예제 #1
0
void goto_inlinet::replace_return(
  goto_programt &dest,
  const exprt &lhs,
  const exprt &constrain)
{
  for(goto_programt::instructionst::iterator
      it=dest.instructions.begin();
      it!=dest.instructions.end();
      it++)
  {
    if(it->is_return())
    {
      if(lhs.is_not_nil())
      {
        goto_programt tmp;
        goto_programt::targett assignment=tmp.add_instruction(ASSIGN);
        
        const code_return2t &ret = to_code_return2t(it->code);
        code_assignt code_assign(lhs, migrate_expr_back(ret.operand));

        // this may happen if the declared return type at the call site
        // differs from the defined return type
        if(code_assign.lhs().type()!=
           code_assign.rhs().type())
          code_assign.rhs().make_typecast(code_assign.lhs().type());

        migrate_expr(code_assign, assignment->code);
        assignment->location=it->location;
        assignment->local_variables=it->local_variables;
        assignment->function=it->location.get_function();

        
        assert(constrain.is_nil()); // bp_constrain gumpf reomved
        
        dest.insert_swap(it, *assignment);
        it++;
      }
      else if(!is_nil_expr(it->code))
      {
        // Encode evaluation of return expr, so that returns with pointer
        // derefs in them still get dereferenced, even when the result is
        // discarded.
        goto_programt tmp;
        goto_programt::targett expression=tmp.add_instruction(OTHER);
        
        expression->make_other();
        expression->location=it->location;
        expression->function=it->location.get_function();
        expression->local_variables=it->local_variables;
        const code_return2t &ret = to_code_return2t(it->code);
        expression->code = code_expression2tc(ret.operand);

        dest.insert_swap(it, *expression);
        it++;
      }

      it->make_goto(--dest.instructions.end());
    }
  }
}
예제 #2
0
void goto_convertt::do_java_new(
  const exprt &lhs,
  const side_effect_exprt &rhs,
  goto_programt &dest)
{
  if(lhs.is_nil())
    throw "do_java_new without lhs is yet to be implemented";
    
  source_locationt location=rhs.source_location();

  assert(rhs.operands().empty());

  if(rhs.type().id()!=ID_pointer)
    throw "do_java_new returns pointer";

  typet object_type=rhs.type().subtype();
  
  // build size expression
  exprt object_size=size_of_expr(object_type, ns);
  
  if(object_size.is_nil())
    throw "do_java_new got nil object_size";

  // we produce a malloc side-effect, which stays
  side_effect_exprt malloc_expr(ID_malloc);
  malloc_expr.copy_to_operands(object_size);
  malloc_expr.type()=pointer_typet(object_type);

  goto_programt::targett t_n=dest.add_instruction(ASSIGN);
  t_n->code=code_assignt(lhs, malloc_expr);
  t_n->source_location=location;
  
  // zero-initialize the object
  dereference_exprt deref(lhs, object_type);
  exprt zero_object=zero_initializer(object_type, location, ns, get_message_handler());
  set_class_identifier(to_struct_expr(zero_object), ns, to_symbol_type(object_type));
  goto_programt::targett t_i=dest.add_instruction(ASSIGN);
  t_i->code=code_assignt(deref, zero_object);
  t_i->source_location=location;
}
예제 #3
0
void goto_convertt::do_java_new_array(
  const exprt &lhs,
  const side_effect_exprt &rhs,
  goto_programt &dest)
{
  if(lhs.is_nil())
    throw "do_java_new_array without lhs is yet to be implemented";
    
  source_locationt location=rhs.source_location();

  assert(rhs.operands().size()>=1); // one per dimension
  
  if(rhs.type().id()!=ID_pointer)
    throw "do_java_new_array returns pointer";

  typet object_type=rhs.type().subtype();
  
  // build size expression
  exprt object_size=size_of_expr(object_type, ns);
  
  if(object_size.is_nil())
    throw "do_java_new_array got nil object_size";

  // we produce a malloc side-effect, which stays
  side_effect_exprt malloc_expr(ID_malloc);
  malloc_expr.copy_to_operands(object_size);
  malloc_expr.type()=pointer_typet(object_type);

  goto_programt::targett t_n=dest.add_instruction(ASSIGN);
  t_n->code=code_assignt(lhs, malloc_expr);
  t_n->source_location=location;
  
