postcondition.cpp

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/*******************************************************************\
 
Module: Symbolic Execution
 
Author: Daniel Kroening, kroening@kroening.com
 
\*******************************************************************/
 
 
#include "postcondition.h"
 
#include <util/find_symbols.h>
#include <util/pointer_expr.h>
#include <util/std_expr.h>
 
#include <pointer-analysis/value_set.h>
 
#include "renaming_level.h"
#include "symex_target_equation.h"
 
class postconditiont
{
public:
  postconditiont(
    const namespacet &_ns,
    const value_sett &_value_set,
    const SSA_stept &_SSA_step,
    const goto_symex_statet &_s)
    : ns(_ns), value_set(_value_set), SSA_step(_SSA_step), s(_s)
  {
  }
 
protected:
  const namespacet &ns;
  const value_sett &value_set;
  const SSA_stept &SSA_step;
  const goto_symex_statet &s;
 
public:
  void compute(exprt &dest);
 
protected:
  void strengthen(exprt &dest);
  void weaken(exprt &dest);
  bool is_used_address_of(const exprt &expr, const irep_idt &identifier);
  bool is_used(const exprt &expr, const irep_idt &identifier);
};
 
void postcondition(
  const namespacet &ns,
  const value_sett &value_set,
  const symex_target_equationt &equation,
  const goto_symex_statet &s,
  exprt &dest)
{
  for(symex_target_equationt::SSA_stepst::const_iterator
      it=equation.SSA_steps.begin();
      it!=equation.SSA_steps.end();
      it++)
  {
    postconditiont postcondition(ns, value_set, *it, s);
    postcondition.compute(dest);
    if(dest.is_false())
      return;
  }
}
 
bool postconditiont::is_used_address_of(
  const exprt &expr,
  const irep_idt &identifier)
{
  if(expr.id()==ID_symbol)
  {
    // leave alone
  }
  else if(expr.id()==ID_index)
  {
    return is_used_address_of(to_index_expr(expr).array(), identifier) ||
           is_used(to_index_expr(expr).index(), identifier);
  }
  else if(expr.id()==ID_member)
  {
    return is_used_address_of(to_member_expr(expr).compound(), identifier);
  }
  else if(expr.id()==ID_dereference)
  {
    return is_used(to_dereference_expr(expr).pointer(), identifier);
  }
 
  return false;
}
 
void postconditiont::compute(exprt &dest)
{
  // weaken due to assignment
  weaken(dest);
 
  // strengthen due to assignment
  strengthen(dest);
}
 
void postconditiont::weaken(exprt &dest)
{
  if(dest.id()==ID_and &&
     dest.type()==bool_typet()) // this distributes over "and"
  {
    Forall_operands(it, dest)
      weaken(*it);
 
    return;
  }
 
  // we are lazy:
  // if lhs is mentioned in dest, we use "true".
 
  const irep_idt &lhs_identifier=SSA_step.ssa_lhs.get_object_name();
 
  if(is_used(dest, lhs_identifier))
    dest=true_exprt();
 
  // otherwise, no weakening needed
}
 
void postconditiont::strengthen(exprt &dest)
{
  const irep_idt &lhs_identifier=SSA_step.ssa_lhs.get_object_name();
 
  if(!is_used(SSA_step.ssa_rhs, lhs_identifier))
  {
    // we don't do arrays or structs
    if(SSA_step.ssa_lhs.type().id()==ID_array ||
       SSA_step.ssa_lhs.type().id()==ID_struct)
      return;
 
    exprt equality =
      get_original_name(equal_exprt{SSA_step.ssa_lhs, SSA_step.ssa_rhs});
 
    if(dest.is_true())
      dest.swap(equality);
    else
      dest=and_exprt(dest, equality);
  }
}
 
bool postconditiont::is_used(
  const exprt &expr,
  const irep_idt &identifier)
{
  if(expr.id()==ID_address_of)
  {
    return is_used_address_of(to_address_of_expr(expr).object(), identifier);
  }
  else if(is_ssa_expr(expr))
  {
    return to_ssa_expr(expr).get_object_name()==identifier;
  }
  else if(expr.id()==ID_symbol)
  {
    return to_symbol_expr(expr).get_identifier() == identifier;
  }
  else if(expr.id()==ID_dereference)
  {
    // aliasing may happen here
    for(const exprt &e :
        value_set.get_value_set(to_dereference_expr(expr).pointer(), ns))
    {
      if(has_symbol_expr(get_original_name(e), identifier, false))
        return true;
    }
 
    return false;
  }
  else
  {
    for(const auto &op : expr.operands())
    {
      if(is_used(op, identifier))
        return true;
    }
  }
 
  return false;
}