Re: [PATCH] c++: non-dependent .* folding [PR112427]

[Patrick Palka <ppalka@redhat.com>]

On Thu, 9 Nov 2023, Jason Merrill wrote:

> On 11/8/23 16:59, Patrick Palka wrote:
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> > trunk?
> > 
> > -- >8 --
> > 
> > Here when building up the non-dependent .* expression, we crash from
> > fold_convert on 'b.a' due to this (templated) COMPONENT_REF having an
> > IDENTIFIER_NODE instead of FIELD_DECL operand that middle-end routines
> > expect.  Like in r14-4899-gd80a26cca02587, this patch fixes this by
> > replacing the problematic piecemeal folding with a single call to
> > cp_fully_fold.
> > 
> > 	PR c++/112427
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* typeck2.cc (build_m_component_ref): Use cp_convert, build2 and
> > 	cp_fully_fold instead of fold_build_pointer_plus and fold_convert.
> 
> > gcc/testsuite/ChangeLog:
> > 
> > 	* g++.dg/template/non-dependent29.C: New test.
> > ---
> >   gcc/cp/typeck2.cc                               |  5 ++++-
> >   gcc/testsuite/g++.dg/template/non-dependent29.C | 13 +++++++++++++
> >   2 files changed, 17 insertions(+), 1 deletion(-)
> >   create mode 100644 gcc/testsuite/g++.dg/template/non-dependent29.C
> > 
> > diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc
> > index 309903afed8..208004221da 100644
> > --- a/gcc/cp/typeck2.cc
> > +++ b/gcc/cp/typeck2.cc
> > @@ -2378,7 +2378,10 @@ build_m_component_ref (tree datum, tree component,
> > tsubst_flags_t complain)
> >         /* Build an expression for "object + offset" where offset is the
> >   	 value stored in the pointer-to-data-member.  */
> >         ptype = build_pointer_type (type);
> > -      datum = fold_build_pointer_plus (fold_convert (ptype, datum),
> > component);
> > +      datum = cp_convert (ptype, datum, complain);
> > +      datum = build2 (POINTER_PLUS_EXPR, ptype,
> > +		      datum, convert_to_ptrofftype (component));
> 
> We shouldn't need to build the POINTER_PLUS_EXPR at all in template context.
> OK with that change.

Hmm, that seems harmless at first glance, but I noticed
build_min_non_dep (called from build_x_binary_op in this case) is
careful to propagate TREE_SIDE_EFFECTS of the given tree, and so eliding
POINTER_PLUS_EXPR here could potentially mean that the tree we
ultimately return from build_x_binary_op when in a template context has
TREE_SIDE_EFFECTS not set when it used to.  Shall we still elide the
POINTER_PLUS_EXPR in a template context despite this?

(The TREE_SIDE_EFFECTS propagation in build_min_non_dep was added in
r71108 to avoid bogus ahead of time -Wunused-value warnings.  But then
r105273 later made us stop issuing -Wunused-value warnings ahead of time
altogether.  So perhaps we don't need to maintain the TREE_SIDE_EFFECTS
flag on templated trees at all anymore?)

> 
> Jason
> 
>