[sledge] Optimize Context.dnf by iterating disjuncts lazily

Summary:
This diff replaces Fml.fold_dnf, which eagerly enumerates a
disjunctive-normal form expansion, with iter_dnf which can be iterated
lazily. Fml.iter_dnf is then used in Context.dnf. This means that the
full DNF expansion does not need to be allocated by
e.g. Smtlib.check_sat, which yields huge peak memory savings.

Reviewed By: jvillard

Differential Revision: D25946111

fbshipit-source-id: e6d179e9f

sledge/src/fol/context.ml

@@ -862,10 +862,8 @@ let dnf f =
     let vs, x = add vs a x in
     (vs, Fml.and_ p a, x)
   in
-  let join1 = Iter.cons in
   let top = (Var.Set.empty, Fml.tt, empty) in
-  let bot = Iter.empty in
+  Iter.from_labelled_iter (Fml.iter_dnf ~meet1 ~top f)
-  Fml.fold_dnf ~meet1 ~join1 ~top ~bot f
 
 let rename x sub =
   [%trace]

sledge/src/fol/fml.ml

@@ -174,7 +174,12 @@ let fold_pos_neg ~pos ~neg s ~f =
   let f_not p s = f (not_ p) s in
   Set.fold ~f:f_not neg (Set.fold ~f pos s)
 
-let fold_dnf ~meet1 ~join1 ~top ~bot fml =
+let iter_pos_neg ~pos ~neg ~f =
+  let f_not p = f (not_ p) in
+  Set.iter ~f pos ;
+  Set.iter ~f:f_not neg
+
+let iter_dnf ~meet1 ~top fml ~f =
   let rec add_conjunct fml (cjn, splits) =
     match fml with
     | Tt | Eq _ | Eq0 _ | Pos _ | Iff _ | Lit _ | Not _ ->
@@ -184,13 +189,13 @@ let fold_dnf ~meet1 ~join1 ~top ~bot fml =
     | Cond {cnd; pos; neg} ->
         add_conjunct (or_ (and_ cnd pos) (and_ (not_ cnd) neg)) (cjn, splits)
   in
-  let rec add_disjunct (cjn, splits) fml djn =
+  let rec add_disjunct (cjn, splits) fml =
     let cjn, splits = add_conjunct fml (cjn, splits) in
     match splits with
     | (pos, neg) :: splits ->
-        fold_pos_neg ~f:(add_disjunct (cjn, splits)) ~pos ~neg djn
+        iter_pos_neg ~f:(add_disjunct (cjn, splits)) ~pos ~neg
-    | [] -> join1 cjn djn
+    | [] -> f cjn
   in
-  add_disjunct (top, []) fml bot
+  add_disjunct (top, []) fml
 
 let vars p = Iter.flat_map ~f:Trm.vars (trms p)

sledge/src/fol/fml.mli

@@ -74,13 +74,8 @@ val map_pos_neg :
 
 val fold_pos_neg : pos:set -> neg:set -> 'a -> f:(t -> 'a -> 'a) -> 'a
 
-val fold_dnf :
+val iter_dnf :
-     meet1:(t -> 'conjunction -> 'conjunction)
+  meet1:(t -> 'a -> 'a) -> top:'a -> t -> f:('a -> unit) -> unit
-  -> join1:('conjunction -> 'disjunction -> 'disjunction)
-  -> top:'conjunction
-  -> bot:'disjunction
-  -> t
-  -> 'disjunction
 
 val vars : t -> Var.t iter
 val trms : t -> Trm.t iter

sledge/src/test/solver_test.ml

@@ -236,19 +236,19 @@ let%test_module _ =
         {|
         ( infer_frame: 12
             %l_6 -[ %l_6, 16 )-> ⟨8×%n_9,%a_2⟩^⟨(16 + -8×%n_9),%a_3⟩
-          * ( (  0 = %n_9 ∧ emp)
+          * ( (  1 = %n_9 ∧ emp)
-            ∨ (  1 = %n_9 ∧ emp)
+            ∨ (  0 = %n_9 ∧ emp)
             ∨ (  2 = %n_9 ∧ emp)
             )
           \- ∃ %a_1, %m_8 .
               %l_6 -[ %l_6, %m_8 )-> ⟨%m_8,%a_1⟩
         ) infer_frame:
-            ( (  0 = %n_9 ∧ 16 = %m_8 ∧ (⟨0,%a_2⟩^⟨16,%a_3⟩) = %a_1 ∧ emp)
+            ( (  1 = %n_9 ∧ 16 = %m_8 ∧ (⟨8,%a_2⟩^⟨8,%a_3⟩) = %a_1 ∧ emp)
             ∨ (  %a_1 = %a_2
                ∧ 2 = %n_9
                ∧ 16 = %m_8
                ∧ (16 + %l_6) -[ %l_6, 16 )-> ⟨0,%a_3⟩)
-            ∨ (  1 = %n_9 ∧ 16 = %m_8 ∧ (⟨8,%a_2⟩^⟨8,%a_3⟩) = %a_1 ∧ emp)
+            ∨ (  0 = %n_9 ∧ 16 = %m_8 ∧ (⟨0,%a_2⟩^⟨16,%a_3⟩) = %a_1 ∧ emp)
             ) |}]
 
     (* Incompleteness: equivalent to above but using ≤ instead of ∨ *)