Re: Extend -fstack-protector-strong to cover calls with return slot

[Florian Weimer <fweimer at redhat dot com>]

On 01/07/2014 02:07 PM, Jakub Jelinek wrote:
> On Tue, Jan 07, 2014 at 01:51:00PM +0100, Florian Weimer wrote:
> > +static bool
> > +stack_protect_return_slot_p ()
> > +{
> > +  basic_block bb;
> > +
> > +  FOR_ALL_BB_FN (bb, cfun)
> > +    for (gimple_stmt_iterator gsi = gsi_start_bb (bb);
> > +	 !gsi_end_p (gsi); gsi_next (&gsi))
> > +      {
> > +	gimple stmt = gsi_stmt (gsi);
> > +	if (is_gimple_call (stmt) && gimple_call_return_slot_opt_p (stmt))
>
> I have to repeat, this is not the right test, it really is just an
> optimization hint, nothing else.  Just look where it is set (unless it is
> C++ where some NRV is performed in the FE as mandated by the languages)
> - in pass_return_slot, which is an optimization pass, not run with -O0 or
> -Og at all.

I don't understand.  The corresponding tree flag is set by 
gimplify_modify_expr_rhs, in the CALL_EXPR case:

	      if (use_target)
		{
		  CALL_EXPR_RETURN_SLOT_OPT (*from_p) = 1;
		  mark_addressable (*to_p);
		}

Isn't this an integral part of the gimplifier which also runs at -O0?  I 
certainly see the flag in the generated gimple at -O0, with the amended 
C test case:

foo11 ()
{
  int D.2037;
  struct B D.2038;

  D.2038 = global3 (); [return slot optimization]
  D.2037 = D.2038.a1;
  return D.2037;
}

As I said, I think the function gimple_call_return_slot_opt_p is 
misnamed because it's not actually an optimization.

--
Florian Weimer / Red Hat Product Security Team