  // multi-dimensional?
  
  assert(ns.follow(object_type).id()==ID_struct);
  const struct_typet &struct_type=to_struct_type(ns.follow(object_type));
  assert(struct_type.components().size()==3);

  // if it's an array, we need to set the length field
  dereference_exprt deref(lhs, object_type);
  member_exprt length(deref, struct_type.components()[1].get_name(), struct_type.components()[1].type());
  goto_programt::targett t_s=dest.add_instruction(ASSIGN);
  t_s->code=code_assignt(length, rhs.op0());
  t_s->source_location=location;
  
  // we also need to allocate space for the data
  member_exprt data(deref, struct_type.components()[2].get_name(), struct_type.components()[2].type());
  side_effect_exprt data_cpp_new_expr(ID_cpp_new_array, data.type());
  data_cpp_new_expr.set(ID_size, rhs.op0());
  goto_programt::targett t_p=dest.add_instruction(ASSIGN);
  t_p->code=code_assignt(data, data_cpp_new_expr);
  t_p->source_location=location;
  
  // zero-initialize the data
  exprt zero_element=gen_zero(data.type().subtype());
  codet array_set(ID_array_set);
  array_set.copy_to_operands(data, zero_element);
  goto_programt::targett t_d=dest.add_instruction(OTHER);
  t_d->code=array_set;
  t_d->source_location=location;

  if(rhs.operands().size()>=2)
  {
    // produce
    // for(int i=0; i<size; i++) tmp[i]=java_new(dim-1);
    // This will be converted recursively.
    
    goto_programt tmp;

    symbol_exprt tmp_i=
      new_tmp_symbol(index_type(), "index", tmp, location).symbol_expr();

    code_fort for_loop;
    
    side_effect_exprt sub_java_new=rhs;
    sub_java_new.operands().erase(sub_java_new.operands().begin());
    
    side_effect_exprt inc(ID_assign);
    inc.operands().resize(2);
    inc.op0()=tmp_i;
    inc.op1()=plus_exprt(tmp_i, gen_one(tmp_i.type()));
    
    dereference_exprt deref_expr(plus_exprt(data, tmp_i), data.type().subtype());
    
    for_loop.init()=code_assignt(tmp_i, gen_zero(tmp_i.type()));
    for_loop.cond()=binary_relation_exprt(tmp_i, ID_lt, rhs.op0());
    for_loop.iter()=inc;
    for_loop.body()=code_skipt();
    for_loop.body()=code_assignt(deref_expr, sub_java_new);

    convert(for_loop, tmp);
    dest.destructive_append(tmp);
  }
}
예제 #4
0
void goto_convertt::do_cpp_new(
  const exprt &lhs,
  const side_effect_exprt &rhs,
  goto_programt &dest)
{
  if(lhs.is_nil())
    throw "do_cpp_new without lhs is yet to be implemented";
  
  // build size expression
  exprt object_size=
    static_cast<const exprt &>(rhs.find(ID_sizeof));

  bool new_array=rhs.get(ID_statement)==ID_cpp_new_array;
  
  exprt count;

  if(new_array)
  {
    count=static_cast<const exprt &>(rhs.find(ID_size));

    if(count.type()!=object_size.type())
      count.make_typecast(object_size.type());

    // might have side-effect
    clean_expr(count, dest);
  }

  exprt tmp_symbol_expr;

  // is this a placement new?
  if(rhs.operands().empty()) // no, "regular" one
  {
    // call __new or __new_array
    exprt new_symbol=
      ns.lookup(new_array?"__new_array":"__new").symbol_expr();
    
    const code_typet &code_type=
      to_code_type(new_symbol.type());

    const typet &return_type=
      code_type.return_type();

    assert(code_type.parameters().size()==1 ||
           code_type.parameters().size()==2);

    const symbolt &tmp_symbol=
      new_tmp_symbol(return_type, "new", dest, rhs.source_location());
    
    tmp_symbol_expr=tmp_symbol.symbol_expr();
    
    code_function_callt new_call;
    new_call.function()=new_symbol;
    if(new_array) new_call.arguments().push_back(count);
    new_call.arguments().push_back(object_size);
    new_call.set("#type", lhs.type().subtype());
    new_call.lhs()=tmp_symbol_expr;
    new_call.add_source_location()=rhs.source_location();
    
