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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug fortran/102043] Wrong array types used for negative stride
 accesses
Date: Wed, 25 Aug 2021 09:16:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102043

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ebotcazou at gcc dot gnu.o=
rg

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
For this testcase the bogus ref is constructed by conv_array_index_offset:

          /* Read the vector to get an index into info->descriptor.  */
          data =3D build_fold_indirect_ref_loc (input_location,
                                          gfc_conv_array_data (desc));
          index =3D gfc_build_array_ref (data, index, NULL);

where ultimatively gfc_conv_descriptor_data_get is what builds the
base the ARRAY_REF is applied to and thus GFC_TYPE_ARRAY_DATAPTR_TYPE
what is "wrong".

This type is built in gfc_get_array_type_bounds as

  /* We define data as an array with the correct size if possible.
     Much better than doing pointer arithmetic.  */
  if (stride)
    rtype =3D build_range_type (gfc_array_index_type, gfc_index_zero_node,
                              int_const_binop (MINUS_EXPR, stride,
                                               build_int_cst (TREE_TYPE
(stride), 1)));
  else
    rtype =3D gfc_array_range_type;
  arraytype =3D build_array_type (etype, rtype);
  arraytype =3D build_pointer_type (arraytype);
  if (restricted)
    arraytype =3D build_qualified_type (arraytype, TYPE_QUAL_RESTRICT);
  GFC_TYPE_ARRAY_DATAPTR_TYPE (fat_type) =3D arraytype;

The issue might also be the source of "bogus" array bound warnings.

Note it might in the end be the issue that the middle-end lacks notion
of a stride for array accesses which the Fortran frontend emulates
by multiplying the index by the stride.  When the stride is negative
then the idea to base all arrays on [0:] no longer works out.  Note
that simply using an unknown TYPE_DOMAIN (aka NULL_TREE) doesn't
work either since we still assume that ARRAY_REFs can only have
positive indices.

Eric - does Ada have something like negative stride array accesses?=