e., the forward functions return the backward functions).
 
diff --git a/compiler/Contexts.ml b/compiler/Contexts.ml
index b1dd9553..54411fd5 100644
--- a/compiler/Contexts.ml
+++ b/compiler/Contexts.ml
@@ -109,6 +109,8 @@ let reset_global_counters () =
   region_id_counter := RegionId.generator_zero;
   abstraction_id_counter := AbstractionId.generator_zero;
   loop_id_counter := LoopId.generator_zero;
+  (* We want the loop id to start at 1 *)
+  let _ = fresh_loop_id () in
   fun_call_id_counter := FunCallId.generator_zero;
   dummy_var_id_counter := DummyVarId.generator_zero
 
diff --git a/compiler/Extract.ml b/compiler/Extract.ml
index dbca4f8f..794a1bfa 100644
--- a/compiler/Extract.ml
+++ b/compiler/Extract.ml
@@ -9,8 +9,7 @@ open TranslateCore
 open Config
 include ExtractTypes
 
-(** Compute the names for all the pure functions generated from a rust function
-    (forward function and backward functions).
+(** Compute the names for all the pure functions generated from a rust function.
  *)
 let extract_fun_decl_register_names (ctx : extraction_ctx)
     (has_decreases_clause : fun_decl -> bool) (def : pure_fun_translation) :
@@ -19,63 +18,36 @@ let extract_fun_decl_register_names (ctx : extraction_ctx)
      method implementations): we do not need to refer to them directly. We will
      only use their type for the fields of the records we generate for the trait
      declarations *)
-  match def.fwd.f.kind with
+  match def.f.kind with
   | TraitMethodDecl _ -> ctx
   | _ -> (
       (* Check if the function is builtin *)
       let builtin =
         let open ExtractBuiltin in
         let funs_map = builtin_funs_map () in
-        match_name_find_opt ctx.trans_ctx def.fwd.f.llbc_name funs_map
+        match_name_find_opt ctx.trans_ctx def.f.llbc_name funs_map
       in
       (* Use the builtin names if necessary *)
       match builtin with
-      | Some (filter_info, info) ->
-          (* Register the filtering information, if there is *)
+      | Some (filter_info, fun_info) ->
+          (* Builtin function: register the filtering information, if there is *)
           let ctx =
             match filter_info with
             | Some keep ->
                 {
                   ctx with
                   funs_filter_type_args_map =
-                    FunDeclId.Map.add def.fwd.f.def_id keep
+                    FunDeclId.Map.add def.f.def_id keep
                       ctx.funs_filter_type_args_map;
                 }
             | _ -> ctx
           in
-          let funs =
-            if !Config.return_back_funs then [ def.fwd.f ]
-            else
-              let backs = List.map (fun f -> f.f) def.backs in
-              if def.keep_fwd then def.fwd.f :: backs else backs
-          in
-          List.fold_left
-            (fun ctx (f : fun_decl) ->
-              let open ExtractBuiltin in
-              let fun_id =
-                (Pure.FunId (FRegular f.def_id), f.loop_id, f.back_id)
-              in
-              let fun_info =
-                List.find_opt
-                  (fun (x : builtin_fun_info) -> x.rg = f.back_id)
-                  info
-              in
-              match fun_info with
-              | Some fun_info ->
-                  ctx_add (FunId (FromLlbc fun_id)) fun_info.extract_name ctx
-              | None ->
-                  raise
-                    (Failure
-                       ("Not found: "
-                       ^ name_to_string ctx f.llbc_name
-                       ^ ", "
-                       ^ Print.option_to_string Pure.show_loop_id f.loop_id
-                       ^ Print.option_to_string Pure.show_region_group_id
-                           f.back_id)))
-            ctx funs
+          let f = def.f in
+          let open ExtractBuiltin in
+          let fun_id = (Pure.FunId (FRegular f.def_id), f.loop_id) in
+          ctx_add (FunId (FromLlbc fun_id)) fun_info.extract_name ctx
       | None ->
-          let fwd = def.fwd in
-          let backs = def.backs in
+          (* Not builtin *)
           (* Register the decrease clauses, if necessary *)
           let register_decreases ctx def =
             if has_decreases_clause def then
@@ -88,21 +60,15 @@ let extract_fun_decl_register_names (ctx : extraction_ctx)
               | Lean -> ctx_add_decreases_proof def ctx
             else ctx
           in
-          let ctx =
-            List.fold_left register_decreases ctx (fwd.f :: fwd.loops)
-          in
-          let register_fun ctx f = ctx_add_fun_decl def f ctx in
+          (* We have to register the function itself, and the loops it
+             may contain (which are extracted as functions) *)
+          let funs = def.f :: def.loops in
+          (* Register the decrease clauses *)
+          let ctx = List.fold_left register_decreases ctx funs in
+          (* Register the name of the function and the loops *)
+          let register_fun ctx f = ctx_add_fun_decl f ctx in
           let register_funs ctx fl = List.fold_left register_fun ctx fl in
-          (* Register the names of the forward functions *)
-          let ctx =
-            if def.keep_fwd then register_funs ctx (fwd.f :: fwd.loops) else ctx
-          in
-          (* Register the names of the backward functions *)
-          List.fold_left
-            (fun ctx { f = back; loops = loop_backs } ->
-              let ctx = register_fun ctx back in
-              register_funs ctx loop_backs)
-            ctx backs)
+          register_funs ctx funs)
 
 (** Simply add the global name to the context. *)
 let extract_global_decl_register_names (ctx : extraction_ctx)
@@ -230,7 +196,7 @@ let fun_builtin_filter_types (id : FunDeclId.id) (types : 'a list)
         let decl = FunDeclId.Map.find id ctx.trans_funs in
         let err =
           "Ill-formed builtin information for function "
-          ^ name_to_string ctx decl.fwd.f.llbc_name
+          ^ name_to_string ctx decl.f.llbc_name
           ^ ": "
           ^ string_of_int (List.length filter)
           ^ " filtering arguments provided for "
@@ -460,8 +426,7 @@ and extract_function_call (ctx : extraction_ctx) (fmt : F.formatter)
          ]}
       *)
       (match fun_id with
-      | FromLlbc
-          (TraitMethod (trait_ref, method_name, _fun_decl_id), lp_id, rg_id) ->
+      | FromLlbc (TraitMethod (trait_ref, method_name, _fun_decl_id), lp_id) ->
           (* We have to check whether the trait method is required or provided *)
           let trait_decl_id = trait_ref.trait_decl_ref.trait_decl_id in
           let trait_decl =
@@ -477,7 +442,7 @@ and extract_function_call (ctx : extraction_ctx) (fmt : F.formatter)
             extract_trait_ref ctx fmt TypeDeclId.Set.empty true trait_ref;
             let fun_name =
               ctx_get_trait_method trait_ref.trait_decl_ref.trait_decl_id
-                method_name rg_id ctx
+                method_name ctx
             in
             let add_brackets (s : string) =
               if !backend = Coq then "(" ^ s ^ ")" else s
@@ -486,9 +451,7 @@ and extract_function_call (ctx : extraction_ctx) (fmt : F.formatter)
           else
             (* Provided method: we see it as a regular function call, and use
                the function name *)
-            let fun_id =
-              FromLlbc (FunId (FRegular method_id.id), lp_id, rg_id)
-            in
+            let fun_id = FromLlbc (FunId (FRegular method_id.id), lp_id) in
             let fun_name = ctx_get_function fun_id ctx in
             F.pp_print_string fmt fun_name;
 
@@ -513,7 +476,7 @@ and extract_function_call (ctx : extraction_ctx) (fmt : F.formatter)
       *)
       let types =
         match fun_id with
-        | FromLlbc (FunId (FRegular id), _, _) ->
+        | FromLlbc (FunId (FRegular id), _) ->
             fun_builtin_filter_types id generics.types ctx
         | _ -> Result.Ok generics.types
       in
@@ -1392,11 +1355,6 @@ let extract_template_lean_termination_and_decreasing (ctx : extraction_ctx)
 
 let extract_fun_comment (ctx : extraction_ctx) (fmt : F.formatter)
     (def : fun_decl) : unit =
-  let { keep_fwd; num_backs } =
-    PureUtils.RegularFunIdMap.find
-      (Pure.FunId (FRegular def.def_id), def.loop_id, def.back_id)
-      ctx.fun_name_info
-  in
   let comment_pre = "[" ^ name_to_string ctx def.llbc_name ^ "]:" in
   let comment =
     let loop_comment =
@@ -1404,23 +1362,7 @@ let extract_fun_comment (ctx : extraction_ctx) (fmt : F.formatter)
       | None -> ""
       | Some id -> " loop " ^ LoopId.to_string id ^ ":"
     in
-    let fwd_back_comment =
-      match def.back_id with
-      | None -> if !Config.return_back_funs then [] else [ "forward function" ]
-      | Some id ->
-          (* Check if there is only one backward function, and no forward function *)
-          if (not keep_fwd) && num_backs = 1 then
-            [
-              "merged forward/backward function";
-              "(there is a single backward function, and the forward function \
-               returns ())";
-            ]
-          else [ "backward function " ^ T.RegionGroupId.to_string id ]
-    in
-    match fwd_back_comment with
-    | [] -> [ comment_pre ^ loop_comment ]
-    | [ s ] -> [ comment_pre ^ loop_comment ^ " " ^ s ]
-    | s :: sl -> (comment_pre ^ loop_comment ^ " " ^ s) :: sl
+    [ comment_pre ^ loop_comment ]
   in
   extract_comment_with_span fmt comment def.meta.span
 
@@ -1435,9 +1377,7 @@ let extract_fun_decl_gen (ctx : extraction_ctx) (fmt : F.formatter)
     (kind : decl_kind) (has_decreases_clause : bool) (def : fun_decl) : unit =
   assert (not def.is_global_decl_body);
   (* Retrieve the function name *)
-  let def_name =
-    ctx_get_local_function def.def_id def.loop_id def.back_id ctx
-  in
+  let def_name = ctx_get_local_function def.def_id def.loop_id ctx in
   (* Add a break before *)
   if !backend <> HOL4 || not (decl_is_first_from_group kind) then
     F.pp_print_break fmt 0 0;
@@ -1681,9 +1621,7 @@ let extract_fun_decl_gen (ctx : extraction_ctx) (fmt : F.formatter)
 let extract_fun_decl_hol4_opaque (ctx : extraction_ctx) (fmt : F.formatter)
     (def : fun_decl) : unit =
   (* Retrieve the definition name *)
-  let def_name =
-    ctx_get_local_function def.def_id def.loop_id def.back_id ctx
-  in
+  let def_name = ctx_get_local_function def.def_id def.loop_id ctx in
   assert (def.signature.generics.const_generics = []);
   (* Add the type/const gen parameters - note that we need those bindings
      only for the generation of the type (they are not top-level) *)
@@ -1870,7 +1808,6 @@ let extract_global_decl_hol4_opaque (ctx : extraction_ctx) (fmt : F.formatter)
 let extract_global_decl (ctx : extraction_ctx) (fmt : F.formatter)
     (global : A.global_decl) (body : fun_decl) (interface : bool) : unit =
   assert body.is_global_decl_body;
-  assert (Option.is_none body.back_id);
   assert (body.signature.inputs = []);
   assert (body.signature.generics = empty_generic_params);
 
