goto_clean_expr.cpp

Go to the documentation of this file.

/*******************************************************************\
 
Module: Program Transformation
 
Author: Daniel Kroening, kroening@kroening.com
 
\*******************************************************************/
 
 
#include "goto_convert_class.h"
 
#include <util/expr_util.h>
#include <util/fresh_symbol.h>
#include <util/mathematical_expr.h>
#include <util/pointer_expr.h>
#include <util/replace_expr.h>
#include <util/std_expr.h>
#include <util/symbol.h>
 
#include <analyses/natural_loops.h>
 
#include "destructor.h"
 
#include <unordered_map>
 
static symbol_exprt find_base_symbol(const exprt &expr)
{
  if(expr.id() == ID_symbol)
  {
    return to_symbol_expr(expr);
  }
  else if(expr.id() == ID_member)
  {
    return find_base_symbol(to_member_expr(expr).struct_op());
  }
  else if(expr.id() == ID_index)
  {
    return find_base_symbol(to_index_expr(expr).array());
  }
  else if(expr.id() == ID_dereference)
  {
    return find_base_symbol(to_dereference_expr(expr).pointer());
  }
  else if(expr.id() == ID_typecast)
  {
    return find_base_symbol(to_typecast_expr(expr).op());
  }
  else if(expr.id() == ID_address_of)
  {
    return find_base_symbol(to_address_of_expr(expr).op());
  }
  else
  {
    throw "unsupported expression type for finding base symbol";
  }
}
 
static exprt convert_statement_expression(
  const quantifier_exprt &qex,
  const code_expressiont &code,
  const irep_idt &mode,
  goto_convertt &converter)
{
  goto_programt where;
  converter.goto_convert(code, where, mode);
  where.compute_location_numbers();
 
  natural_loops_mutablet natural_loops(where);
  INVARIANT(
    natural_loops.loop_map.size() == 0, "quantifier must not contain loops");
 
  std::unordered_set<symbol_exprt, irep_hash> declared_symbols;
  // All bound variables are local.
  declared_symbols.insert(qex.variables().begin(), qex.variables().end());
 
  // `last` is the instruction corresponding to the last expression in the
  // statement expression.
  goto_programt::const_targett last = where.instructions.end();
  for(goto_programt::const_targett it = where.instructions.begin();
      it != where.instructions.end();
      ++it)
  {
    // `last` is an other-instruction.
    if(it->is_other())
    {
      last = it;
    }
 
    if(it->is_decl())
    {
      declared_symbols.insert(it->decl_symbol());
    }
  }
 
  DATA_INVARIANT(
    last != where.instructions.end(),
    "expression statements must contain a terminator expression");
 
  auto last_expr = to_code_expression(last->get_other()).expression();
  if(
    last_expr.id() == ID_typecast &&
    to_typecast_expr(last_expr).type().id() == ID_empty)
  {
    to_typecast_expr(last_expr).type() = bool_typet();
  }
 
  struct pathst
  {
    // `paths` contains all the `targett` we are iterating over.
    std::vector<goto_programt::const_targett> paths;
    std::vector<std::pair<exprt, replace_mapt>> path_conditions_and_value_maps;
 
    pathst(
      std::vector<goto_programt::const_targett> paths,
      std::vector<std::pair<exprt, replace_mapt>>
        path_conditions_and_value_maps)
      : paths(paths),
        path_conditions_and_value_maps(path_conditions_and_value_maps)
    {
    }
 
    bool empty()
    {
      return paths.empty();
    }
 
    goto_programt::const_targett &back_it()
    {
      return paths.back();
    }
 
    exprt &back_path_condition()
    {
      return path_conditions_and_value_maps.back().first;
    }
 
    replace_mapt &back_value_map()
    {
      return path_conditions_and_value_maps.back().second;
    }
 
    void push_back(
      goto_programt::const_targett target,
      exprt path_condition,
      replace_mapt value_map)
    {
      paths.push_back(target);
      path_conditions_and_value_maps.push_back(
        std::make_pair(path_condition, value_map));
    }
 
    void pop_back()
    {
      paths.pop_back();
      path_conditions_and_value_maps.pop_back();
    }
  };
 
  pathst paths(
    {1, where.instructions.begin()},
    {1, std::make_pair(true_exprt(), replace_mapt())});
 
  exprt res = true_exprt();
 
