Improve purity test for dead code elimination

src/passes/7-self_mini_c/self_mini_c.ml

@@ -1,14 +1,62 @@
 open Mini_c
 open Trace
 
-(* Overly conservative for now: ok to treat pure things as impure,
+(* Overly conservative purity test: ok to treat pure things as impure,
    must not treat impure things as pure. *)
-let is_pure : expression -> bool = fun e ->
-  match e.content with
-  | E_closure _ -> true
+
+(* true if the name names a pure constant -- i.e. if uses will be pure
+   assuming arguments are pure *)
+let is_pure_constant : string -> bool =
+  function
+  | "CAR"
+  | "CDR"
+  | "PAIR"
+    -> true
+  (* TODO... *)
   | _ -> false
 
-let rec elim_dead_lambdas : expression -> expression result = fun e ->
+let rec is_pure : expression -> bool = fun e ->
+  match e.content with
+  | E_literal _
+  | E_closure _
+  | E_skip
+  | E_variable _
+  | E_make_empty_map _
+  | E_make_empty_list _
+  | E_make_empty_set _
+  | E_make_none _
+    -> true
+
+  | E_if_bool (cond, bt, bf)
+  | E_if_none (cond, bt, (_, bf))
+  | E_if_cons (cond, bt, (_, bf))
+  | E_if_left (cond, (_, bt), (_, bf))
+    -> List.for_all is_pure [ cond ; bt ; bf ]
+
+  | E_let_in (_, e1, e2)
+  | E_sequence (e1, e2)
+    -> List.for_all is_pure [ e1 ; e2 ]
+
+  | E_constant (c, args)
+    -> is_pure_constant c && List.for_all is_pure args
+
+  (* I'm not sure about these. Maybe can be tested better? *)
+  | E_application _
+  | E_iterator _
+  | E_fold _
+    -> false
+
+  (* Could be pure, but, divergence is an effect, so halting problem
+     is near... *)
+  | E_while _ -> false
+
+  (* definitely not pure *)
+  | E_assignment _ -> false
+
+
+(* Eliminate dead `let` with pure rhs *)
+
+let rec elim_dead_code : expression -> expression result = fun e ->
   let changed = ref false in (* ugh *)
   let mapper : Helpers.mapper = fun e ->
     match e.content with
@@ -22,8 +70,8 @@ let rec elim_dead_lambdas : expression -> expression result = fun e ->
     | _ -> ok e in
   let%bind e = Helpers.map_expression mapper e in
   if !changed
-  then elim_dead_lambdas e
+  then elim_dead_code e
   else ok e
 
 let all_expression : expression -> expression result =
-  elim_dead_lambdas
+  elim_dead_code

src/stages/mini_c/misc.ml

@@ -75,9 +75,8 @@ module Free_variables = struct
                expression (union (singleton v) b) body ;
              ]
     | E_sequence (x, y) -> union (self x) (self y)
-    (* we do not consider the assigned variable free... seems strange,
-       but, matches ast_typed, and does not cause any troubles? *)
-    | E_assignment (_, _, e) -> self e
+    (* NB different from ast_typed... *)
+    | E_assignment (v, _, e) -> unions [ var_name b v ; self e ]
     | E_while (cond , body) -> union (self cond) (self body)
 
   and var_name : bindings -> var_name -> bindings = fun b n ->