@@ -1883,9 +1820,7 @@ let extract_global_decl (ctx : extraction_ctx) (fmt : F.formatter)
 
   let decl_name = ctx_get_global global.def_id ctx in
   let body_name =
-    ctx_get_function
-      (FromLlbc (Pure.FunId (FRegular global.body), None, None))
-      ctx
+    ctx_get_function (FromLlbc (Pure.FunId (FRegular global.body), None)) ctx
   in
 
   let decl_ty, body_ty =
@@ -2058,80 +1993,45 @@ let extract_trait_decl_method_names (ctx : extraction_ctx)
   let required_methods = trait_decl.required_methods in
   (* Compute the names *)
   let method_names =
-    (* We add one field per required forward/backward function *)
-    let get_funs_for_id (id : fun_decl_id) : fun_decl list =
-      let trans : pure_fun_translation = FunDeclId.Map.find id ctx.trans_funs in
-      if !Config.return_back_funs then [ trans.fwd.f ]
-      else List.map (fun f -> f.f) (trans.fwd :: trans.backs)
-    in
     match builtin_info with
     | None ->
-        (* We add one field per required forward/backward function *)
-        let compute_item_names (item_name : string) (id : fun_decl_id) :
-            string * (RegionGroupId.id option * string) list =
-          let compute_fun_name (f : fun_decl) : RegionGroupId.id option * string
-              =
-            (* We do something special to reuse the [ctx_compute_fun_decl]
-               function. TODO: make it cleaner. *)
-            let llbc_name : Types.name =
-              [ Types.PeIdent (item_name, Disambiguator.zero) ]
-            in
-            let f = { f with llbc_name } in
-            let trans = A.FunDeclId.Map.find f.def_id ctx.trans_funs in
-            let name = ctx_compute_fun_name trans f ctx in
-            (* Add a prefix if necessary *)
-            let name =
-              if !Config.record_fields_short_names then name
-              else ctx_compute_trait_decl_name ctx trait_decl ^ "_" ^ name
-            in
-            (f.back_id, name)
+        (* Not a builtin function *)
+        let compute_item_name (item_name : string) (id : fun_decl_id) :
+            string * string =
+          let trans : pure_fun_translation =
+            FunDeclId.Map.find id ctx.trans_funs
+          in
+          let f = trans.f in
+          (* We do something special to reuse the [ctx_compute_fun_decl]
+             function. TODO: make it cleaner. *)
+          let llbc_name : Types.name =
+            [ Types.PeIdent (item_name, Disambiguator.zero) ]
+          in
+          let f = { f with llbc_name } in
+          let name = ctx_compute_fun_name f ctx in
+          (* Add a prefix if necessary *)
+          let name =
+            if !Config.record_fields_short_names then name
+            else ctx_compute_trait_decl_name ctx trait_decl ^ "_" ^ name
           in
-          let funs = get_funs_for_id id in
-          (item_name, List.map compute_fun_name funs)
+          (item_name, name)
         in
-        List.map (fun (name, id) -> compute_item_names name id) required_methods
+        List.map (fun (name, id) -> compute_item_name name id) required_methods
     | Some info ->
+        (* This is a builtin *)
         let funs_map = StringMap.of_list info.methods in
         List.map
-          (fun (item_name, fun_id) ->
+          (fun (item_name, _) ->
             let open ExtractBuiltin in
             let info = StringMap.find item_name funs_map in
-            let trans_funs = get_funs_for_id fun_id in
-            let find (trans_fun : fun_decl) =
-              let info =
-                List.find_opt
-                  (fun (info : builtin_fun_info) -> info.rg = trans_fun.back_id)
-                  info
-              in
-              match info with
-              | Some info -> (info.rg, info.extract_name)
-              | None ->
-                  let err =
-                    "Ill-formed builtin information for trait decl \""
-                    ^ name_to_string ctx trait_decl.llbc_name
-                    ^ "\", method \"" ^ item_name
-                    ^ "\": could not find name for region "
-                    ^ Print.option_to_string Pure.show_region_group_id
-                        trans_fun.back_id
-                  in
-                  log#serror err;
-                  if !Config.fail_hard then raise (Failure err)
-                  else (trans_fun.back_id, "%ERROR_BUILTIN_NAME_NOT_FOUND%")
-            in
-            let rg_with_name_list = List.map find trans_funs in
-            (item_name, rg_with_name_list))
+            let fun_name = info.extract_name in
+            (item_name, fun_name))
           required_methods
   in
   (* Register the names *)
   List.fold_left
-    (fun ctx (item_name, funs) ->
-      (* We add one field per required forward/backward function *)
-      List.fold_left
-        (fun ctx (rg, fun_name) ->
-          ctx_add
-            (TraitMethodId (trait_decl.def_id, item_name, rg))
-            fun_name ctx)
-        ctx funs)
+    (fun ctx (item_name, fun_name) ->
+      ctx_add (TraitMethodId (trait_decl.def_id, item_name)) fun_name ctx)
     ctx method_names
 
 (** Similar to {!extract_type_decl_register_names} *)
@@ -2263,46 +2163,41 @@ let extract_trait_decl_method_items (ctx : extraction_ctx) (fmt : F.formatter)
   (* Lookup the definition *)
   let trans = A.FunDeclId.Map.find id ctx.trans_funs in
   (* Extract the items *)
-  let funs = if trans.keep_fwd then trans.fwd :: trans.backs else trans.backs in
-  let extract_method (f : fun_and_loops) =
-    let f = f.f in
-    let fun_name = ctx_get_trait_method decl.def_id item_name f.back_id ctx in
-    let ty () =
-      (* Extract the generics *)
-      (* We need to add the generics specific to the method, by removing those
-         which actually apply to the trait decl *)
-      let generics =
-        let drop_trait_clauses = false in
-        generic_params_drop_prefix ~drop_trait_clauses decl.generics
-          f.signature.generics
-      in
-      (* Note that we do not filter the LLBC generic parameters.
-         This is ok because:
-         - we only use them to find meaningful names for the trait clauses
-         - we only generate trait clauses for the clauses we find in the
-           pure generics *)
-      let ctx, type_params, cg_params, trait_clauses =
-        ctx_add_generic_params f.llbc_name f.signature.llbc_generics generics
-          ctx
-      in
-      let backend_uses_forall =
-        match !backend with Coq | Lean -> true | FStar | HOL4 -> false
-      in
-      let generics_not_empty = generics <> empty_generic_params in
-      let use_forall = generics_not_empty && backend_uses_forall in
-      let use_arrows = generics_not_empty && not backend_uses_forall in
-      let use_forall_use_sep = false in
-      extract_generic_params ctx fmt TypeDeclId.Set.empty ~use_forall
-        ~use_forall_use_sep ~use_arrows generics type_params cg_params
-        trait_clauses;
-      if use_forall then F.pp_print_string fmt ",";
-      (* Extract the inputs and output *)
-      F.pp_print_space fmt ();
-      extract_fun_inputs_output_parameters_types ctx fmt f
+  let f = trans.f in
+  let fun_name = ctx_get_trait_method decl.def_id item_name ctx in
+  let ty () =
+    (* Extract the generics *)
+    (* We need to add the generics specific to the method, by removing those
+       which actually apply to the trait decl *)
+    let generics =
+      let drop_trait_clauses = false in
+      generic_params_drop_prefix ~drop_trait_clauses decl.generics
+        f.signature.generics
+    in
+    (* Note that we do not filter the LLBC generic parameters.
+       This is ok because:
+       - we only use them to find meaningful names for the trait clauses
+       - we only generate trait clauses for the clauses we find in the
+         pure generics *)
+    let ctx, type_params, cg_params, trait_clauses =
+      ctx_add_generic_params f.llbc_name f.signature.llbc_generics generics ctx
     in
-    extract_trait_decl_item ctx fmt fun_name ty
+    let backend_uses_forall =
+      match !backend with Coq | Lean -> true | FStar | HOL4 -> false
+    in
+    let generics_not_empty = generics <> empty_generic_params in
+    let use_forall = generics_not_empty && backend_uses_forall in
+    let use_arrows = generics_not_empty && not backend_uses_forall in
+    let use_forall_use_sep = false in
+    extract_generic_params ctx fmt TypeDeclId.Set.empty ~use_forall
+      ~use_forall_use_sep ~use_arrows generics type_params cg_params
+      trait_clauses;
+    if use_forall then F.pp_print_string fmt ",";
+    (* Extract the inputs and output *)
+    F.pp_print_space fmt ();
+    extract_fun_inputs_output_parameters_types ctx fmt f
   in
-  List.iter extract_method funs
+  extract_trait_decl_item ctx fmt fun_name ty
 