  // Visit the quantifier body along `paths`.
  while(!paths.empty())
  {
    auto &current_it = paths.back_it();
    auto &path_condition = paths.back_path_condition();
    auto &value_map = paths.back_value_map();
 
    if(current_it == where.instructions.end())
    {
      paths.pop_back();
      continue;
    }
 
    switch(current_it->type())
    {
    // Add all local-declared symbols into `declared_symbols`.
    case goto_program_instruction_typet::DECL:
      declared_symbols.insert(current_it->decl_symbol());
      break;
 
    // ASSIGN lhs := rhs
    // Add the replace lhr <- value_map(rhs) to the current value_map.
    case goto_program_instruction_typet::ASSIGN:
    {
      // Check that if lhs is a declared symbol.
      auto lhs = current_it->assign_lhs();
      INVARIANT(
        declared_symbols.count(find_base_symbol(lhs)),
        "quantifier must not contain side effects");
      exprt rhs = current_it->assign_rhs();
      replace_expr(value_map, rhs);
      value_map[lhs] = rhs;
    }
    break;
 
    // GOTO label
    // -----------
    // Move the current targett to label.
    // or
    // IF cond GOTO label
    // ----------
    // Move the current targett to targett+1 with path condition
    // path_condition && !cond;
    // and add a new path starting from label with path condition
    // path_condition && cond.
    case goto_program_instruction_typet::GOTO:
    {
      exprt condition = current_it->condition();
      replace_expr(value_map, condition);
      if(!condition.is_true())
      {
        auto next_it = current_it->targets.front();
        exprt copy_path_condition = path_condition;
        path_condition = and_exprt(path_condition, not_exprt(condition));
        current_it++;
        paths.push_back(
          next_it, and_exprt(copy_path_condition, condition), value_map);
      }
      else
      {
        current_it = current_it->targets.front();
      }
      continue;
    }
 
    // EXPRESSION(expr)
    // The last instruction is an expression statement.
    // We add the predicate path_condition ==> value_map(expr) to res.
    case goto_program_instruction_typet::OTHER:
    {
      if(current_it == last)
      {
        exprt copy_of_last_expr = last_expr;
        replace_expr(value_map, copy_of_last_expr);
        res = and_exprt(res, implies_exprt(path_condition, copy_of_last_expr));
        paths.pop_back();
        continue;
      }
    }
    break;
 
    // Ignored instructions.
    case goto_program_instruction_typet::ASSERT:
    case goto_program_instruction_typet::ASSUME:
    case goto_program_instruction_typet::ATOMIC_BEGIN:
    case goto_program_instruction_typet::ATOMIC_END:
    case goto_program_instruction_typet::DEAD:
    case goto_program_instruction_typet::LOCATION:
    case goto_program_instruction_typet::SKIP:
    case goto_program_instruction_typet::THROW:
      break;
 
    // Unsupported instructions.
    case goto_program_instruction_typet::CATCH:
    case goto_program_instruction_typet::END_FUNCTION:
    case goto_program_instruction_typet::END_THREAD:
    case goto_program_instruction_typet::FUNCTION_CALL:
    case goto_program_instruction_typet::INCOMPLETE_GOTO:
    case goto_program_instruction_typet::SET_RETURN_VALUE:
    case goto_program_instruction_typet::START_THREAD:
    case goto_program_instruction_typet::NO_INSTRUCTION_TYPE:
      UNREACHABLE;
    }
 
    current_it++;
  }
  return res;
}
 
symbol_exprt goto_convertt::make_compound_literal(
  const exprt &expr,
  goto_programt &dest,
  const irep_idt &mode)
{
  const source_locationt source_location = expr.find_source_location();
 
  symbolt &new_symbol = get_fresh_aux_symbol(
    expr.type(),
    tmp_symbol_prefix,
    "literal",
    source_location,
    mode,
    symbol_table);
  new_symbol.is_static_lifetime = lifetime != lifetimet::AUTOMATIC_LOCAL;
  new_symbol.value = expr;
 
