Ejemplo n.º 1
0
bool all_paths_enumeratort::next(patht &path)
{
  if(last_path.empty())
  {
    // This is the first time we've been called -- build an initial
    // path.
    last_path.push_back(path_nodet(loop_header));

    // This shouldn't be able to fail.
    complete_path(last_path, 0);

    if(is_looping(last_path))
    {
      // If this was a loop path, we're good.  If it wasn't,
      // we'll keep enumerating paths until we hit a looping one.
      // This case is exactly the same as if someone just called
      // next() on us.
      path.clear();
      path.insert(path.begin(), last_path.begin(), last_path.end());
      return true;
    }
  }

  do
  {
#ifdef DEBUG
    std::cout << "Enumerating next path...\n";
#endif

    int decision=backtrack(last_path);
    complete_path(last_path, decision);

    if(is_looping(last_path))
    {
      path.clear();
      path.insert(path.begin(), last_path.begin(), last_path.end());
      return true;
    }
  }
  while(!last_path.empty());

  // We've enumerated all the paths.
  return false;
}
Ejemplo n.º 2
0
bool polynomial_acceleratort::accelerate(patht &loop,
    path_acceleratort &accelerator) {
  goto_programt::instructionst body;
  accelerator.clear();

  for (patht::iterator it = loop.begin();
       it != loop.end();
       ++it) {
    body.push_back(*(it->loc));
  }

  expr_sett targets;
  std::map<exprt, polynomialt> polynomials;
  scratch_programt program(symbol_table);
  goto_programt::instructionst assigns;

  utils.find_modified(body, targets);

#ifdef DEBUG
  std::cout << "Polynomial accelerating program:" << std::endl;

  for (goto_programt::instructionst::iterator it = body.begin();
       it != body.end();
       ++it) {
    program.output_instruction(ns, "scratch", std::cout, it);
  }

  std::cout << "Modified:" << std::endl;

  for (expr_sett::iterator it = targets.begin();
       it != targets.end();
       ++it) {
    std::cout << expr2c(*it, ns) << std::endl;
  }
#endif

  for (goto_programt::instructionst::iterator it = body.begin();
       it != body.end();
       ++it) {
    if (it->is_assign() || it->is_decl()) {
      assigns.push_back(*it);
    }
  }

  if (loop_counter.is_nil()) {
    symbolt loop_sym = utils.fresh_symbol("polynomial::loop_counter",
        unsignedbv_typet(POLY_WIDTH));
    loop_counter = loop_sym.symbol_expr();
  }

  for (expr_sett::iterator it = targets.begin();
       it != targets.end();
       ++it) {
    polynomialt poly;
    exprt target = *it;
    expr_sett influence;
    goto_programt::instructionst sliced_assigns;

    if (target.type() == bool_typet()) {
      // Hack: don't accelerate booleans.
      continue;
    }

    cone_of_influence(assigns, target, sliced_assigns, influence);

    if (influence.find(target) == influence.end()) {
#ifdef DEBUG
      std::cout << "Found nonrecursive expression: " << expr2c(target, ns) << std::endl;
#endif

      nonrecursive.insert(target);
      continue;
    }

    if (target.id() == ID_index ||
        target.id() == ID_dereference) {
      // We can't accelerate a recursive indirect access...
      accelerator.dirty_vars.insert(target);
      continue;
    }

    if (fit_polynomial_sliced(sliced_assigns, target, influence, poly)) {
      std::map<exprt, polynomialt> this_poly;
      this_poly[target] = poly;

      if (check_inductive(this_poly, assigns)) {
        polynomials.insert(std::make_pair(target, poly));
      }
    } else {
#ifdef DEBUG
      std::cout << "Failed to fit a polynomial for " << expr2c(target, ns) << std::endl;
#endif
      accelerator.dirty_vars.insert(*it);
    }
  }

  if (polynomials.empty()) {
    //return false;
  }

  /*
  if (!utils.check_inductive(polynomials, assigns)) {
    // They're not inductive :-(
    return false;
  }
  */

  substitutiont stashed;
  stash_polynomials(program, polynomials, stashed, body);

  exprt guard;
  exprt guard_last;

  bool path_is_monotone;
  
  try {
    path_is_monotone = utils.do_assumptions(polynomials, loop, guard);
  } catch (std::string s) {
    // Couldn't do WP.
    std::cout << "Assumptions error: " << s << std::endl;
    return false;
  }

  guard_last = guard;

  for (std::map<exprt, polynomialt>::iterator it = polynomials.begin();
       it != polynomials.end();
       ++it) {
    replace_expr(it->first, it->second.to_expr(), guard_last);
  }

  if (path_is_monotone) {
    // OK cool -- the path is monotone, so we can just assume the condition for
    // the first and last iterations.
    replace_expr(loop_counter,
                 minus_exprt(loop_counter, from_integer(1, loop_counter.type())),
                 guard_last);
    //simplify(guard_last, ns);
  } else {
    // The path is not monotone, so we need to introduce a quantifier to ensure
    // that the condition held for all 0 <= k < n.
    symbolt k_sym = utils.fresh_symbol("polynomial::k", unsignedbv_typet(POLY_WIDTH));
    exprt k = k_sym.symbol_expr();

    exprt k_bound = and_exprt(binary_relation_exprt(from_integer(0, k.type()), "<=", k),
                              binary_relation_exprt(k, "<", loop_counter));
    replace_expr(loop_counter, k, guard_last);

    implies_exprt implies(k_bound, guard_last);
    //simplify(implies, ns);

    exprt forall(ID_forall);
    forall.type() = bool_typet();
    forall.copy_to_operands(k);
    forall.copy_to_operands(implies);

    guard_last = forall;
  }

  // All our conditions are met -- we can finally build the accelerator!
  // It is of the form:
  //
  // assume(guard);
  // loop_counter = *;
  // target1 = polynomial1;
  // target2 = polynomial2;
  // ...
  // assume(guard);
  // assume(no overflows in previous code);

  program.add_instruction(ASSUME)->guard = guard;

  program.assign(loop_counter, side_effect_expr_nondett(loop_counter.type()));

  for (std::map<exprt, polynomialt>::iterator it = polynomials.begin();
       it != polynomials.end();
       ++it) {
    program.assign(it->first, it->second.to_expr());
  }

  // Add in any array assignments we can do now.
  if (!utils.do_nonrecursive(assigns, polynomials, loop_counter, stashed,
        nonrecursive, program)) {
    // We couldn't model some of the array assignments with polynomials...
    // Unfortunately that means we just have to bail out.
#ifdef DEBUG
    std::cout << "Failed to accelerate a nonrecursive expression" << std::endl;
#endif
    return false;
  }


  program.add_instruction(ASSUME)->guard = guard_last;
  program.fix_types();

  if (path_is_monotone) {
    utils.ensure_no_overflows(program);
  }

  accelerator.pure_accelerator.instructions.swap(program.instructions);

  return true;
}