 (** Extract a trait declaration *)
 let extract_trait_decl (ctx : extraction_ctx) (fmt : F.formatter)
@@ -2494,21 +2389,10 @@ let extract_trait_decl_coq_arguments (ctx : extraction_ctx) (fmt : F.formatter)
       decl.parent_clauses;
     (* The required methods *)
     List.iter
-      (fun (item_name, id) ->
-        (* Lookup the definition *)
-        let trans = A.FunDeclId.Map.find id ctx.trans_funs in
+      (fun (item_name, _) ->
         (* Extract the items *)
-        let funs =
-          if trans.keep_fwd then trans.fwd :: trans.backs else trans.backs
-        in
-        let extract_for_method (f : fun_and_loops) =
-          let f = f.f in
-          let item_name =
-            ctx_get_trait_method decl.def_id item_name f.back_id ctx
-          in
-          extract_coq_arguments_instruction ctx fmt item_name num_params
-        in
-        List.iter extract_for_method funs)
+        let item_name = ctx_get_trait_method decl.def_id item_name ctx in
+        extract_coq_arguments_instruction ctx fmt item_name num_params)
       decl.required_methods;
     (* Add a space *)
     F.pp_print_space fmt ())
@@ -2531,75 +2415,71 @@ let extract_trait_impl_method_items (ctx : extraction_ctx) (fmt : F.formatter)
   (* Lookup the definition *)
   let trans = A.FunDeclId.Map.find id ctx.trans_funs in
   (* Extract the items *)
-  let funs = if trans.keep_fwd then trans.fwd :: trans.backs else trans.backs in
-  let extract_method (f : fun_and_loops) =
-    let f = f.f in
-    let fun_name = ctx_get_trait_method trait_decl_id item_name f.back_id ctx in
-    let ty () =
-      (* Filter the generics if the method is a builtin *)
-      let i_tys, _, _ = impl_generics in
-      let impl_types, i_tys, f_tys =
-        match FunDeclId.Map.find_opt f.def_id ctx.funs_filter_type_args_map with
-        | None -> (impl.generics.types, i_tys, f.signature.generics.types)
-        | Some filter ->
-            let filter_list filter ls =
-              let ls = List.combine filter ls in
-              List.filter_map (fun (b, ty) -> if b then Some ty else None) ls
-            in
-            let impl_types = impl.generics.types in
-            let impl_filter =
-              Collections.List.prefix (List.length impl_types) filter
-            in
-            let i_tys = i_tys in
-            let i_filter = Collections.List.prefix (List.length i_tys) filter in
-            ( filter_list impl_filter impl_types,
-              filter_list i_filter i_tys,
-              filter_list filter f.signature.generics.types )
-      in
-      let f_generics = { f.signature.generics with types = f_tys } in
-      (* Extract the generics - we need to quantify over the generics which
-         are specific to the method, and call it will all the generics
-         (trait impl + method generics) *)
-      let f_generics =
-        let drop_trait_clauses = true in
-        generic_params_drop_prefix ~drop_trait_clauses
-          { impl.generics with types = impl_types }
-          f_generics
-      in
-      (* Register and print the quantified generics.
-
-         Note that we do not filter the LLBC generic parameters.
-         This is ok because:
-         - we only use them to find meaningful names for the trait clauses
-         - we only generate trait clauses for the clauses we find in the
-           pure generics *)
-      let ctx, f_tys, f_cgs, f_tcs =
-        ctx_add_generic_params f.llbc_name f.signature.llbc_generics f_generics
-          ctx
-      in
-      let use_forall = f_generics <> empty_generic_params in
-      extract_generic_params ctx fmt TypeDeclId.Set.empty ~use_forall f_generics
-        f_tys f_cgs f_tcs;
-      if use_forall then F.pp_print_string fmt ",";
-      (* Extract the function call *)
-      F.pp_print_space fmt ();
-      let fun_name = ctx_get_local_function f.def_id None f.back_id ctx in
-      F.pp_print_string fmt fun_name;
-      let all_generics =
-        let _, i_cgs, i_tcs = impl_generics in
-        List.concat [ i_tys; f_tys; i_cgs; f_cgs; i_tcs; f_tcs ]
-      in
-
-      (* Filter the generics if the function is builtin *)
-      List.iter
-        (fun p ->
-          F.pp_print_space fmt ();
-          F.pp_print_string fmt p)
-        all_generics
+  let f = trans.f in
+  let fun_name = ctx_get_trait_method trait_decl_id item_name ctx in
+  let ty () =
+    (* Filter the generics if the method is a builtin *)
+    let i_tys, _, _ = impl_generics in
+    let impl_types, i_tys, f_tys =
+      match FunDeclId.Map.find_opt f.def_id ctx.funs_filter_type_args_map with
+      | None -> (impl.generics.types, i_tys, f.signature.generics.types)
+      | Some filter ->
+          let filter_list filter ls =
+            let ls = List.combine filter ls in
+            List.filter_map (fun (b, ty) -> if b then Some ty else None) ls
+          in
+          let impl_types = impl.generics.types in
+          let impl_filter =
+            Collections.List.prefix (List.length impl_types) filter
+          in
+          let i_tys = i_tys in
+          let i_filter = Collections.List.prefix (List.length i_tys) filter in
+          ( filter_list impl_filter impl_types,
+            filter_list i_filter i_tys,
+            filter_list filter f.signature.generics.types )
+    in
+    let f_generics = { f.signature.generics with types = f_tys } in
+    (* Extract the generics - we need to quantify over the generics which
+       are specific to the method, and call it will all the generics
+       (trait impl + method generics) *)
+    let f_generics =
+      let drop_trait_clauses = true in
+      generic_params_drop_prefix ~drop_trait_clauses
+        { impl.generics with types = impl_types }
+        f_generics
+    in
+    (* Register and print the quantified generics.
+
+       Note that we do not filter the LLBC generic parameters.
+       This is ok because:
+       - we only use them to find meaningful names for the trait clauses
+       - we only generate trait clauses for the clauses we find in the
+         pure generics *)
+    let ctx, f_tys, f_cgs, f_tcs =
+      ctx_add_generic_params f.llbc_name f.signature.llbc_generics f_generics
+        ctx
+    in
+    let use_forall = f_generics <> empty_generic_params in
+    extract_generic_params ctx fmt TypeDeclId.Set.empty ~use_forall f_generics
+      f_tys f_cgs f_tcs;
+    if use_forall then F.pp_print_string fmt ",";
+    (* Extract the function call *)
+    F.pp_print_space fmt ();
+    let fun_name = ctx_get_local_function f.def_id None ctx in
+    F.pp_print_string fmt fun_name;
+    let all_generics =
+      let _, i_cgs, i_tcs = impl_generics in
+      List.concat [ i_tys; f_tys; i_cgs; f_cgs; i_tcs; f_tcs ]
     in
-    extract_trait_impl_item ctx fmt fun_name ty
+
+    (* Filter the generics if the function is builtin *)
+    List.iter
+      (fun p ->
+        F.pp_print_space fmt ();
+        F.pp_print_string fmt p)
+      all_generics
   in
-  List.iter extract_method funs
+  extract_trait_impl_item ctx fmt fun_name ty
 
 (** Extract a trait implementation *)
 let extract_trait_impl (ctx : extraction_ctx) (fmt : F.formatter)
@@ -2766,8 +2646,6 @@ let extract_trait_impl (ctx : extraction_ctx) (fmt : F.formatter)
  *)
 let extract_unit_test_if_unit_fun (ctx : extraction_ctx) (fmt : F.formatter)
     (def : fun_decl) : unit =
-  (* We only insert unit tests for forward functions *)
-  assert (def.back_id = None);
   (* Check if this is a unit function *)
   let sg = def.signature in
   if
@@ -2791,9 +2669,7 @@ let extract_unit_test_if_unit_fun (ctx : extraction_ctx) (fmt : F.formatter)
         F.pp_print_string fmt "assert_norm";
         F.pp_print_space fmt ();
         F.pp_print_string fmt "(";
-        let fun_name =
-          ctx_get_local_function def.def_id def.loop_id def.back_id ctx
-        in
+        let fun_name = ctx_get_local_function def.def_id def.loop_id ctx in
         F.pp_print_string fmt fun_name;
         if sg.inputs <> [] then (
           F.pp_print_space fmt ();
@@ -2807,9 +2683,7 @@ let extract_unit_test_if_unit_fun (ctx : extraction_ctx) (fmt : F.formatter)
         F.pp_print_string fmt "Check";
         F.pp_print_space fmt ();
         F.pp_print_string fmt "(";
-        let fun_name =
-          ctx_get_local_function def.def_id def.loop_id def.back_id ctx
-        in
+        let fun_name = ctx_get_local_function def.def_id def.loop_id ctx in
         F.pp_print_string fmt fun_name;
         if sg.inputs <> [] then (
           F.pp_print_space fmt ();
@@ -2820,9 +2694,7 @@ let extract_unit_test_if_unit_fun (ctx : extraction_ctx) (fmt : F.formatter)
         F.pp_print_string fmt "#assert";
         F.pp_print_space fmt ();
         F.pp_print_string fmt "(";
-        let fun_name =
-          ctx_get_local_function def.def_id def.loop_id def.back_id ctx
-        in
+        let fun_name = ctx_get_local_function def.def_id def.loop_id ctx in
         F.pp_print_string fmt fun_name;
         if sg.inputs <> [] then (
           F.pp_print_space fmt ();
@@ -2835,9 +2707,7 @@ let extract_unit_test_if_unit_fun (ctx : extraction_ctx) (fmt : F.formatter)
     | HOL4 ->
         F.pp_print_string fmt "val _ = assert_return (";
         F.pp_print_string fmt "“";
-        let fun_name =
-          ctx_get_local_function def.def_id def.loop_id def.back_id ctx
-        in
+        let fun_name = ctx_get_local_function def.def_id def.loop_id ctx in
         F.pp_print_string fmt fun_name;
         if sg.inputs <> [] then (
           F.pp_print_space fmt ();
diff --git a/compiler/ExtractBase.ml b/compiler/ExtractBase.ml
index 04686705..591e8aab 100644
--- a/compiler/ExtractBase.ml
+++ b/compiler/ExtractBase.ml
@@ -167,11 +167,7 @@ type id =
   | TraitImplId of TraitImplId.id
   | LocalTraitClauseId of TraitClauseId.id
   | TraitDeclConstructorId of TraitDeclId.id
-  | TraitMethodId of TraitDeclId.id * string * T.RegionGroupId.id option
-      (** Something peculiar with trait methods: because we have to take into
-          account forward/backward functions, we may need to generate fields
-          items per method.
-       *)
+  | TraitMethodId of TraitDeclId.id * string
   | TraitItemId of TraitDeclId.id * string
       (** A trait associated item which is not a method *)
   | TraitParentClauseId of TraitDeclId.id * TraitClauseId.id
@@ -353,8 +349,6 @@ let basename_to_unique (names_set : StringSet.t)
   in
   if StringSet.mem basename names_set then gen 1 else basename
 