    convert(new_call, dest);
  }
  else if(rhs.operands().size()==1)
  {
    // call __placement_new
    exprt new_symbol=
      ns.lookup(new_array?"__placement_new_array":"__placement_new").symbol_expr();
    
    const code_typet &code_type=
      to_code_type(new_symbol.type());

    const typet &return_type=code_type.return_type();
    
    assert(code_type.parameters().size()==2 ||
           code_type.parameters().size()==3);

    const symbolt &tmp_symbol=
      new_tmp_symbol(return_type, "new", dest, rhs.source_location());

    tmp_symbol_expr=tmp_symbol.symbol_expr();

    code_function_callt new_call;
    new_call.function()=new_symbol;
    if(new_array) new_call.arguments().push_back(count);
    new_call.arguments().push_back(object_size);
    new_call.arguments().push_back(rhs.op0()); // memory location
    new_call.set("#type", lhs.type().subtype());
    new_call.lhs()=tmp_symbol_expr;
    new_call.add_source_location()=rhs.source_location();

    for(unsigned i=0; i<code_type.parameters().size(); i++)
      if(new_call.arguments()[i].type()!=code_type.parameters()[i].type())
        new_call.arguments()[i].make_typecast(code_type.parameters()[i].type());
    
    convert(new_call, dest);
  }
  else
    throw "cpp_new expected to have 0 or 1 operands";

  goto_programt::targett t_n=dest.add_instruction(ASSIGN);
  t_n->code=code_assignt(
    lhs, typecast_exprt(tmp_symbol_expr, lhs.type()));
  t_n->source_location=rhs.find_source_location();
    
  // grab initializer
  goto_programt tmp_initializer;
  cpp_new_initializer(lhs, rhs, tmp_initializer);

  dest.destructive_append(tmp_initializer);
}
예제 #5
0
void goto_convertt::do_prob_uniform(
  const exprt &lhs,
  const exprt &function,
  const exprt::operandst &arguments,
  goto_programt &dest)
{
  const irep_idt &identifier=function.get(ID_identifier);

  // make it a side effect if there is an LHS
  if(arguments.size()!=2)
  {
    err_location(function);
    throw "`"+id2string(identifier)+"' expected to have two arguments";
  }

  if(lhs.is_nil())
  {
    err_location(function);
    throw "`"+id2string(identifier)+"' expected to have LHS";
  }

  exprt rhs=side_effect_exprt("prob_uniform", lhs.type());
  rhs.add_source_location()=function.source_location();

  if(lhs.type().id()!=ID_unsignedbv &&
     lhs.type().id()!=ID_signedbv)
  {
    err_location(function);
    throw "`"+id2string(identifier)+"' expected other type";
  }

  if(arguments[0].type().id()!=lhs.type().id() ||
     arguments[1].type().id()!=lhs.type().id())
  {
    err_location(function);
    throw "`"+id2string(identifier)+"' expected operands to be of same type as LHS";
  }

  if(!arguments[0].is_constant() ||
     !arguments[1].is_constant())
  {
    err_location(function);
    throw "`"+id2string(identifier)+"' expected operands to be constant literals";
  }

  mp_integer lb, ub;

  if(to_integer(arguments[0], lb) ||
     to_integer(arguments[1], ub))
  {
    err_location(function);
    throw "error converting operands";
  }

  if(lb > ub)
  {
    err_location(function);
    throw "expected lower bound to be smaller or equal to the upper bound";
  }

  rhs.copy_to_operands(arguments[0], arguments[1]);

  code_assignt assignment(lhs, rhs);
  assignment.add_source_location()=function.source_location();
  copy(assignment, ASSIGN, dest);
}
예제 #6
0
void goto_convertt::do_prob_coin(
  const exprt &lhs,
  const exprt &function,
  const exprt::operandst &arguments,
  goto_programt &dest)
{
  const irep_idt &identifier=function.get(ID_identifier);

