refactor: add reset_tasks function to clear caches on backtrack

also refactor CC a bit

src/sat/Internal.ml

@@ -960,6 +960,7 @@ module Make (Th : Theory_intf.S) = struct
       Vec.shrink st.elt_levels lvl;
       Vec.shrink st.backtrack_levels lvl;
       (* Recover the right theory state. *)
+      Th.reset_tasks (theory st);
       backtrack_down_to st b_lvl;
     );
     assert (Vec.size st.elt_levels = Vec.size st.backtrack_levels);

src/sat/Theory_intf.ml

@@ -127,6 +127,9 @@ module type S = sig
   val add_formula : t -> formula -> unit
   (** Internalize formula for later use *)
 
+  val reset_tasks : t -> unit
+  (** Clear caches/queues (when we backtrack) *)
+
   val if_sat : t -> formula slice_actions -> (formula, proof) res
   (** Called at the end of the search in case a model has been found. If no new clause is
       pushed, then 'sat' is returned, else search is resumed. *)

src/smt/Congruence_closure.ml

@@ -56,8 +56,8 @@ type t = {
   (* proof state *)
   ps_queue: (node*node) Vec.t;
   (* pairs to explain *)
-  true_ : node lazy_t;
-  false_ : node lazy_t;
+  mutable true_ : node;
+  mutable false_ : node;
 }
 (* TODO: an additional union-find to keep track, for each term,
    of the terms they are known to be equal to, according
@@ -97,8 +97,8 @@ let rec find_rec cc (n:node) : repr =
     root
   )
 
-let[@inline] true_ cc = Lazy.force cc.true_
-let[@inline] false_ cc = Lazy.force cc.false_
+let[@inline] true_ cc = cc.true_
+let[@inline] false_ cc = cc.false_
 
 (* get term that should be there *)
 let[@inline] get_ cc (t:term) : node =
@@ -398,7 +398,7 @@ and update_combine cc =
         let r_into_old_tags = r_into.n_tags in
         on_backtrack cc
           (fun () ->
-             Log.debugf 5
+             Log.debugf 15
                (fun k->k "(@[cc.undo_merge@ :of %a :into %a@])"
                    Term.pp r_from.n_term Term.pp r_into.n_term);
              r_from.n_root <- r_from;
@@ -483,11 +483,11 @@ let add_seq cc seq =
   seq (fun t -> ignore @@ add_ cc t);
   update_pending cc
 
-(* before we push tasks into [pending], ensure they are removed when we backtrack *)
-let reset_on_backtrack cc : unit =
-  if Vec.is_empty cc.pending then (
-    on_backtrack cc (fun () -> Vec.clear cc.pending);
-  )
+(* to do after backtracking: reset task lists *)
+let reset_tasks cc : unit =
+  Vec.clear cc.pending;
+  Vec.clear cc.combine;
+  ()
 
 (* assert that this boolean literal holds *)
 let assert_lit cc lit : unit = match Lit.view lit with
@@ -503,7 +503,6 @@ let assert_lit cc lit : unit = match Lit.view lit with
        basically, just have [n] point to true/false and thus acquire
        the corresponding value, so its superterms (like [ite]) can evaluate
        properly *)
-    reset_on_backtrack cc;
     push_combine cc n rhs (E_lit lit);
     ()
 
@@ -511,7 +510,6 @@ let assert_eq cc (t:term) (u:term) expl : unit =
   let n1 = add cc t in
   let n2 = add cc u in
   if not (same_class cc n1 n2) then (
-    reset_on_backtrack cc;
     union cc n1 n2 expl
   )
 
@@ -560,7 +558,7 @@ end
 
 let create ?(size=2048) ~actions (tst:Term.state) : t =
   let nd = Equiv_class.dummy in
-  let rec cc = {
+  let cc = {
     tst;
     acts=actions;
     tbl = Term.Tbl.create size;
@@ -569,11 +567,11 @@ let create ?(size=2048) ~actions (tst:Term.state) : t =
     combine= Vec.make_empty (nd,nd,E_reduction);
     ps_lits=Lit.Set.empty;
     ps_queue=Vec.make_empty (nd,nd);
-    true_ = lazy (add cc (Term.true_ tst));
-    false_ = lazy (add cc (Term.false_ tst));
+    true_ = Equiv_class.dummy;
+    false_ = Equiv_class.dummy;
   } in
-  ignore (Lazy.force cc.true_);
-  ignore (Lazy.force cc.false_);
+  cc.true_ <- add cc (Term.true_ tst);
+  cc.false_ <- add cc (Term.false_ tst);
   cc
 (* check satisfiability, update congruence closure *)
 let check (cc:t) : unit =
@@ -597,8 +595,8 @@ let mk_model (cc:t) (m:Model.t) : Model.t =
          let v = match Model.eval m t with
            | Some v -> v
            | None ->
-             if same_class cc r (Lazy.force cc.true_) then Value.true_
-             else if same_class cc r (Lazy.force cc.false_) then Value.false_
+             if same_class cc r cc.true_ then Value.true_
+             else if same_class cc r cc.false_ then Value.false_
              else (
                Value.mk_elt
                  (ID.makef "v_%d" @@ Term.id t)

src/smt/Congruence_closure.mli

@@ -71,6 +71,9 @@ val assert_distinct : t -> term list -> neq:term -> Lit.t -> unit
     with explanation [e]
     precond: [u = distinct l] *)
 
+val reset_tasks : t -> unit
+(** Reset the queue of pending tasks *)
+
 val check : t -> unit
 
 val final_check : t -> unit

src/smt/Theory_combine.ml

@@ -21,6 +21,7 @@ type conflict = Theory.conflict
 (* raise upon conflict *)
 exception Exn_conflict of conflict
 
+(* TODO: first-class module instead *)
 type t = {
   cdcl_acts: (formula,proof) Sat_solver.actions;
   (** actions provided by the SAT solver *)
@@ -226,3 +227,5 @@ let add_theory (self:t) (th:Theory.t) : unit =
 
 let add_theory_l self = List.iter (add_theory self)
 
+let reset_tasks self =
+  Congruence_closure.reset_tasks (cc self)