-type fun_name_info = { keep_fwd : bool; num_backs : int }
-
 type names_maps = {
   names_map : names_map;
       (** The map for id to names, where we forbid name collisions
@@ -384,7 +378,7 @@ let allow_collisions (id : id) : bool =
   | FieldId _ | TraitItemClauseId _ | TraitParentClauseId _ | TraitItemId _
   | TraitMethodId _ ->
       !Config.record_fields_short_names
-  | FunId (Pure _ | FromLlbc (FunId (FAssumed _), _, _)) ->
+  | FunId (Pure _ | FromLlbc (FunId (FAssumed _), _)) ->
       (* We map several assumed functions to the same id *)
       true
   | _ -> false
@@ -549,15 +543,6 @@ type extraction_ctx = {
             -- makes the if then else dependent
           ]}
         *)
-  fun_name_info : fun_name_info PureUtils.RegularFunIdMap.t;
-      (** Information used to filter and name functions - we use it
-          to print comments in the generated code, to help link
-          the generated code to the original code (information such
-          as: "this function is the backward function of ...", or
-          "this function is the merged forward/backward function of ..."
-          in case a Rust function only has one backward translation
-          and we filter the forward function because it returns unit.
-        *)
   trait_decl_id : trait_decl_id option;
       (** If we are extracting a trait declaration, identifies it *)
   is_provided_method : bool;
@@ -668,14 +653,8 @@ let id_to_string (id : id) (ctx : extraction_ctx) : string =
       ^ TraitClauseId.to_string clause_id
   | TraitItemId (id, name) ->
       "trait_item_id: " ^ trait_decl_id_to_string id ^ ", type name: " ^ name
-  | TraitMethodId (trait_decl_id, fun_name, rg_id) ->
-      let fwd_back_kind =
-        match rg_id with
-        | None -> "forward"
-        | Some rg_id -> "backward " ^ RegionGroupId.to_string rg_id
-      in
-      trait_decl_id_to_string trait_decl_id
-      ^ ", method name (" ^ fwd_back_kind ^ "): " ^ fun_name
+  | TraitMethodId (trait_decl_id, fun_name) ->
+      trait_decl_id_to_string trait_decl_id ^ ", method name: " ^ fun_name
   | TraitSelfClauseId -> "trait_self_clause"
 
 let ctx_add (id : id) (name : string) (ctx : extraction_ctx) : extraction_ctx =
@@ -694,8 +673,8 @@ let ctx_get_function (id : fun_id) (ctx : extraction_ctx) : string =
   ctx_get (FunId id) ctx
 
 let ctx_get_local_function (id : A.FunDeclId.id) (lp : LoopId.id option)
-    (rg : RegionGroupId.id option) (ctx : extraction_ctx) : string =
-  ctx_get_function (FromLlbc (FunId (FRegular id), lp, rg)) ctx
+    (ctx : extraction_ctx) : string =
+  ctx_get_function (FromLlbc (FunId (FRegular id), lp)) ctx
 
 let ctx_get_type (id : type_id) (ctx : extraction_ctx) : string =
   assert (id <> TTuple);
@@ -733,8 +712,8 @@ let ctx_get_trait_type (id : trait_decl_id) (item_name : string)
   ctx_get_trait_item id item_name ctx
 
 let ctx_get_trait_method (id : trait_decl_id) (item_name : string)
-    (rg_id : T.RegionGroupId.id option) (ctx : extraction_ctx) : string =
-  ctx_get (TraitMethodId (id, item_name, rg_id)) ctx
+    (ctx : extraction_ctx) : string =
+  ctx_get (TraitMethodId (id, item_name)) ctx
 
 let ctx_get_trait_parent_clause (id : trait_decl_id) (clause : trait_clause_id)
     (ctx : extraction_ctx) : string =
@@ -1164,8 +1143,7 @@ let initialize_names_maps () : names_maps =
   in
   let assumed_functions =
     List.map
-      (fun (fid, rg, name) ->
-        (FromLlbc (Pure.FunId (FAssumed fid), None, rg), name))
+      (fun (fid, name) -> (FromLlbc (Pure.FunId (FAssumed fid), None), name))
       init.assumed_llbc_functions
     @ List.map (fun (fid, name) -> (Pure fid, name)) init.assumed_pure_functions
   in
@@ -1408,61 +1386,12 @@ let default_fun_loop_suffix (num_loops : int) (loop_id : LoopId.id option) :
          If this function admits only one loop, we omit it. *)
       if num_loops = 1 then "_loop" else "_loop" ^ LoopId.to_string loop_id
 
-(** A helper function: generates a function suffix from a region group
-    information.
+(** A helper function: generates a function suffix.
     TODO: move all those helpers.
 *)
-let default_fun_suffix (num_loops : int) (loop_id : LoopId.id option)
-    (num_region_groups : int) (rg : region_group_info option)
-    ((keep_fwd, num_backs) : bool * int) : string =
-  let lp_suff = default_fun_loop_suffix num_loops loop_id in
-
-  (* There are several cases:
-     - [rg] is [Some]: this is a forward function:
-       - we add "_fwd"
-     - [rg] is [None]: this is a backward function:
-       - this function has one extracted backward function:
-         - if the forward function has been filtered, we add nothing:
-           the forward function is useless, so the unique backward function
-           takes its place, in a way (in effect, we "merge" the forward
-           and the backward functions).
-         - otherwise we add "_back"
-       - this function has several backward functions: we add "_back" and an
-         additional suffix to identify the precise backward function
-     Note that we always add a suffix (in case there are no region groups,
-     we could not add the "_fwd" suffix) to prevent name clashes between
-     definitions (in particular between type and function definitions).
-  *)
-  let rg_suff =
-    (* TODO: make all the backends match what is done for Lean *)
-    match rg with
-    | None ->
-        if
-          (* In order to avoid name conflicts:
-           * - if the forward is eliminated, we add the suffix "_fwd" (it won't be used)
-           * - otherwise, no suffix (because the backward functions will have a suffix)
-           *)
-          num_backs = 1 && not keep_fwd
-        then "_fwd"
-        else ""
-    | Some rg ->
-        assert (num_region_groups > 0 && num_backs > 0);
-        if num_backs = 1 then
-          (* Exactly one backward function *)
-          if not keep_fwd then "" else "_back"
-        else if
-          (* Several region groups/backward functions:
-             - if all the regions in the group have names, we use those names
-             - otherwise we use an index
-          *)
-          List.for_all Option.is_some rg.region_names
-        then
-          (* Concatenate the region names *)
-          "_back" ^ String.concat "" (List.map Option.get rg.region_names)
-        else (* Use the region index *)
-          "_back" ^ RegionGroupId.to_string rg.id
-  in
-  lp_suff ^ rg_suff
+let default_fun_suffix (num_loops : int) (loop_id : LoopId.id option) : string =
+  (* We only generate a suffix for the functions we generate from the loops *)
+  default_fun_loop_suffix num_loops loop_id
 
 (** Compute the name of a regular (non-assumed) function.
 
@@ -1472,24 +1401,13 @@ let default_fun_suffix (num_loops : int) (loop_id : LoopId.id option)
       indices to derive unique names for the loops for instance - if there is
       exactly one loop, we don't need to use indices)
     - loop id (if pertinent)
-    - number of region groups
-    - region group information in case of a backward function
-      ([None] if forward function)
-    - pair:
-      - do we generate the forward function (it may have been filtered)?
-      - the number of *extracted backward functions* (same comment as for
-        the number of loops)
-        The number of extracted backward functions if not necessarily
-        equal to the number of region groups, because we may have
-        filtered some of them.
     TODO: use the fun id for the assumed functions.
  *)
 let ctx_compute_fun_name (ctx : extraction_ctx) (fname : llbc_name)
-    (num_loops : int) (loop_id : LoopId.id option) (num_rgs : int)
-    (rg : region_group_info option) (filter_info : bool * int) : string =
+    (num_loops : int) (loop_id : LoopId.id option) : string =
   let fname = ctx_compute_fun_name_no_suffix ctx fname in
   (* Compute the suffix *)
-  let suffix = default_fun_suffix num_loops loop_id num_rgs rg filter_info in
+  let suffix = default_fun_suffix num_loops loop_id in
   (* Concatenate *)
   fname ^ suffix
 
@@ -1963,61 +1881,26 @@ let ctx_add_global_decl_and_body (def : A.global_decl) (ctx : extraction_ctx) :
   | None ->
       (* Not the case: "standard" registration *)
       let name = ctx_compute_global_name ctx def.name in
-      let body = FunId (FromLlbc (FunId (FRegular def.body), None, None)) in
+      let body = FunId (FromLlbc (FunId (FRegular def.body), None)) in
       let ctx = ctx_add decl (name ^ "_c") ctx in
       let ctx = ctx_add body (name ^ "_body") ctx in
       ctx
 
-let ctx_compute_fun_name (trans_group : pure_fun_translation) (def : fun_decl)
-    (ctx : extraction_ctx) : string =
-  (* Lookup the LLBC def to compute the region group information *)
-  let def_id = def.def_id in
-  let llbc_def = A.FunDeclId.Map.find def_id ctx.trans_ctx.fun_ctx.fun_decls in
-  let sg = llbc_def.signature in
-  let regions_hierarchy =
-    LlbcAstUtils.FunIdMap.find (FRegular def_id)
-      ctx.trans_ctx.fun_ctx.regions_hierarchies
-  in
-  let num_rgs = List.length regions_hierarchy in
-  let { keep_fwd; fwd = _; backs } = trans_group in
-  let num_backs = List.length backs in
-  let rg_info =
-    match def.back_id with
-    | None -> None
-    | Some rg_id ->
-        let rg = T.RegionGroupId.nth regions_hierarchy rg_id in
-        let region_names =
-          List.map
-            (fun rid -> (T.RegionVarId.nth sg.generics.regions rid).name)
-            rg.regions
-        in
-        Some { id = rg_id; region_names }
-  in
+let ctx_compute_fun_name (def : fun_decl) (ctx : extraction_ctx) : string =
   (* Add the function name *)
-  ctx_compute_fun_name ctx def.llbc_name def.num_loops def.loop_id num_rgs
-    rg_info (keep_fwd, num_backs)
+  ctx_compute_fun_name ctx def.llbc_name def.num_loops def.loop_id
 
 (* TODO: move to Extract *)
-let ctx_add_fun_decl (trans_group : pure_fun_translation) (def : fun_decl)
-    (ctx : extraction_ctx) : extraction_ctx =
+let ctx_add_fun_decl (def : fun_decl) (ctx : extraction_ctx) : extraction_ctx =
   (* Sanity check: the function should not be a global body - those are handled
    * separately *)
   assert (not def.is_global_decl_body);
   (* Lookup the LLBC def to compute the region group information *)
   let def_id = def.def_id in
-  let { keep_fwd; fwd = _; backs } = trans_group in
-  let num_backs = List.length backs in
   (* Add the function name *)
-  let def_name = ctx_compute_fun_name trans_group def ctx in
-  let fun_id = (Pure.FunId (FRegular def_id), def.loop_id, def.back_id) in
-  let ctx = ctx_add (FunId (FromLlbc fun_id)) def_name ctx in
-  (* Add the name info *)
-  {
-    ctx with
-    fun_name_info =
-      PureUtils.RegularFunIdMap.add fun_id { keep_fwd; num_backs }
-        ctx.fun_name_info;
-  }
+  let def_name = ctx_compute_fun_name def ctx in
+  let fun_id = (Pure.FunId (FRegular def_id), def.loop_id) in
+  ctx_add (FunId (FromLlbc fun_id)) def_name ctx
 
 let ctx_compute_type_decl_name (ctx : extraction_ctx) (def : type_decl) : string
     =
diff --git a/compiler/ExtractBuiltin.ml b/compiler/ExtractBuiltin.ml
index 3ea5655a..88de31fe 100644
--- a/compiler/ExtractBuiltin.ml
+++ b/compiler/ExtractBuiltin.ml
@@ -221,12 +221,11 @@ type builtin_fun_info = { extract_name : string } [@@deriving show]
     parameters. For instance, in the case of the `Vec` functions, there is
     a type parameter for the allocator to use, which we want to filter.
  *)
-let builtin_funs () : (pattern * bool list option * builtin_fun_info list) list
-    =
+let builtin_funs () : (pattern * bool list option * builtin_fun_info) list =
   (* Small utility *)
   let mk_fun (rust_name : string) (extract_name : string option)
       (filter : bool list option) :
-      pattern * bool list option * builtin_fun_info list =
+      pattern * bool list option * builtin_fun_info =
     let rust_name =
       try parse_pattern rust_name
       with Failure _ ->
@@ -238,7 +237,7 @@ let builtin_funs () : (pattern * bool list option * builtin_fun_info list) list
       | Some name -> split_on_separator name
     in
     let basename = flatten_name extract_name in
-    let f = [ { extract_name = basename } ] in
+    let f = { extract_name = basename } in
     (rust_name, filter, f)
   in
   [
@@ -377,7 +376,7 @@ type builtin_trait_decl_info = {
           - a Rust name
           - an extraction name
           - a list of clauses *)
-  methods : (string * builtin_fun_info list) list;
+  methods : (string * builtin_fun_info) list;
 }
 [@@deriving show]
 