  // make it a side effect if there is an LHS
  if(arguments.size()!=2) 
  {
    err_location(function);
    throw "`"+id2string(identifier)+"' expected to have two arguments";
  }
  
  if(lhs.is_nil())
  {
    err_location(function);
    throw "`"+id2string(identifier)+"' expected to have LHS";
  }

  exprt rhs=side_effect_exprt("prob_coin", lhs.type());
  rhs.add_source_location()=function.source_location();

  if(lhs.type()!=bool_typet())
  {
    err_location(function);
    throw "`"+id2string(identifier)+"' expected bool";
  }

  if(arguments[0].type().id()!=ID_unsignedbv ||
     arguments[0].id()!=ID_constant)
  {
    err_location(function);
    throw "`"+id2string(identifier)+"' expected first "
          "operand to be a constant literal of type unsigned long";
  }

  mp_integer num, den;

  if(to_integer(arguments[0], num) ||
     to_integer(arguments[1], den))
  {
    err_location(function);
    throw "error converting operands";
  }

  if(num-den > mp_integer(0))
  {
    err_location(function);
    throw "probability has to be smaller than 1";
  }

  if(den == mp_integer(0))
  {
    err_location(function);
    throw "denominator may not be zero";
  }

  rationalt numerator(num), denominator(den);
  rationalt prob = numerator / denominator;

  rhs.copy_to_operands(from_rational(prob));

  code_assignt assignment(lhs, rhs);
  assignment.add_source_location()=function.source_location();
  copy(assignment, ASSIGN, dest);
}
예제 #7
0
void goto_convertt::do_function_call_symbol(
  const exprt &lhs,
  const symbol_exprt &function,
  const exprt::operandst &arguments,
  goto_programt &dest)
{
  if(function.get_bool("#invalid_object"))
    return; // ignore

  // lookup symbol
  const irep_idt &identifier=function.get_identifier();

  const symbolt *symbol;
  if(ns.lookup(identifier, symbol))
  {
    err_location(function);
    throw "error: function `"+id2string(identifier)+"' not found";
  }

  if(symbol->type.id()!=ID_code)
  {
    err_location(function);
    throw "error: function `"+id2string(identifier)+"' type mismatch: expected code";
  }
  
  if(identifier==CPROVER_PREFIX "assume" ||
     identifier=="__VERIFIER_assume")
  {
    if(arguments.size()!=1)
    {
      err_location(function);
      throw "`"+id2string(identifier)+"' expected to have one argument";
    }

    goto_programt::targett t=dest.add_instruction(ASSUME);
    t->guard=arguments.front();
    t->source_location=function.source_location();
    t->source_location.set("user-provided", true);
    
    // let's double-check the type of the argument
    if(t->guard.type().id()!=ID_bool)
      t->guard.make_typecast(bool_typet());

    if(lhs.is_not_nil())
    {
      err_location(function);
      throw id2string(identifier)+" expected not to have LHS";
    }
  }
  else if(identifier=="__VERIFIER_error")
  {
    if(!arguments.empty())
    {
      err_location(function);
      throw "`"+id2string(identifier)+"' expected to have no arguments";
    }

    goto_programt::targett t=dest.add_instruction(ASSERT);
    t->guard=false_exprt();
    t->source_location=function.source_location();
    t->source_location.set("user-provided", true);
    t->source_location.set_property_class(ID_assertion);

    if(lhs.is_not_nil())
    {
      err_location(function);
      throw id2string(identifier)+" expected not to have LHS";
    }
  }
  else if(has_prefix(id2string(identifier), "java::java.lang.AssertionError.<init>:"))
  {
    // insert function call anyway
    code_function_callt function_call;
    function_call.lhs()=lhs;
    function_call.function()=function;
    function_call.arguments()=arguments;
    function_call.add_source_location()=function.source_location();

    copy(function_call, FUNCTION_CALL, dest);

    if(arguments.size()!=1 && arguments.size()!=2)
    {
      err_location(function);
      throw "`"+id2string(identifier)+"' expected to have one or two arguments";
    }

    goto_programt::targett t=dest.add_instruction(ASSERT);
    t->guard=false_exprt();
    t->source_location=function.source_location();
    t->source_location.set("user-provided", true);
    t->source_location.set_property_class(ID_assertion);    
    t->source_location.set_comment("assertion at "+function.source_location().as_string());
  }
  else if(identifier=="assert" &&
          !ns.lookup(identifier).location.get_function().empty())
  {
    if(arguments.size()!=1)
    {
      err_location(function);
      throw "`"+id2string(identifier)+"' expected to have one argument";
    }

    goto_programt::targett t=dest.add_instruction(ASSERT);
    t->guard=arguments.front();
    t->source_location=function.source_location();
    t->source_location.set("user-provided", true);
    t->source_location.set_property_class(ID_assertion);
    t->source_location.set_comment("assertion "+id2string(from_expr(ns, "", t->guard)));
    
