Perform simplification of not_exprt as preorder step

Each of the possible simplifications has an impact on its operands,
which in turn mostly needs to be simplified. Instead of doing this again
(as the operand has already been simplified), do most of the work as a
preorder step so that only the modified operand is subject to
simplification.

Author: tautschnig

src/util/simplify_expr.cpp

@@ -2795,6 +2795,10 @@ simplify_exprt::simplify_node_preorder(const exprt &expr)
   {
     result = simplify_unary_pointer_predicate_preorder(to_unary_expr(expr));
   }
+  else if(expr.id() == ID_not)
+  {
+    result = simplify_not_preorder(to_not_expr(expr));
+  }
   else if(expr.has_operands())
   {
     optionalt<exprt::operandst> new_operands;

src/util/simplify_expr_boolean.cpp

@@ -377,3 +377,66 @@ simplify_exprt::resultt<> simplify_exprt::simplify_not(const not_exprt &expr)
 
   return unchanged(expr);
 }
+
+simplify_exprt::resultt<>
+simplify_exprt::simplify_not_preorder(const not_exprt &expr)
+{
+  const exprt &op = expr.op();
+
+  if(!expr.is_boolean() || !op.is_boolean())
+  {
+    return unchanged(expr);
+  }
+
+  if(op.id() == ID_not) // (not not a) == a
+  {
+    return changed(simplify_rec(to_not_expr(op).op()));
+  }
+  else if(op.is_false())
+  {
+    return true_exprt{};
+  }
+  else if(op.is_true())
+  {
+    return false_exprt{};
+  }
+  else if(op.id() == ID_and || op.id() == ID_or)
+  {
+    exprt tmp = op;
+
+    Forall_operands(it, tmp)
+    {
+      *it = boolean_negate(*it);
+    }
+
+    tmp.id(tmp.id() == ID_and ? ID_or : ID_and);
+
+    return changed(simplify_rec(tmp));
+  }
+  else if(op.id() == ID_notequal) // !(a!=b) <-> a==b
+  {
+    exprt tmp = op;
+    tmp.id(ID_equal);
+    return changed(simplify_rec(tmp));
+  }
+  else if(op.id() == ID_exists) // !(exists: a) <-> forall: not a
+  {
+    auto const &op_as_exists = to_exists_expr(op);
+    return changed(simplify_rec(
+      forall_exprt{op_as_exists.symbol(), not_exprt{op_as_exists.where()}}));
+  }
+  else if(op.id() == ID_forall) // !(forall: a) <-> exists: not a
+  {
+    auto const &op_as_forall = to_forall_expr(op);
+    return changed(simplify_rec(
+      exists_exprt{op_as_forall.symbol(), not_exprt{op_as_forall.where()}}));
+  }
+
+  auto op_result = simplify_rec(op);
+  if(!op_result.has_changed())
+    return op_result;
+
+  not_exprt tmp = expr;
+  tmp.op() = std::move(op_result.expr);
+  return std::move(tmp);
+}

src/util/simplify_expr_class.h

@@ -167,6 +167,7 @@ class simplify_exprt
   NODISCARD resultt<> simplify_if(const if_exprt &);
   NODISCARD resultt<> simplify_bitnot(const bitnot_exprt &);
   NODISCARD resultt<> simplify_not(const not_exprt &);
+  NODISCARD resultt<> simplify_not_preorder(const not_exprt &expr);
   NODISCARD resultt<> simplify_boolean(const exprt &);
   NODISCARD resultt<> simplify_inequality(const binary_relation_exprt &);
   NODISCARD resultt<>