Re: [PATCH][v2] Adjust volatile handling of the operand scanner

[Richard Biener <rguenther@suse.de>]

On Tue, 10 Aug 2021, Eric Botcazou wrote:

> > The question is whether we instead want to amend build3 to
> > set TREE_THIS_VOLATILE automatically when the FIELD_DECL has
> > it set.  At least for the Fortran FE cases the gimplifier
> > fails to see some volatile references and thus can generate
> > wrong code which is a latent issue.
> 
> What do we do for other similar flags, e.g. TREE_READONLY?

build3 currently does no special processing for the FIELD_DECL operand,
it just sets TREE_THIS_VOLATILE from operand zero for tcc_references.

The C and C++ frontends have repeated patterns like

          ref = build3 (COMPONENT_REF, subtype, datum, subdatum,
                        NULL_TREE);
          SET_EXPR_LOCATION (ref, loc);
          if (TREE_READONLY (subdatum)
              || (use_datum_quals && TREE_READONLY (datum)))
            TREE_READONLY (ref) = 1;
          if (TREE_THIS_VOLATILE (subdatum)
              || (use_datum_quals && TREE_THIS_VOLATILE (datum)))
            TREE_THIS_VOLATILE (ref) = 1;

Leaving out TREE_READONLY shouldn't have any correctness issue.  It's
just that when adjusting the SSA operand scanner to correctly interpret
GENERIC that this uncovers pre-existing issues in the Fortran frontend
(one manifests in a testsuite FAIL - otherwise I wouldn't have noticed).

I'm fine with just plugging the Fortran FE holes as we discover them
but I did not check other frontends and testsuite coverage is weak.

Now - I wonder if there's a reason a frontend might _not_ want to
set TREE_THIS_VOLATILE on a COMPONENT_REF when the FIELD_DECL has
TREE_THIS_VOLATILE set.

I guess I'll do one more experiment and add verification that
TREE_THIS_VOLATILE on COMPONENT_REFs and FIELD_DECLs is consistent
and see where that trips.

Richard.