Re: [PATCH RFA (fold)] c++: constexpr bit_cast with empty field

[Jason Merrill <jason@redhat.com>]

On 7/18/23 07:31, Richard Biener wrote:
> On Mon, Jul 17, 2023 at 11:20 PM Jason Merrill via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>>
>> Tested x86_64-pc-linux-gnu, OK for trunk?
>>
>> -- 8< --
>>
>> The change to only cache constexpr calls that are
>> reduced_constant_expression_p tripped on bit-cast3.C, which failed that
>> predicate due to the presence of an empty field in the result of
>> native_interpret_aggregate, which reduced_constant_expression_p rejects to
>> avoid confusing output_constructor.
>>
>> This patch proposes to skip such fields in native_interpret_aggregate, since
>> they aren't actually involved in the value representation.
>>
>> gcc/ChangeLog:
>>
>>          * fold-const.cc (native_interpret_aggregate): Skip empty fields.
>>
>> gcc/cp/ChangeLog:
>>
>>          * constexpr.cc (cxx_eval_bit_cast): Check that the result of
>>          native_interpret_aggregate doesn't need more evaluation.
>> ---
>>   gcc/cp/constexpr.cc | 9 +++++++++
>>   gcc/fold-const.cc   | 3 ++-
>>   2 files changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
>> index 9d85c3be5cc..6e8f1c2b61e 100644
>> --- a/gcc/cp/constexpr.cc
>> +++ b/gcc/cp/constexpr.cc
>> @@ -1440,6 +1440,8 @@ enum value_cat {
>>
>>   static tree cxx_eval_constant_expression (const constexpr_ctx *, tree,
>>                                            value_cat, bool *, bool *, tree * = NULL);
>> +static tree cxx_eval_bare_aggregate (const constexpr_ctx *, tree,
>> +                                    value_cat, bool *, bool *);
>>   static tree cxx_fold_indirect_ref (const constexpr_ctx *, location_t, tree, tree,
>>                                     bool * = NULL);
>>   static tree find_heap_var_refs (tree *, int *, void *);
>> @@ -4803,6 +4805,13 @@ cxx_eval_bit_cast (const constexpr_ctx *ctx, tree t, bool *non_constant_p,
>>          {
>>            clear_type_padding_in_mask (TREE_TYPE (t), mask);
>>            clear_uchar_or_std_byte_in_mask (loc, r, mask);
>> +         if (CHECKING_P)
>> +           {
>> +             tree e = cxx_eval_bare_aggregate (ctx, r, vc_prvalue,
>> +                                               non_constant_p, overflow_p);
>> +             gcc_checking_assert (e == r);
>> +             r = e;
>> +           }
>>          }
>>       }
>>
>> diff --git a/gcc/fold-const.cc b/gcc/fold-const.cc
>> index a02ede79fed..db8f7de5680 100644
>> --- a/gcc/fold-const.cc
>> +++ b/gcc/fold-const.cc
>> @@ -8935,7 +8935,8 @@ native_interpret_aggregate (tree type, const unsigned char *ptr, int off,
>>       return NULL_TREE;
>>     for (tree field = TYPE_FIELDS (type); field; field = DECL_CHAIN (field))
>>       {
>> -      if (TREE_CODE (field) != FIELD_DECL || DECL_PADDING_P (field))
>> +      if (TREE_CODE (field) != FIELD_DECL || DECL_PADDING_P (field)
>> +         || integer_zerop (DECL_SIZE (field)))
> 
> The gimplifier uses is_empty_type (TREE_TYPE  (field)), wouldn't that be better
> for consistency reasons at least?

Good point, thanks.  Pushed with that change.

> OK with that change or with integer_zerop if there is a reason to not
> use is_empty_type
> here.
> 
> Thanks,
> Richard.
> 
>>          continue;
>>         tree fld = field;
>>         HOST_WIDE_INT bitoff = 0, pos = 0, sz = 0;
>>
>> base-commit: caabf0973a4e9a26421c94d540e3e20051e93e77
>> --
>> 2.39.3
>>
>