has_volatile_ops and early optimization w/o alias information

[Richard Guenther]

On Mon, 3 Sep 2007, Richard Guenther wrote:

> 
> We set has_volatile_ops on all(?) memory references during early
> optimization because we don't have alias information.  But we
> do it inconsistently for loads.  For example I see
> 
>   D.2574_23 = *D.2573_22;
> 
> (no volatile) and
> 
>   D.2565_28 ={v} tab[D.2560_27].__delta;
> 
> (volatile).  Because for indirect references we also check
> 
>           /* Aliasing information is missing; mark statement as
>              volatile so we won't optimize it out too actively.  */
>           else if (!gimple_aliases_computed_p (cfun)
>                    && (flags & opf_def))
>             s_ann->has_volatile_ops = true;
> 
> so only add has_volatile_ops if we would create a DEF.  Now the
> other place is in the generic add_virtual_operand like
> 
>   if (aliases == NULL)
>     {
>       if (!gimple_aliases_computed_p (cfun))
>         s_ann->has_volatile_ops = true;
> 
> and so also marks load.  Which one is safe?  I suppose it is safe
> to DCE loads even without alias information?  So I'd add the check
> for a DEF also in the generic add_virtual_operand code.

Which leads to the following patch.  Bootstrapped and tested on
x86_64-unknown-linux-gnu.

Richard.


2007-09-03  Richard Guenther  <rguenther@suse.de>

	* tree-ssa-operands.c (add_virtual_operand): Only mark
	stores as has_volatile_ops if alias information is not available.

Index: tree-ssa-operands.c
===================================================================
*** tree-ssa-operands.c	(revision 128037)
--- tree-ssa-operands.c	(working copy)
*************** add_virtual_operand (tree var, stmt_ann_
*** 1494,1500 ****
  
    if (aliases == NULL)
      {
!       if (!gimple_aliases_computed_p (cfun))
          s_ann->has_volatile_ops = true;
  
        /* The variable is not aliased or it is an alias tag.  */
--- 1494,1501 ----
  
    if (aliases == NULL)
      {
!       if (!gimple_aliases_computed_p (cfun)
! 	  && (flags & opf_def))
          s_ann->has_volatile_ops = true;
  
        /* The variable is not aliased or it is an alias tag.  */