[pulse] normalize any linear atom

Summary:
Linear arithmetic is able to simplify more atoms, eg `x+y <= x+y`
becomes `True` by normalising to "lhs - rhs <= 0". This does the first
step of normalisation, but to get True in this example we also need to
substitute inside atoms according to the linear equalities, which is the
next diff (for now we only substitute variables inside atoms for other
variables or for constants).

Reviewed By: skcho

Differential Revision: D23241457

fbshipit-source-id: 0da0b545c

infer/src/pulse/PulseFormula.ml

@@ -660,8 +660,8 @@ module Term = struct
 
   (** more or less syntactic attempt at detecting when an arbitrary term is a linear formula; call
       {!Atom.eval_term} first for best results *)
-  let rec to_lin_arith t =
+  let linearize t =
-    (* NOTE: don't duplicate simplifications between here and {!Atom.eval_term} *)
+    let rec aux_linearize t =
       let open IOption.Let_syntax in
       match t with
       | Var v ->
@@ -671,17 +671,17 @@ module Term = struct
       | Linear l ->
           Some l
       | Minus t ->
-        let+ l = to_lin_arith t in
+          let+ l = aux_linearize t in
           LinArith.minus l
       | Add (t1, t2) ->
-        let* l1 = to_lin_arith t1 in
+          let* l1 = aux_linearize t1 in
-        let+ l2 = to_lin_arith t2 in
+          let+ l2 = aux_linearize t2 in
           LinArith.add l1 l2
       | Mult (Const c, t) | Mult (t, Const c) ->
-        let+ l = to_lin_arith t in
+          let+ l = aux_linearize t in
           LinArith.mult c l
       | Div (t, Const c) when Q.is_not_zero c ->
-        let+ l = to_lin_arith t in
+          let+ l = aux_linearize t in
           LinArith.mult (Q.inv c) l
       | Mult _
       | Div _
@@ -700,6 +700,15 @@ module Term = struct
       | Equal _
       | NotEqual _ ->
           None
+    in
+    match aux_linearize t with None -> t | Some l -> Linear l
+
+
+  let simplify_linear = function
+    | Linear l -> (
+      match LinArith.get_as_const l with Some c -> Const c | None -> Linear l )
+    | t ->
+        t
 end
 
 (** Basically boolean terms, used to build the part of a formula that is not equalities between
@@ -731,8 +740,6 @@ module Atom = struct
         F.fprintf fmt "%a ≠ %a" pp_term t1 pp_term t2
 
 
-  let pp = pp_with_pp_var Var.pp
-
   let get_terms atom =
     let (LessEqual (t1, t2) | LessThan (t1, t2) | Equal (t1, t2) | NotEqual (t1, t2)) = atom in
     (t1, t2)
@@ -787,7 +794,8 @@ module Atom = struct
 
   let rec eval_term t =
     let t =
-      Term.map_direct_subterms ~f:eval_term t |> Term.simplify_shallow |> Term.eval_const_shallow
+      Term.map_direct_subterms ~f:eval_term t
+      |> Term.simplify_shallow |> Term.eval_const_shallow |> Term.linearize |> Term.simplify_linear
     in
     match (t : Term.t) with
     (* terms that are atoms can be simplified in [eval_atom] *)
@@ -803,11 +811,7 @@ module Atom = struct
         t
 
 
-  (** This assumes that the terms in the atom have been normalized/evaluated already.
+  (** This assumes that the terms in the atom have been normalized/evaluated already. *)
-
-      TODO: probably a better way to implement this would be to rely entirely on
-      [eval_term (term_of_atom (atom))], possibly implementing it as something about observing the
-      sign/zero-ness of [t1 - t2]. *)
   and eval_atom (atom : t) =
     let t1, t2 = get_terms atom in
     match (t1, t2) with
@@ -821,6 +825,19 @@ module Atom = struct
           eval_result_of_bool (Q.leq c1 c2)
       | LessThan _ ->
           eval_result_of_bool (Q.lt c1 c2) )
+    | Linear l1, Linear l2 ->
+        let l = LinArith.subtract l1 l2 in
+        let t = Term.simplify_linear (Linear l) in
+        eval_atom
+          ( match atom with
+          | Equal _ ->
+              Equal (t, Term.zero)
+          | NotEqual _ ->
+              NotEqual (t, Term.zero)
+          | LessEqual _ ->
+              LessEqual (t, Term.zero)
+          | LessThan _ ->
+              LessThan (t, Term.zero) )
     | _ ->
         if Term.equal_syntax t1 t2 then
           match atom with
@@ -865,6 +882,10 @@ module SatUnsatMonad = struct
   let ( let* ) phi f = bind_normalized f phi
 end
 