  // The value might depend on a variable, thus
  // generate code for this.
 
  symbol_exprt result = new_symbol.symbol_expr();
  result.add_source_location() = source_location;
 
  // The lifetime of compound literals is really that of
  // the block they are in.
  if(!new_symbol.is_static_lifetime)
    copy(code_declt(result), DECL, dest);
 
  code_assignt code_assign(result, expr);
  code_assign.add_source_location() = source_location;
  convert(code_assign, dest, mode);
 
  // now create a 'dead' instruction
  if(!new_symbol.is_static_lifetime)
  {
    code_deadt code_dead(result);
    targets.scope_stack.add(std::move(code_dead), {});
  }
 
  return result;
}
 
bool goto_convertt::needs_cleaning(const exprt &expr)
{
  if(
    expr.id() == ID_side_effect || expr.id() == ID_compound_literal ||
    expr.id() == ID_comma)
  {
    return true;
  }
 
  // We can't flatten quantified expressions by introducing new literals for
  // conditional expressions.  This is because the body of the quantified
  // may refer to bound variables, which are not visible outside the scope
  // of the quantifier.
  //
  // For example, the following transformation would not be valid:
  //
  // forall (i : int) (i == 0 || i > 10)
  //
  //   transforming to
  //
  // g1 = (i == 0)
  // g2 = (i > 10)
  // forall (i : int) (g1 || g2)
  if(expr.id() == ID_forall || expr.id() == ID_exists)
  {
    code_expressiont where{to_quantifier_expr(expr).where()};
    // Need cleaning when the quantifier body is a side-effect expression.
    if(has_subexpr(expr, ID_side_effect))
      return true;
 
    return false;
  }
 
  for(const auto &op : expr.operands())
  {
    if(needs_cleaning(op))
      return true;
  }
 
  return false;
}
 
void goto_convertt::rewrite_boolean(exprt &expr)
{
  PRECONDITION(
    expr.id() == ID_and || expr.id() == ID_or || expr.id() == ID_implies);
  PRECONDITION_WITH_DIAGNOSTICS(
    expr.is_boolean(),
    expr.find_source_location(),
    "'",
    expr.id(),
    "' must be Boolean, but got ",
    irep_pretty_diagnosticst{expr});
 
  const source_locationt source_location = expr.find_source_location();
 
  // re-write "a ==> b" into a?b:1
  if(auto implies = expr_try_dynamic_cast<implies_exprt>(expr))
  {
    expr = if_exprt{
      std::move(implies->lhs()),
      std::move(implies->rhs()),
      true_exprt{}.with_source_location(source_location),
      bool_typet{}};
    return;
  }
 
  // re-write "a && b" into nested a?b:0
  // re-write "a || b" into nested a?1:b
 
  exprt tmp;
 
  if(expr.id() == ID_and)
    tmp = true_exprt();
  else // ID_or
    tmp = false_exprt();
 
  tmp.add_source_location() = source_location;
 
  exprt::operandst &ops = expr.operands();
 
  // start with last one
  for(exprt::operandst::reverse_iterator it = ops.rbegin(); it != ops.rend();
      ++it)
  {
    exprt &op = *it;
 
    DATA_INVARIANT_WITH_DIAGNOSTICS(
      op.is_boolean(),
      "boolean operators must have only boolean operands",
      source_location);
 
    if(expr.id() == ID_and)
    {
      exprt if_e =
        if_exprt{op, tmp, false_exprt{}.with_source_location(source_location)}
          .with_source_location(source_location);
      tmp.swap(if_e);
      continue;
    }
    if(expr.id() == ID_or)
    {
      exprt if_e =
        if_exprt{op, true_exprt{}.with_source_location(source_location), tmp}
          .with_source_location(source_location);
      tmp.swap(if_e);
      continue;
    }
    UNREACHABLE;
  }
 
  expr.swap(tmp);
}
 
goto_convertt::clean_expr_resultt goto_convertt::clean_expr(
  exprt &expr,
  const irep_idt &mode,
  bool result_is_used)
{
  // this cleans:
  //   && || ==> ?: comma (control-dependency)
  //   function calls
  //   object constructors like arrays, string constants, structs
  //   ++ -- (pre and post)
  //   compound assignments
  //   compound literals
 
  if(!needs_cleaning(expr))
    return {};
 
  if(expr.id() == ID_and || expr.id() == ID_or || expr.id() == ID_implies)
  {
    // rewrite into ?:
    rewrite_boolean(expr);
 
