[Bug c++/71372] [6/7 Regression] C++ FE drops TREE_THIS_VOLATILE in cp_fold on all tcc_reference trees

["rguenther at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71372

--- Comment #8 from rguenther at suse dot de <rguenther at suse dot de> ---
On Thu, 2 Jun 2016, rguenther at suse dot de wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71372
> 
> --- Comment #7 from rguenther at suse dot de <rguenther at suse dot de> ---
> On Wed, 1 Jun 2016, jakub at gcc dot gnu.org wrote:
> 
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71372
> > 
> > Jakub Jelinek <jakub at gcc dot gnu.org> changed:
> > 
> >            What    |Removed                     |Added
> > ----------------------------------------------------------------------------
> >                  CC|                            |jakub at gcc dot gnu.org
> > 
> > --- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> --- Isn't 
> > this generally a problem of the whole folder, not just cp_fold? I mean, 
> > if you have say MEM[&MEM[p, CST1], CST2] and the outer MEM_REF has e.g. 
> > TREE_THIS_VOLATILE, TREE_THIS_NOTRAP, TREE_SIDE_EFFECTS, TREE_READONLY, 
> > TREE_CONSTANT set on it, and you call fold on it, then I don't see what 
> > would preserve those bits (not sure if all of them are applicable). I 
> 
> True, but that's a bug of that specific folder then.

Actually it's not a bug of those but of the callers (given fold_binary
doesn't get to see that flag).

> > see just
> >       switch (TREE_CODE_LENGTH (code))
> >         {
> >         case 1:
> >           op0 = TREE_OPERAND (t, 0);
> >           tem = fold_unary_loc (loc, code, type, op0);
> >           return tem ? tem : expr;
> >         case 2:
> >           op0 = TREE_OPERAND (t, 0);
> >           op1 = TREE_OPERAND (t, 1);
> >           tem = fold_binary_loc (loc, code, type, op0, op1);
> >           return tem ? tem : expr;
> >         case 3:
> >           op0 = TREE_OPERAND (t, 0);
> >           op1 = TREE_OPERAND (t, 1);
> >           op2 = TREE_OPERAND (t, 2);
> >           tem = fold_ternary_loc (loc, code, type, op0, op1, op2);
> >           return tem ? tem : expr;
> > without really trying to preserve anything.

This would need to do the fixup in case of tcc_reference codes.  
Fortunately fold () calls are rare.