    // let's double-check the type of the argument
    if(t->guard.type().id()!=ID_bool)
      t->guard.make_typecast(bool_typet());

    if(lhs.is_not_nil())
    {
      err_location(function);
      throw id2string(identifier)+" expected not to have LHS";
    }
  }
  else if(identifier==CPROVER_PREFIX "assert")
  {
    if(arguments.size()!=2)
    {
      err_location(function);
      throw "`"+id2string(identifier)+"' expected to have two arguments";
    }
    
    const irep_idt description=
      get_string_constant(arguments[1]);

    goto_programt::targett t=dest.add_instruction(ASSERT);
    t->guard=arguments[0];
    t->source_location=function.source_location();
    t->source_location.set("user-provided", true);
    t->source_location.set_property_class(ID_assertion);
    t->source_location.set_comment(description);
    
    // let's double-check the type of the argument
    if(t->guard.type().id()!=ID_bool)
      t->guard.make_typecast(bool_typet());

    if(lhs.is_not_nil())
    {
      err_location(function);
      throw id2string(identifier)+" expected not to have LHS";
    }
  }
  else if(identifier==CPROVER_PREFIX "printf")
  {
    do_printf(lhs, function, arguments, dest);
  }
  else if(identifier==CPROVER_PREFIX "scanf")
  {
    do_scanf(lhs, function, arguments, dest);
  }
  else if(identifier==CPROVER_PREFIX "input" ||
          identifier=="__CPROVER::input")
  {
    do_input(lhs, function, arguments, dest);
  }
  else if(identifier==CPROVER_PREFIX "output" ||
          identifier=="__CPROVER::output")
  {
    do_output(lhs, function, arguments, dest);
  }
  else if(identifier==CPROVER_PREFIX "atomic_begin" ||
          identifier=="__CPROVER::atomic_begin" ||
          identifier=="__VERIFIER_atomic_begin")
  {
    do_atomic_begin(lhs, function, arguments, dest);
  }
  else if(identifier==CPROVER_PREFIX "atomic_end" ||
          identifier=="__CPROVER::atomic_end" ||
          identifier=="__VERIFIER_atomic_end")
  {
    do_atomic_end(lhs, function, arguments, dest);
  }
  else if(identifier==CPROVER_PREFIX "prob_biased_coin")
  {
    do_prob_coin(lhs, function, arguments, dest);
  }
  else if(has_prefix(id2string(identifier), CPROVER_PREFIX "prob_uniform_"))
  {
    do_prob_uniform(lhs, function, arguments, dest);
  }
  else if(has_prefix(id2string(identifier), "nondet_") ||
          has_prefix(id2string(identifier), "__VERIFIER_nondet_"))
  {
    // make it a side effect if there is an LHS
    if(lhs.is_nil()) return;
    
    exprt rhs;
    
    // We need to special-case for _Bool, which
    // can only be 0 or 1.
    if(lhs.type().id()==ID_c_bool)
    {
      rhs=side_effect_expr_nondett(bool_typet());
      rhs.add_source_location()=function.source_location();
      rhs.set(ID_C_identifier, identifier);
      rhs=typecast_exprt(rhs, lhs.type());
    } 
    else
    {
      rhs=side_effect_expr_nondett(lhs.type());
      rhs.add_source_location()=function.source_location();
      rhs.set(ID_C_identifier, identifier);
    }

    code_assignt assignment(lhs, rhs);
    assignment.add_source_location()=function.source_location();
    copy(assignment, ASSIGN, dest);
  }
  else if(has_prefix(id2string(identifier), CPROVER_PREFIX "uninterpreted_"))
  {
    // make it a side effect if there is an LHS
    if(lhs.is_nil()) return;

    function_application_exprt rhs;
    rhs.type()=lhs.type();
    rhs.add_source_location()=function.source_location();
    rhs.function()=function;
    rhs.arguments()=arguments;