    // recursive call
    return clean_expr(expr, mode, result_is_used);
  }
  else if(expr.id() == ID_if)
  {
    // first clean condition
    clean_expr_resultt side_effects =
      clean_expr(to_if_expr(expr).cond(), mode, true);
 
    // possibly done now
    if(
      !needs_cleaning(to_if_expr(expr).true_case()) &&
      !needs_cleaning(to_if_expr(expr).false_case()))
    {
      return side_effects;
    }
 
    // copy expression
    if_exprt if_expr = to_if_expr(expr);
 
    DATA_INVARIANT_WITH_DIAGNOSTICS(
      if_expr.cond().is_boolean(),
      "condition for an 'if' must be boolean",
      if_expr.find_source_location());
 
    const source_locationt source_location = expr.find_source_location();
 
#if 0
    // We do some constant-folding here, to mimic
    // what typical compilers do.
    {
      exprt tmp_cond=if_expr.cond();
      simplify(tmp_cond, ns);
      if(tmp_cond.is_true())
      {
        clean_expr(if_expr.true_case(), dest, result_is_used);
        expr=if_expr.true_case();
        return;
      }
      else if(tmp_cond.is_false())
      {
        clean_expr(if_expr.false_case(), dest, result_is_used);
        expr=if_expr.false_case();
        return;
      }
    }
#endif
 
    clean_expr_resultt tmp_true(
      clean_expr(if_expr.true_case(), mode, result_is_used));
 
    clean_expr_resultt tmp_false(
      clean_expr(if_expr.false_case(), mode, result_is_used));
 
    if(result_is_used)
    {
      symbolt &new_symbol = new_tmp_symbol(
        expr.type(),
        "if_expr",
        side_effects.side_effects,
        source_location,
        mode);
 
      code_assignt assignment_true;
      assignment_true.lhs() = new_symbol.symbol_expr();
      assignment_true.rhs() = if_expr.true_case();
      assignment_true.add_source_location() = source_location;
      convert(assignment_true, tmp_true.side_effects, mode);
 
      code_assignt assignment_false;
      assignment_false.lhs() = new_symbol.symbol_expr();
      assignment_false.rhs() = if_expr.false_case();
      assignment_false.add_source_location() = source_location;
      convert(assignment_false, tmp_false.side_effects, mode);
 
      // overwrites expr
      expr = new_symbol.symbol_expr();
    }
    else
    {
      // preserve the expressions for possible later checks
      if(if_expr.true_case().is_not_nil())
      {
        // add a (void) type cast so that is_skip catches it in case the
        // expression is just a constant
        code_expressiont code_expression(
          typecast_exprt(if_expr.true_case(), empty_typet()));
        convert(code_expression, tmp_true.side_effects, mode);
      }
 
      if(if_expr.false_case().is_not_nil())
      {
        // add a (void) type cast so that is_skip catches it in case the
        // expression is just a constant
        code_expressiont code_expression(
          typecast_exprt(if_expr.false_case(), empty_typet()));
        convert(code_expression, tmp_false.side_effects, mode);
      }
 
      expr = nil_exprt();
    }
 
    // generate guard for argument side-effects
    generate_ifthenelse(
      if_expr.cond(),
      source_location,
      tmp_true.side_effects,
      if_expr.true_case().source_location(),
      tmp_false.side_effects,
      if_expr.false_case().source_location(),
      side_effects.side_effects,
      mode);
 
    destruct_locals(tmp_false.temporaries, side_effects.side_effects, ns);
    destruct_locals(tmp_true.temporaries, side_effects.side_effects, ns);
    destruct_locals(side_effects.temporaries, side_effects.side_effects, ns);
    side_effects.temporaries.clear();
 
    if(expr.is_not_nil())
      side_effects.add_temporary(to_symbol_expr(expr).get_identifier());
 
    return side_effects;
  }
  else if(expr.id() == ID_comma)
  {
    clean_expr_resultt side_effects;
 
    if(result_is_used)
    {
      exprt result;
 