@@ -418,7 +417,7 @@ let builtin_trait_decls_info () =
           if !record_fields_short_names then item_name
           else extract_name ^ "_" ^ item_name
         in
-        let fwd = [ { extract_name = basename } ] in
+        let fwd = { extract_name = basename } in
         (item_name, fwd)
       in
       List.map mk_method methods
diff --git a/compiler/ExtractTypes.ml b/compiler/ExtractTypes.ml
index a3dbf3cc..05b71b9f 100644
--- a/compiler/ExtractTypes.ml
+++ b/compiler/ExtractTypes.ml
@@ -272,7 +272,7 @@ let start_fun_decl_group (ctx : extraction_ctx) (fmt : F.formatter)
       if is_single_opaque_fun_decl_group dg then ()
       else
         let compute_fun_def_name (def : Pure.fun_decl) : string =
-          ctx_get_local_function def.def_id def.loop_id def.back_id ctx ^ "_def"
+          ctx_get_local_function def.def_id def.loop_id ctx ^ "_def"
         in
         let names = List.map compute_fun_def_name dg in
         (* Add a break before *)
diff --git a/compiler/Main.ml b/compiler/Main.ml
index 664ec067..3f5e62ad 100644
--- a/compiler/Main.ml
+++ b/compiler/Main.ml
@@ -72,12 +72,6 @@ let () =
         Arg.Symbol (backend_names, set_backend),
         " Specify the target backend" );
       ("-dest", Arg.Set_string dest_dir, " Specify the output directory");
-      ( "-no-filter-useless-calls",
-        Arg.Clear filter_useless_monadic_calls,
-        " Do not filter the useless function calls" );
-      ( "-no-filter-useless-funs",
-        Arg.Clear filter_useless_functions,
-        " Do not filter the useless forward/backward functions" );
       ( "-test-units",
         Arg.Set test_unit_functions,
         " Test the unit functions with the concrete (i.e., not symbolic) \
diff --git a/compiler/PrintPure.ml b/compiler/PrintPure.ml
index 66475d02..21ca7f08 100644
--- a/compiler/PrintPure.ml
+++ b/compiler/PrintPure.ml
@@ -462,21 +462,13 @@ let inst_fun_sig_to_string (env : fmt_env) (sg : inst_fun_sig) : string =
   let all_types = List.append inputs [ output ] in
   String.concat " -> " all_types
 
-let fun_suffix (lp_id : LoopId.id option) (rg_id : T.RegionGroupId.id option) :
-    string =
+let fun_suffix (lp_id : LoopId.id option) : string =
   let lp_suff =
     match lp_id with
     | None -> ""
     | Some lp_id -> "^loop" ^ LoopId.to_string lp_id
   in
-
-  let rg_suff =
-    match rg_id with
-    | None -> ""
-    | Some rg_id -> "@" ^ T.RegionGroupId.to_string rg_id
-  in
-
-  lp_suff ^ rg_suff
+  lp_suff
 
 let llbc_assumed_fun_id_to_string (fid : A.assumed_fun_id) : string =
   match fid with
@@ -505,7 +497,7 @@ let pure_assumed_fun_id_to_string (fid : pure_assumed_fun_id) : string =
 
 let regular_fun_id_to_string (env : fmt_env) (fun_id : fun_id) : string =
   match fun_id with
-  | FromLlbc (fid, lp_id, rg_id) ->
+  | FromLlbc (fid, lp_id) ->
       let f =
         match fid with
         | FunId (FRegular fid) -> fun_decl_id_to_string env fid
@@ -513,7 +505,7 @@ let regular_fun_id_to_string (env : fmt_env) (fun_id : fun_id) : string =
         | TraitMethod (trait_ref, method_name, _) ->
             trait_ref_to_string env true trait_ref ^ "." ^ method_name
       in
-      f ^ fun_suffix lp_id rg_id
+      f ^ fun_suffix lp_id
   | Pure fid -> pure_assumed_fun_id_to_string fid
 
 let unop_to_string (unop : unop) : string =
@@ -746,7 +738,7 @@ and emeta_to_string (env : fmt_env) (meta : emeta) : string =
 
 let fun_decl_to_string (env : fmt_env) (def : fun_decl) : string =
   let env = { env with generics = def.signature.generics } in
-  let name = def.name ^ fun_suffix def.loop_id def.back_id in
+  let name = def.name ^ fun_suffix def.loop_id in
   let signature = fun_sig_to_string env def.signature in
   match def.body with
   | None -> "val " ^ name ^ " :\n  " ^ signature
diff --git a/compiler/Pure.ml b/compiler/Pure.ml
index a879ba37..dd7a4acf 100644
--- a/compiler/Pure.ml
+++ b/compiler/Pure.ml
@@ -560,8 +560,7 @@ type fun_id_or_trait_method_ref =
 [@@deriving show, ord]
 
 (** A function id for a non-assumed function *)
-type regular_fun_id =
-  fun_id_or_trait_method_ref * LoopId.id option * RegionGroupId.id option
+type regular_fun_id = fun_id_or_trait_method_ref * LoopId.id option
 [@@deriving show, ord]
 
 (** A function identifier *)
@@ -1078,7 +1077,6 @@ type fun_decl = {
        *)
   loop_id : LoopId.id option;
       (** [Some] if this definition was generated for a loop *)
-  back_id : RegionGroupId.id option;
   llbc_name : llbc_name;  (** The original LLBC name. *)
   name : string;
       (** We use the name only for printing purposes (for debugging):
diff --git a/compiler/PureMicroPasses.ml b/compiler/PureMicroPasses.ml
index ec64df21..04bc90d7 100644
--- a/compiler/PureMicroPasses.ml
+++ b/compiler/PureMicroPasses.ml
@@ -925,156 +925,9 @@ let inline_useless_var_reassignments (ctx : trans_ctx) ~(inline_named : bool)
       in
       { def with body = Some body }
 
-(** For the cases where we split the forward/backward functions.
-
-    Given a forward or backward function call, is there, for every execution
-    path, a child backward function called later with exactly the same input
-    list prefix. We use this to filter useless function calls: if there are
-    such child calls, we can remove this one (in case its outputs are not
-    used).
-    We do this check because we can't simply remove function calls whose
-    outputs are not used, as they might fail. However, if a function fails,
-    its children backward functions then fail on the same inputs (ignoring
-    the additional inputs those receive).
-    
-    For instance, if we have:
-    {[
-      fn f<'a>(x : &'a mut T);
-    ]}
-    
-    We often have things like this in the synthesized code:
-    {[
-      _ <-- f@fwd x;
-      ...
-      nx <-- f@back'a x y;
-      ...
-    ]}
-
-    If [f@back'a x y] fails, then necessarily [f@fwd x] also fails.
-    In this situation, we can remove the call [f@fwd x].
- *)
-let expression_contains_child_call_in_all_paths (ctx : trans_ctx)
-    (id0 : fun_id_or_trait_method_ref) (lp_id0 : LoopId.id option)
-    (rg_id0 : T.RegionGroupId.id option) (generics0 : generic_args)
-    (args0 : texpression list) (e : texpression) : bool =
-  let check_call (fun_id1 : fun_or_op_id) (generics1 : generic_args)
-      (args1 : texpression list) : bool =
-    (* Check the fun_ids, to see if call1's function is a child of call0's function *)
-    match fun_id1 with
-    | Fun (FromLlbc (id1, lp_id1, rg_id1)) ->
-        (* Both are "regular" calls: check if they come from the same rust function *)
-        if id0 = id1 && lp_id0 = lp_id1 then
-          (* Same rust functions: check the regions hierarchy *)
-          let call1_is_child =
-            match (rg_id0, rg_id1) with
-            | None, _ ->
-                (* The function used in call0 is the forward function: the one
-                 * used in call1 is necessarily a child *)
-                true
-            | Some _, None ->
-                (* Opposite of previous case *)
-                false
-            | Some rg_id0, Some rg_id1 ->
-                if rg_id0 = rg_id1 then true
-                else
-                  (* We need to use the regions hierarchy *)
-                  let regions_hierarchy =
-                    let id0 =
-                      match id0 with
-                      | FunId fun_id -> fun_id
-                      | TraitMethod (_, _, fun_decl_id) -> FRegular fun_decl_id
-                    in
-                    LlbcAstUtils.FunIdMap.find id0
-                      ctx.fun_ctx.regions_hierarchies
-                  in
-                  (* Compute the set of ancestors of the function in call1 *)
-                  let call1_ancestors =
-                    LlbcAstUtils.list_ancestor_region_groups regions_hierarchy
-                      rg_id1
-                  in
-                  (* Check if the function used in call0 is inside *)
-                  T.RegionGroupId.Set.mem rg_id0 call1_ancestors
-          in
-          (* If call1 is a child, then we need to check if the input arguments
-           * used in call0 are a prefix of the input arguments used in call1
-           * (note call1 being a child, it will likely consume strictly more
-           * given back values).
-           * *)
-          if call1_is_child then
-            let call1_args =
-              Collections.List.prefix (List.length args0) args1
-            in
-            let args = List.combine args0 call1_args in
-            (* Note that the input values are expressions, *which may contain
-             * meta-values* (which we need to ignore). *)
-            let input_eq (v0, v1) =
-              PureUtils.remove_meta v0 = PureUtils.remove_meta v1
-            in
-            (* Compare the generics and the prefix of the input arguments *)
-            generics0 = generics1 && List.for_all input_eq args
-          else (* Not a child *)
-            false
-        else (* Not the same function *)
-          false
-    | _ -> false
-  in
-
-  let visitor =
-    object (self)
-      inherit [_] reduce_expression
-      method zero _ = false
-      method plus b0 b1 _ = b0 () && b1 ()
-
-      method! visit_texpression env e =
-        match e.e with
-        | Var _ | CVar _ | Const _ -> fun _ -> false
-        | StructUpdate _ ->
-            (* There shouldn't be monadic calls in structure updates - also
-               note that by returning [false] we are conservative: we might
-               *prevent* possible optimisations (i.e., filtering some function
-               calls), which is sound. *)
-            fun _ -> false
-        | Let (_, _, re, e) -> (
-            match opt_destruct_function_call re with
-            | None -> fun () -> self#visit_texpression env e ()
-            | Some (func1, generics1, args1) ->
-                let call_is_child = check_call func1 generics1 args1 in
-                if call_is_child then fun () -> true
-                else fun () -> self#visit_texpression env e ())
-        | Lambda (_, e) -> self#visit_texpression env e
-        | App _ -> (
-            fun () ->
-              match opt_destruct_function_call e with
-              | Some (func1, tys1, args1) -> check_call func1 tys1 args1
-              | None -> false)
-        | Qualif _ ->
-            (* Note that this case includes functions without arguments *)
-            fun () -> false
-        | Meta (_, e) -> self#visit_texpression env e
-        | Loop loop ->
-            (* We only visit the *function end* *)
-            self#visit_texpression env loop.fun_end
-        | Switch (_, body) -> self#visit_switch_body env body
-
-      method! visit_switch_body env body =
-        match body with
-        | If (e1, e2) ->
-            fun () ->
-              self#visit_texpression env e1 ()
-              && self#visit_texpression env e2 ()
-        | Match branches ->
-            fun () ->
-              List.for_all
-                (fun br -> self#visit_texpression env br.branch ())
-                branches
-    end
-  in
-  visitor#visit_texpression () e ()
-
 (** Filter the useless assignments (removes the useless variables, filters
     the function calls) *)
-let filter_useless (filter_monadic_calls : bool) (ctx : trans_ctx)
-    (def : fun_decl) : fun_decl =
+let filter_useless (_ctx : trans_ctx) (def : fun_decl) : fun_decl =
   (* We first need a transformation on *left-values*, which filters the useless
    * variables and tells us whether the value contains any variable which has
    * not been replaced by [_] (in which case we need to keep the assignment,
@@ -1166,30 +1019,8 @@ let filter_useless (filter_monadic_calls : bool) (ctx : trans_ctx)
               if not monadic then
                 (* Not a monadic let-binding: simple case *)
                 (e.e, fun _ -> used)
-              else
-                (* Monadic let-binding: trickier.
-                 * We can filter if the right-expression is a function call,
-                 * under some conditions. *)
-                match (filter_monadic_calls, opt_destruct_function_call re) with
-                | true, Some (Fun (FromLlbc (fid, lp_id, rg_id)), tys, args) ->
-                    (* If we split the forward/backward functions.
-
-                       We need to check if there is a child call - see
-                       the comments for:
-                       [expression_contains_child_call_in_all_paths] *)
-                    if not !Config.return_back_funs then
-                      let has_child_call =
-                        expression_contains_child_call_in_all_paths ctx fid
-                          lp_id rg_id tys args e
-                      in
-                      if has_child_call then (* Filter *)
-                        (e.e, fun _ -> used)
-                      else (* No child call: don't filter *)
-                        dont_filter ()
-                    else dont_filter ()
-                | _ ->
-                    (* Not an LLBC function call or not allowed to filter: we can't filter *)
-                    dont_filter ()
+              else (* Monadic let-binding: can't filter *)
+                dont_filter ()
             else (* There are used variables: don't filter *)
               dont_filter ()
         | Loop loop ->
@@ -1442,22 +1273,6 @@ let simplify_aggregates (ctx : trans_ctx) (def : fun_decl) : fun_decl =
       let body = { body with body = body_exp } in
       { def with body = Some body }
 
