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

[Richard Biener <richard.guenther@gmail.com>]

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?

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
>