      Forall_operands(it, expr)
      {
        bool last = (it == --expr.operands().end());
 
        // special treatment for last one
        if(last)
        {
          result.swap(*it);
          side_effects.add(clean_expr(result, mode, true));
        }
        else
        {
          side_effects.add(clean_expr(*it, mode, false));
 
          // remember these for later checks
          if(it->is_not_nil())
            convert(code_expressiont(*it), side_effects.side_effects, mode);
        }
      }
 
      expr.swap(result);
    }
    else // result not used
    {
      Forall_operands(it, expr)
      {
        side_effects.add(clean_expr(*it, mode, false));
 
        // remember as expression statement for later checks
        if(it->is_not_nil())
          convert(code_expressiont(*it), side_effects.side_effects, mode);
      }
 
      expr = nil_exprt();
    }
 
    return side_effects;
  }
  else if(expr.id() == ID_typecast)
  {
    typecast_exprt &typecast = to_typecast_expr(expr);
 
    // preserve 'result_is_used'
    clean_expr_resultt side_effects =
      clean_expr(typecast.op(), mode, result_is_used);
 
    if(typecast.op().is_nil())
      expr.make_nil();
 
    return side_effects;
  }
  else if(expr.id() == ID_side_effect)
  {
    // some of the side-effects need special treatment!
    const irep_idt statement = to_side_effect_expr(expr).get_statement();
 
    if(statement == ID_gcc_conditional_expression)
    {
      // need to do separately
      return remove_gcc_conditional_expression(expr, mode);
    }
    else if(statement == ID_statement_expression)
    {
      // need to do separately to prevent that
      // the operands of expr get 'cleaned'
      return remove_statement_expression(
        to_side_effect_expr(expr), mode, result_is_used);
    }
    else if(statement == ID_assign)
    {
      // we do a special treatment for x=f(...)
      INVARIANT(
        expr.operands().size() == 2,
        "side-effect assignment expressions must have two operands");
 
      auto &side_effect_assign = to_side_effect_expr_assign(expr);
 
      if(
        side_effect_assign.rhs().id() == ID_side_effect &&
        to_side_effect_expr(side_effect_assign.rhs()).get_statement() ==
          ID_function_call)
      {
        clean_expr_resultt side_effects =
          clean_expr(side_effect_assign.lhs(), mode);
        exprt lhs = side_effect_assign.lhs();
 
        const bool must_use_rhs = assignment_lhs_needs_temporary(lhs);
        if(must_use_rhs)
        {
          side_effects.add(remove_function_call(
            to_side_effect_expr_function_call(side_effect_assign.rhs()),
            mode,
            true));
        }
 
        // turn into code
        exprt new_lhs = skip_typecast(lhs);
        exprt new_rhs = typecast_exprt::conditional_cast(
          side_effect_assign.rhs(), new_lhs.type());
        code_assignt assignment(std::move(new_lhs), new_rhs);
        assignment.add_source_location() = expr.source_location();
        convert_assign(assignment, side_effects.side_effects, mode);
 
        if(result_is_used)
          expr = must_use_rhs ? new_rhs : lhs;
        else
          expr.make_nil();
 
        return side_effects;
      }
    }
  }
  else if(expr.id() == ID_forall || expr.id() == ID_exists)
  {
    quantifier_exprt &qex = to_quantifier_expr(expr);
    code_expressiont code{qex.where()};
    DATA_INVARIANT(
      !has_subexpr(expr, ID_side_effect) ||
        (code.operands()[0].id() == ID_side_effect &&
         code.operands()[0].get_named_sub()[ID_statement].id() ==
           ID_statement_expression),
      "quantifier must not contain side effects");
 