+let sat_of_eval_result (eval_result : Atom.eval_result) =
+  match eval_result with True -> Sat None | False -> Unsat | Atom atom -> Sat (Some atom)
+
+
 module VarUF =
   UnionFind.Make
     (struct
@@ -1055,14 +1076,7 @@ end = struct
           match q_opt with None -> VarSubst v_canon | Some q -> QSubst q )
     in
     let atom' = Atom.map_terms atom ~f:(fun t -> normalize_term phi t) in
-    match Atom.eval atom' with
+    Atom.eval atom' |> sat_of_eval_result
-    | True ->
-        Sat None
-    | False ->
-        L.d_printfln "Contradiction detected! %a ~> %a is False" Atom.pp atom Atom.pp atom' ;
-        Unsat
-    | Atom atom ->
-        Sat (Some atom)
 
 
   let normalize_atoms phi =
@@ -1075,15 +1089,12 @@ end = struct
           >>= function
           | None ->
               acc
-          | Some (Atom.Equal (t1, t2) as atom') -> (
+          | Some (Atom.Equal (Linear l, Const c)) | Some (Atom.Equal (Const c, Linear l)) ->
-            match Option.both (Term.to_lin_arith t1) (Term.to_lin_arith t2) with
+              (* NOTE: {!normalize_atom} calls {!Atom.eval}, which normalizes linear equalities so
-            | None ->
+                 they end up only on one side, hence only this match case is needed to detect linear
-                Sat (phi, Atom.Set.add atom' facts)
-            | Some (l1, l2) ->
-                (* an atom has been found to have become a linear equality, move it to the linear
                  equalities *)
-                let+ phi = solve_eq ~fuel:5 l1 l2 phi in
+              let+ phi = solve_eq ~fuel:5 l (LinArith.of_q c) phi in
-                (phi, facts) )
+              (phi, facts)
           | Some atom' ->
               Sat (phi, Atom.Set.add atom' facts) )
     in

infer/src/pulse/unit/PulseFormulaTest.ml

@@ -130,7 +130,7 @@ let%test_module "normalization" =
   ( module struct
     let%expect_test _ =
       normalize (x < y) ;
-      [%expect {|true (no var=var) && true (no linear) && {x < y}|}]
+      [%expect {|true (no var=var) && true (no linear) && {[x + -y] < 0}|}]
 
     let%expect_test _ =
       normalize (x + i 1 - i 1 < x) ;
@@ -166,6 +166,10 @@ let%test_module "normalization" =
       normalize (x = i 0 && x < i 0) ;
       [%expect {|unsat|}]
 
+    let%expect_test _ =
+      normalize (x + y < x + y) ;
+      [%expect {|true (no var=var) && v6 = x + y ∧ v7 = x + y && {[v6 + -v7] < 0}|}]
+
     let%expect_test "nonlinear arithmetic" =
       normalize (z * (x + (v * y) + i 1) / w = i 0) ;
       [%expect
@@ -174,7 +178,7 @@ let%test_module "normalization" =
         &&
         v7 = x + v6 ∧ v8 = x + v6 +1 ∧ v10 = 0
         &&
-        {0 = v9÷w}∧{v6 = v×y}∧{v9 = z×v8} |}]
+        {0 = [v9]÷[w]}∧{[v6] = [v]×[y]}∧{[v9] = [z]×[v8]} |}]
 
     (* check that this becomes all linear equalities *)
     let%expect_test _ =