    code_assignt assignment(lhs, rhs);
    assignment.add_source_location()=function.source_location();
    copy(assignment, ASSIGN, dest);
  }
  else if(has_prefix(id2string(identifier), CPROVER_PREFIX "array_set"))
  {
    do_array_set(lhs, function, arguments, dest);
  }
  else if(identifier==CPROVER_PREFIX "array_equal" ||
          identifier=="__CPROVER::array_equal")
  {
    do_array_equal(lhs, function, arguments, dest);
  }
  else if(identifier==CPROVER_PREFIX "array_copy" ||
          identifier=="__CPROVER::array_equal")
  {
    do_array_copy(lhs, function, arguments, dest);
  }
  else if(identifier=="printf")
  /*
          identifier=="fprintf" ||
          identifier=="sprintf" ||
          identifier=="snprintf")
  */
  {
    do_printf(lhs, function, arguments, dest);
  }
  else if(identifier=="__assert_fail" ||
          identifier=="_assert" ||
          identifier=="__assert_c99" ||
          identifier=="_wassert")
  {
    // __assert_fail is Linux
    // These take four arguments:
    // "expression", "file.c", line, __func__
    // klibc has __assert_fail with 3 arguments
    // "expression", "file.c", line

    // MingW has
    // void _assert (const char*, const char*, int);
    // with three arguments:
    // "expression", "file.c", line

    // This has been seen in Solaris 11.
    // Signature:
    // void __assert_c99(const char *desc, const char *file, int line, const char *func);

    // _wassert is Windows. The arguments are
    // L"expression", L"file.c", line

    if(arguments.size()!=4 &&
       arguments.size()!=3)
    {
      err_location(function);
      throw "`"+id2string(identifier)+"' expected to have four arguments";
    }
    
    const irep_idt description=
      "assertion "+id2string(get_string_constant(arguments[0]));

    goto_programt::targett t=dest.add_instruction(ASSERT);
    t->guard=false_exprt();
    t->source_location=function.source_location();
    t->source_location.set("user-provided", true);
    t->source_location.set_property_class(ID_assertion);
    t->source_location.set_comment(description);
    // we ignore any LHS
  }
  else if(identifier=="__assert_rtn" ||
          identifier=="__assert")
  {
    // __assert_rtn has been seen on MacOS;
    // __assert is FreeBSD and Solaris 11.
    // These take four arguments:
    // __func__, "file.c", line, "expression"
    // On Solaris 11, it's three arguments:
    // "expression", "file", line
    
    irep_idt description;
    
    if(arguments.size()==4)
    {
      description=
        "assertion "+id2string(get_string_constant(arguments[3]));
    }
    else if(arguments.size()==3)
    {
      description=
        "assertion "+id2string(get_string_constant(arguments[1]));
    }
    else
    {
      err_location(function);
      throw "`"+id2string(identifier)+"' expected to have four arguments";
    }

    goto_programt::targett t=dest.add_instruction(ASSERT);
    t->guard=false_exprt();
    t->source_location=function.source_location();
    t->source_location.set("user-provided", true);
    t->source_location.set_property_class(ID_assertion);
    t->source_location.set_comment(description);
    // we ignore any LHS
  }
  else if(identifier=="__assert_func")
  {
    // __assert_func is newlib (used by, e.g., cygwin)
    // These take four arguments:
    // "file.c", line, __func__, "expression"
    if(arguments.size()!=4)
    {
      err_location(function);
      throw "`"+id2string(identifier)+"' expected to have four arguments";
    }

    const irep_idt description=
      "assertion "+id2string(get_string_constant(arguments[3]));

    goto_programt::targett t=dest.add_instruction(ASSERT);
    t->guard=false_exprt();
    t->source_location=function.source_location();
    t->source_location.set("user-provided", true);
    t->source_location.set_property_class(ID_assertion);
    t->source_location.set_comment(description);
    // we ignore any LHS
  }
  else if(identifier==CPROVER_PREFIX "fence")
  {
    if(arguments.size()<1)
    {
      err_location(function);
      throw "`"+id2string(identifier)+"' expected to have at least one argument";
    }

    goto_programt::targett t=dest.add_instruction(OTHER);
    t->source_location=function.source_location();
    t->code.set(ID_statement, ID_fence);

    forall_expr(it, arguments)
    {
      const irep_idt kind=get_string_constant(*it);
      t->code.set(kind, true);
    }
  }
  else if(identifier=="__builtin_prefetch")