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

[Jason Merrill <jason@redhat.com>]

On 11/10/23 10:28, Patrick Palka wrote:
> On Fri, 10 Nov 2023, Patrick Palka wrote:
> 
>> 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?

True, we would need build_min_non_dep to also get TREE_SIDE_EFFECTS from 
the operands.  That might be useful in general for similar situations?

I also note that convert_to_ptrofftype uses fold_convert, so the new 
code could have the same problem if the pointer to member operand is 
also a COMPONENT_REF.

Jason