-(** Return [None] if the function is a backward function with no outputs (so
-    that we eliminate the definition which is useless).
-
-    Note that the calls to such functions are filtered when translating from
-    symbolic to pure. Here, we remove the definitions altogether, because they
-    are now useless
-  *)
-let filter_if_backward_with_no_outputs (def : fun_decl) : fun_decl option =
-  if
-    !Config.filter_useless_functions
-    && Option.is_some def.back_id
-    && def.signature.output = mk_result_ty mk_unit_ty
-    || def.signature.output = mk_unit_ty
-  then None
-  else Some def
-
 (** Retrieve the loop definitions from the function definition.
 
     {!SymbolicToPure} generates an AST in which the loop bodies are part of
@@ -1530,14 +1345,7 @@ let decompose_loops (_ctx : trans_ctx) (def : fun_decl) :
                   info.num_inputs_with_fuel_no_state
                   info.num_inputs_with_fuel_with_state
               in
-              let back_inputs =
-                if !Config.return_back_funs then []
-                else
-                  snd
-                    (Collections.List.split_at fun_sig.inputs
-                       info.num_inputs_with_fuel_with_state)
-              in
-              List.concat [ fuel; fwd_inputs; fwd_state; back_inputs ]
+              List.concat [ fuel; fwd_inputs; fwd_state ]
             in
 
             let output = loop.output_ty in
@@ -1618,7 +1426,6 @@ let decompose_loops (_ctx : trans_ctx) (def : fun_decl) :
                 kind = def.kind;
                 num_loops;
                 loop_id = Some loop.loop_id;
-                back_id = def.back_id;
                 llbc_name = def.llbc_name;
                 name = def.name;
                 signature = loop_sig;
@@ -1640,35 +1447,6 @@ let decompose_loops (_ctx : trans_ctx) (def : fun_decl) :
       let loops = List.map snd (LoopId.Map.bindings !loops) in
       (def, loops)
 
-(** Return [false] if the forward function is useless and should be filtered.
-
-    - a forward function with no output (comes from a Rust function with
-      unit return type)
-    - the function has mutable borrows as inputs (which is materialized
-      by the fact we generated backward functions which were not filtered).
-
-    In such situation, every call to the Rust function will be translated to:
-    - a call to the forward function which returns nothing
-    - calls to the backward functions
-    As a failing backward function implies the forward function also fails,
-    we can filter the calls to the forward function, which thus becomes
-    useless.
-    In such situation, we can remove the forward function definition
-    altogether.
-  *)
-let keep_forward (fwd : fun_and_loops) (backs : fun_and_loops list) : bool =
-  (* The question of filtering the forward functions arises only if we split
-     the forward/backward functions *)
-  if !Config.return_back_funs then true
-  else if
-    (* Note that at this point, the output types are no longer seen as tuples:
-     * they should be lists of length 1. *)
-    !Config.filter_useless_functions
-    && fwd.f.signature.output = mk_result_ty mk_unit_ty
-    && backs <> []
-  then false
-  else true
-
 (** Convert the unit variables to [()] if they are used as right-values or
     [_] if they are used as left values in patterns. *)
 let unit_vars_to_unit (def : fun_decl) : fun_decl =
@@ -1724,19 +1502,17 @@ let eliminate_box_functions (_ctx : trans_ctx) (def : fun_decl) : fun_decl =
              * could have: [box_new f x])
              * *)
             match fun_id with
-            | Fun (FromLlbc (FunId (FAssumed aid), _lp_id, rg_id)) -> (
-                match (aid, rg_id) with
-                | BoxNew, _ ->
-                    assert (rg_id = None);
+            | Fun (FromLlbc (FunId (FAssumed aid), _lp_id)) -> (
+                match aid with
+                | BoxNew ->
                     let arg, args = Collections.List.pop args in
                     mk_apps arg args
-                | BoxFree, _ ->
+                | BoxFree ->
                     assert (args = []);
                     mk_unit_rvalue
-                | ( ( SliceIndexShared | SliceIndexMut | ArrayIndexShared
-                    | ArrayIndexMut | ArrayToSliceShared | ArrayToSliceMut
-                    | ArrayRepeat ),
-                    _ ) ->
+                | SliceIndexShared | SliceIndexMut | ArrayIndexShared
+                | ArrayIndexMut | ArrayToSliceShared | ArrayToSliceMut
+                | ArrayRepeat ->
                     super#visit_texpression env e)
             | _ -> super#visit_texpression env e)
         | _ -> super#visit_texpression env e
@@ -1989,7 +1765,7 @@ let apply_end_passes_to_def (ctx : trans_ctx) (def : fun_decl) : fun_decl =
     (lazy ("eliminate_box_functions:\n\n" ^ fun_decl_to_string ctx def ^ "\n"));
 
   (* Filter the useless variables, assignments, function calls, etc. *)
-  let def = filter_useless !Config.filter_useless_monadic_calls ctx def in
+  let def = filter_useless ctx def in
   log#ldebug (lazy ("filter_useless:\n\n" ^ fun_decl_to_string ctx def ^ "\n"));
 
   (* Simplify the lets immediately followed by a return.
@@ -2130,16 +1906,7 @@ let filter_loop_inputs (transl : pure_fun_translation list) :
   *)
   let all_decls =
     List.concat
-      (List.concat
-         (List.concat
-            (List.map
-               (fun { fwd; backs; _ } ->
-                 [ fwd.f :: fwd.loops ]
-                 :: List.map
-                      (fun { f = back; loops = loops_back } ->
-                        [ back :: loops_back ])
-                      backs)
-               transl)))
+      (List.concat (List.map (fun { f; loops } -> [ f :: loops ]) transl))
   in
   let subgroups = ReorderDecls.group_reorder_fun_decls all_decls in
 
@@ -2207,7 +1974,7 @@ let filter_loop_inputs (transl : pure_fun_translation list) :
               match e_app.e with
               | Qualif qualif -> (
                   match qualif.id with
-                  | FunOrOp (Fun (FromLlbc (FunId fun_id', loop_id', _))) ->
+                  | FunOrOp (Fun (FromLlbc (FunId fun_id', loop_id'))) ->
                       if (fun_id', loop_id') = fun_id then (
                         (* For each argument, check if it is exactly the original
                            input parameter. Note that there shouldn't be partial
@@ -2357,8 +2124,7 @@ let filter_loop_inputs (transl : pure_fun_translation list) :
                     match e_app.e with
                     | Qualif qualif -> (
                         match qualif.id with
-                        | FunOrOp (Fun (FromLlbc (FunId fun_id, loop_id, _)))
-                          -> (
+                        | FunOrOp (Fun (FromLlbc (FunId fun_id, loop_id))) -> (
                             match
                               FunLoopIdMap.find_opt (fun_id, loop_id) !used_map
                             with
@@ -2400,13 +2166,8 @@ let filter_loop_inputs (transl : pure_fun_translation list) :
   in
   let transl =
     List.map
-      (fun trans ->
-        let filter_fun_and_loops f =
-          { f = filter_in_one f.f; loops = List.map filter_in_one f.loops }
-        in
-        let fwd = filter_fun_and_loops trans.fwd in
-        let backs = List.map filter_fun_and_loops trans.backs in
-        { trans with fwd; backs })
+      (fun f ->
+        { f = filter_in_one f.f; loops = List.map filter_in_one f.loops })
       transl
   in
 
@@ -2420,18 +2181,11 @@ let filter_loop_inputs (transl : pure_fun_translation list) :
     it thus returns the pair: (function def, loop defs). See {!decompose_loops}
     for more information.
 