    // Handle the case that quantifier body is a statement expression.
    if(
      code.operands()[0].id() == ID_side_effect &&
      code.operands()[0].get_named_sub()[ID_statement].id() ==
        ID_statement_expression)
    {
      auto res = convert_statement_expression(qex, code, mode, *this);
      qex.where() = res;
      return clean_expr(res, mode, result_is_used);
    }
  }
  else if(expr.id() == ID_address_of)
  {
    address_of_exprt &addr = to_address_of_expr(expr);
    return clean_expr_address_of(addr.object(), mode);
  }
 
  clean_expr_resultt side_effects;
 
  // TODO: evaluation order
 
  Forall_operands(it, expr)
    side_effects.add(clean_expr(*it, mode));
 
  if(expr.id() == ID_side_effect)
  {
    side_effects.add(remove_side_effect(
      to_side_effect_expr(expr), mode, result_is_used, false));
  }
  else if(expr.id() == ID_compound_literal)
  {
    // This is simply replaced by the literal
    DATA_INVARIANT(
      expr.operands().size() == 1, "ID_compound_literal has a single operand");
    expr = to_unary_expr(expr).op();
  }
 
  return side_effects;
}
 
goto_convertt::clean_expr_resultt
goto_convertt::clean_expr_address_of(exprt &expr, const irep_idt &mode)
{
  clean_expr_resultt side_effects;
 
  // The address of object constructors can be taken,
  // which is re-written into the address of a variable.
 
  if(expr.id() == ID_compound_literal)
  {
    DATA_INVARIANT(
      expr.operands().size() == 1, "ID_compound_literal has a single operand");
    side_effects.add(clean_expr(to_unary_expr(expr).op(), mode));
    expr = make_compound_literal(
      to_unary_expr(expr).op(), side_effects.side_effects, mode);
  }
  else if(expr.id() == ID_string_constant)
  {
    // Leave for now, but long-term these might become static symbols.
    // LLVM appears to do precisely that.
  }
  else if(expr.id() == ID_index)
  {
    index_exprt &index_expr = to_index_expr(expr);
    side_effects.add(clean_expr_address_of(index_expr.array(), mode));
    side_effects.add(clean_expr(index_expr.index(), mode));
  }
  else if(expr.id() == ID_dereference)
  {
    dereference_exprt &deref_expr = to_dereference_expr(expr);
    side_effects.add(clean_expr(deref_expr.pointer(), mode));
  }
  else if(expr.id() == ID_comma)
  {
    // Yes, one can take the address of a comma expression.
    // Treatment is similar to clean_expr() above.
 
    exprt result;
 
    Forall_operands(it, expr)
    {
      bool last = (it == --expr.operands().end());
 
      // special treatment for last one
      if(last)
        result.swap(*it);
      else
      {
        side_effects.add(clean_expr(*it, mode, false));
 
        // get any side-effects
        if(it->is_not_nil())
          convert(code_expressiont(*it), side_effects.side_effects, mode);
      }
    }
 
    expr.swap(result);
 
    // do again
    side_effects.add(clean_expr_address_of(expr, mode));
  }
  else if(expr.id() == ID_side_effect)
  {
    side_effects.add(
      remove_side_effect(to_side_effect_expr(expr), mode, true, true));
  }
  else
    Forall_operands(it, expr)
      side_effects.add(clean_expr_address_of(*it, mode));
 
  return side_effects;
}
 
goto_convertt::clean_expr_resultt
goto_convertt::remove_gcc_conditional_expression(
  exprt &expr,
  const irep_idt &mode)
{
  clean_expr_resultt side_effects;
 
  {
    auto &binary_expr = to_binary_expr(expr);
 
    // first remove side-effects from condition
    side_effects = clean_expr(to_binary_expr(expr).op0(), mode);
 
    // now we can copy op0 safely
    if_exprt if_expr(
      typecast_exprt::conditional_cast(binary_expr.op0(), bool_typet()),
      binary_expr.op0(),
      binary_expr.op1(),
      expr.type());
    if_expr.add_source_location() = expr.source_location();
 
    expr.swap(if_expr);
  }
 
  // there might still be junk in expr.op2()
  side_effects.add(clean_expr(expr, mode));
 
  return side_effects;
}