-    Will return [None] if the function is a backward function with no outputs.
-
     [ctx]: used only for printing.
  *)
-let apply_passes_to_def (ctx : trans_ctx) (def : fun_decl) :
-    fun_and_loops option =
+let apply_passes_to_def (ctx : trans_ctx) (def : fun_decl) : fun_and_loops =
   (* Debug *)
-  log#ldebug
-    (lazy
-      ("PureMicroPasses.apply_passes_to_def: " ^ def.name ^ " ("
-      ^ Print.option_to_string T.RegionGroupId.to_string def.back_id
-      ^ ")"));
+  log#ldebug (lazy ("PureMicroPasses.apply_passes_to_def: " ^ def.name));
 
   log#ldebug (lazy ("original decl:\n\n" ^ fun_decl_to_string ctx def ^ "\n"));
 
@@ -2451,29 +2205,13 @@ let apply_passes_to_def (ctx : trans_ctx) (def : fun_decl) :
   let def = remove_meta def in
   log#ldebug (lazy ("remove_meta:\n\n" ^ fun_decl_to_string ctx def ^ "\n"));
 
-  (* Remove the backward functions with no outputs.
+  (* Extract the loop definitions by removing the {!Loop} node *)
+  let def, loops = decompose_loops ctx def in
 
-     Note that the *calls* to those functions should already have been removed,
-     when translating from symbolic to pure. Here, we remove the definitions
-     altogether, because they are now useless *)
-  let name = def.name ^ PrintPure.fun_suffix def.loop_id def.back_id in
-  let opt_def = filter_if_backward_with_no_outputs def in
-
-  match opt_def with
-  | None ->
-      log#ldebug (lazy ("filtered (backward with no outputs): " ^ name ^ "\n"));
-      None
-  | Some def ->
-      log#ldebug
-        (lazy ("not filtered (not backward with no outputs): " ^ name ^ "\n"));
-
-      (* Extract the loop definitions by removing the {!Loop} node *)
-      let def, loops = decompose_loops ctx def in
-
-      (* Apply the remaining passes *)
-      let f = apply_end_passes_to_def ctx def in
-      let loops = List.map (apply_end_passes_to_def ctx) loops in
-      Some { f; loops }
+  (* Apply the remaining passes *)
+  let f = apply_end_passes_to_def ctx def in
+  let loops = List.map (apply_end_passes_to_def ctx) loops in
+  { f; loops }
 
 (** Apply the micro-passes to a list of forward/backward translations.
 
@@ -2489,18 +2227,11 @@ let apply_passes_to_def (ctx : trans_ctx) (def : fun_decl) :
     but convenient.
  *)
 let apply_passes_to_pure_fun_translations (ctx : trans_ctx)
-    (transl : (fun_decl * fun_decl list) list) : pure_fun_translation list =
-  let apply_to_one (trans : fun_decl * fun_decl list) : pure_fun_translation =
-    (* Apply the passes to the individual functions *)
-    let fwd, backs = trans in
-    let fwd = Option.get (apply_passes_to_def ctx fwd) in
-    let backs = List.filter_map (apply_passes_to_def ctx) backs in
-    (* Compute whether we need to filter the forward function or not *)
-    let keep_fwd = keep_forward fwd backs in
-    { keep_fwd; fwd; backs }
-  in
-
-  let transl = List.map apply_to_one transl in
+    (transl : fun_decl list) : pure_fun_translation list =
+  (* Apply the micro-passes *)
+  let transl = List.map (apply_passes_to_def ctx) transl in
 
-  (* Filter the useless inputs in the loop functions *)
+  (* Filter the useless inputs in the loop functions (loops are initially
+     parameterized by *all* the symbolic values in the context, because
+     they may access any of them). *)
   filter_loop_inputs transl
diff --git a/compiler/ReorderDecls.ml b/compiler/ReorderDecls.ml
index 53c94ff4..f5443e03 100644
--- a/compiler/ReorderDecls.ml
+++ b/compiler/ReorderDecls.ml
@@ -5,11 +5,7 @@ open Pure
 (** The local logger *)
 let log = Logging.reorder_decls_log
 
-type fun_id = {
-  def_id : FunDeclId.id;
-  lp_id : LoopId.id option;
-  rg_id : T.RegionGroupId.id option;
-}
+type fun_id = { def_id : FunDeclId.id; lp_id : LoopId.id option }
 [@@deriving show, ord]
 
 module FunIdOrderedType : OrderedType with type t = fun_id = struct
@@ -43,11 +39,11 @@ let compute_body_fun_deps (e : texpression) : FunIdSet.t =
         | FunOrOp (Fun fid) -> (
             match fid with
             | Pure _ -> ()
-            | FromLlbc (fid, lp_id, rg_id) -> (
+            | FromLlbc (fid, lp_id) -> (
                 match fid with
                 | FunId (FAssumed _) -> ()
                 | TraitMethod (_, _, fid) | FunId (FRegular fid) ->
-                    let id = { def_id = fid; lp_id; rg_id } in
+                    let id = { def_id = fid; lp_id } in
                     ids := FunIdSet.add id !ids))
     end
   in
@@ -71,7 +67,7 @@ let group_reorder_fun_decls (decls : fun_decl list) :
     (bool * fun_decl list) list =
   let module IntMap = MakeMap (OrderedInt) in
   let get_fun_id (decl : fun_decl) : fun_id =
-    { def_id = decl.def_id; lp_id = decl.loop_id; rg_id = decl.back_id }
+    { def_id = decl.def_id; lp_id = decl.loop_id }
   in
   (* Compute the list/set of identifiers *)
   let idl = List.map get_fun_id decls in
diff --git a/compiler/SymbolicToPure.ml b/compiler/SymbolicToPure.ml
index 859d6f17..2db5f66c 100644
--- a/compiler/SymbolicToPure.ml
+++ b/compiler/SymbolicToPure.ml
@@ -2035,7 +2035,7 @@ and translate_function_call (call : S.call) (e : S.expression) (ctx : bs_ctx) :
     | S.Fun (fid, call_id) ->
         (* Regular function call *)
         let fid_t = translate_fun_id_or_trait_method_ref ctx fid in
-        let func = Fun (FromLlbc (fid_t, None, None)) in
+        let func = Fun (FromLlbc (fid_t, None)) in
         (* Retrieve the effect information about this function (can fail,
          * takes a state as input, etc.) *)
         let effect_info = get_fun_effect_info ctx fid None None in
@@ -2539,8 +2539,6 @@ and translate_end_abstraction_loop (ectx : C.eval_ctx) (abs : V.abs)
         get_fun_effect_info ctx (FunId fun_id) (Some vloop_id) (Some rg_id)
       in
       let loop_info = LoopId.Map.find loop_id ctx.loops in
-      let generics = loop_info.generics in
-      let fwd_inputs = Option.get loop_info.forward_inputs in
       (* Retrieve the additional backward inputs. Note that those are actually
          the backward inputs of the function we are synthesizing (and that we
          need to *transmit* to the loop backward function): they are not the
@@ -2582,10 +2580,6 @@ and translate_end_abstraction_loop (ectx : C.eval_ctx) (abs : V.abs)
           (fun (arg, mp) -> mk_opt_mplace_texpression mp arg)
           (List.combine inputs args_mplaces)
       in
-      let input_tys = (List.map (fun (x : texpression) -> x.ty)) args in
-      let ret_ty =
-        if effect_info.can_fail then mk_result_ty output.ty else output.ty
-      in
       (* Create the expression for the function:
          - it is either a call to a top-level function, if we split the
            forward/backward functions
@@ -3218,7 +3212,7 @@ and translate_forward_end (ectx : C.eval_ctx)
       let out_pat = mk_simpl_tuple_pattern out_pats in
 
       let loop_call =
-        let fun_id = Fun (FromLlbc (FunId fid, Some loop_id, None)) in
+        let fun_id = Fun (FromLlbc (FunId fid, Some loop_id)) in
         let func = { id = FunOrOp fun_id; generics = loop_info.generics } in
         let input_tys = (List.map (fun (x : texpression) -> x.ty)) args in
         let ret_ty =
@@ -3567,9 +3561,6 @@ let translate_fun_decl (ctx : bs_ctx) (body : S.expression option) : fun_decl =
   let name = name_to_string ctx llbc_name in
   (* Translate the signature *)
   let signature = translate_fun_sig_from_decomposed ctx.sg in
-  let regions_hierarchy =
-    FunIdMap.find (FRegular def_id) ctx.fun_ctx.regions_hierarchies
-  in
   (* Translate the body, if there is *)
   let body =
     match body with
diff --git a/compiler/Translate.ml b/compiler/Translate.ml
index 55a94302..c12de045 100644
--- a/compiler/Translate.ml
+++ b/compiler/Translate.ml
@@ -1,5 +1,4 @@
 open Interpreter
-open Expressions
 open Types
 open Values
 open LlbcAst
@@ -49,8 +48,6 @@ let translate_function_to_pure (trans_ctx : trans_ctx)
   log#ldebug
     (lazy ("translate_function_to_pure: " ^ name_to_string trans_ctx fdef.name));
 
-  let def_id = fdef.def_id in
-
   (* Compute the symbolic ASTs, if the function is transparent *)
   let symbolic_trans = translate_function_to_symbolics trans_ctx fdef in
 
@@ -124,20 +121,7 @@ let translate_function_to_pure (trans_ctx : trans_ctx)
     SymbolicToPure.translate_fun_sig_from_decl_to_decomposed trans_ctx fdef
   in
 
-  let regions_hierarchy =
-    LlbcAstUtils.FunIdMap.find (FRegular def_id) fun_ctx.regions_hierarchies
-  in
-
-  let var_counter, back_state_vars =
-    if !Config.return_back_funs then (var_counter, [])
-    else
-      List.fold_left_map
-        (fun var_counter (region_vars : region_var_group) ->
-          let gid = region_vars.id in
-          let var, var_counter = Pure.VarId.fresh var_counter in
-          (var_counter, (gid, var)))
-        var_counter regions_hierarchy
-  in
+  let var_counter, back_state_vars = (var_counter, []) in
   let back_state_vars = RegionGroupId.Map.of_list back_state_vars in
 
   let ctx =
@@ -195,28 +179,7 @@ let translate_function_to_pure (trans_ctx : trans_ctx)
   in
 
   (* Add the backward inputs *)
-  let ctx, backward_inputs_no_state, backward_inputs_with_state =
-    if !Config.return_back_funs then (ctx, [], [])
-    else
-      let ctx, inputs_no_with_state =
-        List.fold_left_map
-          (fun ctx (region_vars : region_var_group) ->
-            let gid = region_vars.id in
-            let back_sg = RegionGroupId.Map.find gid sg.back_sg in
-            let ctx, no_state =
-              SymbolicToPure.fresh_vars back_sg.inputs_no_state ctx
-            in
-            let ctx, with_state =
-              SymbolicToPure.fresh_vars back_sg.inputs ctx
-            in
-            (ctx, ((gid, no_state), (gid, with_state))))
-          ctx regions_hierarchy
-      in
-      let inputs_no_state, inputs_with_state =
-        List.split inputs_no_with_state
-      in
-      (ctx, inputs_no_state, inputs_with_state)
-  in
+  let backward_inputs_no_state, backward_inputs_with_state = ([], []) in
   let backward_inputs_no_state =
     RegionGroupId.Map.of_list backward_inputs_no_state
   in
@@ -225,40 +188,10 @@ let translate_function_to_pure (trans_ctx : trans_ctx)
   in
   let ctx = { ctx with backward_inputs_no_state; backward_inputs_with_state } in
 
-  (* Translate the forward function *)
-  let pure_forward =
-    match symbolic_trans with
-    | None -> SymbolicToPure.translate_fun_decl ctx None
-    | Some (_, ast) -> SymbolicToPure.translate_fun_decl ctx (Some ast)
-  in
-
-  (* Translate the backward functions, if we split the forward and backward functions *)
-  let translate_backward (rg : region_var_group) : Pure.fun_decl =
-    (* For the backward inputs/outputs initialization: we use the fact that
-     * there are no nested borrows for now, and so that the region groups
-     * can't have parents *)
-    assert (rg.parents = []);
-    let back_id = rg.id in
-
-    match symbolic_trans with
-    | None ->
-        (* Initialize the context *)
-        let ctx = { ctx with bid = Some back_id } in
-        (* Translate *)
-        SymbolicToPure.translate_fun_decl ctx None
-    | Some (_, symbolic) ->
-        (* Initialize the context *)
-        let ctx = { ctx with bid = Some back_id } in
-        (* Translate *)
-        SymbolicToPure.translate_fun_decl ctx (Some symbolic)
-  in
-  let pure_backwards =
-    if !Config.return_back_funs then []
-    else List.map translate_backward regions_hierarchy
-  in
-
-  (* Return *)
-  (pure_forward, pure_backwards)
+  (* Translate the function *)
+  match symbolic_trans with
+  | None -> SymbolicToPure.translate_fun_decl ctx None
+  | Some (_, ast) -> SymbolicToPure.translate_fun_decl ctx (Some ast)
 
 (* TODO: factor out the return type *)
 let translate_crate_to_pure (crate : crate) :
@@ -513,9 +446,8 @@ let export_global (fmt : Format.formatter) (config : gen_config) (ctx : gen_ctx)
   let global_decls = ctx.trans_ctx.global_ctx.global_decls in
   let global = GlobalDeclId.Map.find id global_decls in
   let trans = FunDeclId.Map.find global.body ctx.trans_funs in
-  assert (trans.fwd.loops = []);
-  assert (trans.backs = []);
-  let body = trans.fwd.f in
+  assert (trans.loops = []);
+  let body = trans.f in
 
   let is_opaque = Option.is_none body.Pure.body in
   (* Check if we extract the global *)
@@ -643,7 +575,7 @@ let export_functions_group (fmt : Format.formatter) (config : gen_config)
     let funs_map = builtin_funs_map () in
     List.map
       (fun (trans : pure_fun_translation) ->
-        match_name_find_opt ctx.trans_ctx trans.fwd.f.llbc_name funs_map <> None)
+        match_name_find_opt ctx.trans_ctx trans.f.llbc_name funs_map <> None)
       pure_ls
   in
 
@@ -660,7 +592,7 @@ let export_functions_group (fmt : Format.formatter) (config : gen_config)
     (* Extract the decrease clauses template bodies *)
     if config.extract_template_decreases_clauses then
       List.iter
-        (fun { fwd; _ } ->
+        (fun f ->
           (* We only generate decreases clauses for the forward functions, because
              the termination argument should only depend on the forward inputs.
              The backward functions thus use the same decreases clauses as the
@@ -687,27 +619,14 @@ let export_functions_group (fmt : Format.formatter) (config : gen_config)
                   raise
                     (Failure "HOL4 doesn't have decreases/termination clauses")
           in
-          extract_decrease fwd.f;
-          List.iter extract_decrease fwd.loops)
+          extract_decrease f.f;
+          List.iter extract_decrease f.loops)
         pure_ls;
 
-    (* Concatenate the function definitions, filtering the useless forward
-     * functions. *)
+    (* Flatten the translated functions (concatenate the functions with
+       the declarations introduced for the loops) *)
     let decls =
-      List.concat
-        (List.map
-           (fun { keep_fwd; fwd; backs } ->
-             let fwd =
-               if keep_fwd then List.append fwd.loops [ fwd.f ] else []
-             in
-             let backs : Pure.fun_decl list =
-               List.concat
-                 (List.map
-                    (fun back -> List.append back.loops [ back.f ])
-                    backs)
-             in
-             List.append fwd backs)
-           pure_ls)
+      List.concat (List.map (fun f -> List.append f.loops [ f.f ]) pure_ls)
     in
 
     (* Extract the function definitions *)
@@ -724,9 +643,7 @@ let export_functions_group (fmt : Format.formatter) (config : gen_config)
     (* Insert unit tests if necessary *)
     if config.test_trans_unit_functions then
       List.iter
-        (fun trans ->
-          if trans.keep_fwd then
-            Extract.extract_unit_test_if_unit_fun ctx fmt trans.fwd.f)
+        (fun trans -> Extract.extract_unit_test_if_unit_fun ctx fmt trans.f)
         pure_ls
 
 (** Export a trait declaration. *)
@@ -812,7 +729,7 @@ let extract_definitions (fmt : Format.formatter) (config : gen_config)
            extract their type directly in the records we generate for
            the trait declarations themselves, there is no point in having
            separate type definitions) *)
-        match pure_fun.fwd.f.Pure.kind with
+        match pure_fun.f.Pure.kind with
         | TraitMethodDecl _ -> ()
         | _ ->
             (* Translate *)
@@ -1001,18 +918,18 @@ let translate_crate (filename : string) (dest_dir : string) (crate : crate) :
    * whether we should generate a decrease clause or not. *)
   let rec_functions =
     List.map
-      (fun { fwd; _ } ->
-        let fwd_f =
-          if fwd.f.Pure.signature.fwd_info.effect_info.is_rec then
-            [ (fwd.f.def_id, None) ]
+      (fun trans ->
+        let f =
+          if trans.f.Pure.signature.fwd_info.effect_info.is_rec then
+            [ (trans.f.def_id, None) ]
           else []
         in
-        let loop_fwds =
+        let loops =
           List.map
             (fun (def : Pure.fun_decl) -> [ (def.def_id, def.loop_id) ])
-            fwd.loops
+            trans.loops
         in
-        fwd_f :: loop_fwds)
+        f :: loops)
       trans_funs
   in
   let rec_functions : PureUtils.fun_loop_id list =
@@ -1028,7 +945,7 @@ let translate_crate (filename : string) (dest_dir : string) (crate : crate) :
   let trans_funs : pure_fun_translation FunDeclId.Map.t =
     FunDeclId.Map.of_list
       (List.map
-         (fun (trans : pure_fun_translation) -> (trans.fwd.f.def_id, trans))
+         (fun (trans : pure_fun_translation) -> (trans.f.def_id, trans))
          trans_funs)
   in
 
@@ -1052,7 +969,6 @@ let translate_crate (filename : string) (dest_dir : string) (crate : crate) :
       names_maps;
       indent_incr = 2;
       use_dep_ite = !Config.backend = Lean && !Config.extract_decreases_clauses;
-      fun_name_info = PureUtils.RegularFunIdMap.empty;
       trait_decl_id = None (* None by default *);
       is_provided_method = false (* false by default *);
       trans_trait_decls;
@@ -1082,7 +998,6 @@ let translate_crate (filename : string) (dest_dir : string) (crate : crate) :
       (fun ctx (trans : pure_fun_translation) ->
         (* If requested by the user, register termination measures and decreases
            proofs for all the recursive functions *)
-        let fwd_def = trans.fwd.f in
         let gen_decr_clause (def : Pure.fun_decl) =
           !Config.extract_decreases_clauses
           && PureUtils.FunLoopIdSet.mem
@@ -1091,7 +1006,7 @@ let translate_crate (filename : string) (dest_dir : string) (crate : crate) :
         in
         (* Register the names, only if the function is not a global body -
          * those are handled later *)
-        let is_global = fwd_def.Pure.is_global_decl_body in
+        let is_global = trans.f.Pure.is_global_decl_body in
         if is_global then ctx
         else Extract.extract_fun_decl_register_names ctx gen_decr_clause trans)
       ctx
@@ -1171,13 +1086,7 @@ let translate_crate (filename : string) (dest_dir : string) (crate : crate) :
     let exe_dir = Filename.dirname Sys.argv.(0) in
     let primitives_src_dest =
       match !Config.backend with
-      | FStar ->
-          let src =
-            if !Config.return_back_funs then
-              "/backends/fstar/merge/Primitives.fst"
-            else "/backends/fstar/split/Primitives.fst"
-          in
-          Some (src, "Primitives.fst")
+      | FStar -> Some ("/backends/fstar/merge/Primitives.fst", "Primitives.fst")
       | Coq -> Some ("/backends/coq/Primitives.v", "Primitives.v")
       | Lean -> None
       | HOL4 -> None
diff --git a/compiler/TranslateCore.ml b/compiler/TranslateCore.ml
index 88438872..05877b5a 100644
--- a/compiler/TranslateCore.ml
+++ b/compiler/TranslateCore.ml
@@ -8,19 +8,8 @@ let log = Logging.translate_log
 
 type trans_ctx = decls_ctx [@@deriving show]
 type fun_and_loops = { f : Pure.fun_decl; loops : Pure.fun_decl list }
-type pure_fun_translation_no_loops = Pure.fun_decl * Pure.fun_decl list
-
-type pure_fun_translation = {
-  keep_fwd : bool;
-      (** Should we extract the forward function?
-
-          If the forward function returns `()` and there is exactly one
-          backward function, we may merge the forward into the backward
-          function and thus don't extract the forward function)?
-       *)
-  fwd : fun_and_loops;
-  backs : fun_and_loops list;
-}
+type pure_fun_translation_no_loops = Pure.fun_decl
+type pure_fun_translation = fun_and_loops
 
 let trans_ctx_to_fmt_env (ctx : trans_ctx) : Print.fmt_env =
   Print.Contexts.decls_ctx_to_fmt_env ctx