Values of WIDE_INT_MAX_ELTS in gcc11 and gcc12 are different

[Richard Biener]

On Sat, Nov 6, 2021 at 10:56 AM Jakub Jelinek <jakub@redhat.com> wrote:
>
> On Fri, Nov 05, 2021 at 05:37:25PM +0000, Qing Zhao wrote:
> > > On Nov 5, 2021, at 11:17 AM, Jakub Jelinek <jakub@redhat.com> wrote:
> > >
> > > On Fri, Nov 05, 2021 at 04:11:36PM +0000, Qing Zhao wrote:
> > >> 3076       if (TREE_CODE (TREE_TYPE (lhs)) != BOOLEAN_TYPE
> > >> 3077           && tree_fits_uhwi_p (var_size)
> > >> 3078           && (init_type == AUTO_INIT_PATTERN
> > >> 3079               || !is_gimple_reg_type (var_type))
> > >> 3080           && int_mode_for_size (tree_to_uhwi (var_size) * BITS_PER_UNIT,
> > >> 3081                                 0).exists ())
> > >> 3082         {
> > >> 3083           unsigned HOST_WIDE_INT total_bytes = tree_to_uhwi (var_size);
> > >> 3084           unsigned char *buf = (unsigned char *) xmalloc (total_bytes);
> > >> 3085           memset (buf, (init_type == AUTO_INIT_PATTERN
> > >> 3086                         ? INIT_PATTERN_VALUE : 0), total_bytes);
> > >> 3087           tree itype = build_nonstandard_integer_type
> > >> 3088                          (total_bytes * BITS_PER_UNIT, 1);
> > >>
> > >> The exact failing point is at function “set_min_and_max_values_for_integral_type”:
> > >>
> > >> 2851   gcc_assert (precision <= WIDE_INT_MAX_PRECISION);
> > >>
> > >> For _Complex long double,  “precision” is 256.
> > >> In GCC11, “WIDE_INT_MAX_PRECISION” is 192,  in GCC12, it’s 512.
> > >> As a result, the above assertion failed on GCC11.
> > >>
> > >> I am wondering what’s the best fix for this issue in gcc11?
> > >
> > > Even for gcc 12 the above is wrong, you can't blindly assume that
> > > build_nonstandard_integer_type will work for arbitrary precisions,
> > > and even if it works that it will actually work.
> > > The fact that such a mode exist is one thing, but
> > > targetm.scalar_mode_supported_p should be tested for whether the mode
> > > is actually supported.
> >
> > You mean “int_mode_for_size().exists()” is not enough to make sure
> > “build_nonstandard_integer_type” to be valid?  We should add
> > “targetm.scalar_mode_supported_p” too ?
>
> Yeah.  The former says whether the backend has that mode at all.
> But some modes may be there only in some specific patterns but
> without support for mov, add, etc.  Only for
> targetm.scalar_mode_supported_p modes the backend guarantees that
> one can use them e.g. in mode attribute and can expect expansion
> to expand everything with that mode that is needed in some way.
> E.g. only if targetm.scalar_mode_supported_p (TImode) the FEs
> support __int128_t type, etc.

The memcpy folding code now checks

              scalar_int_mode mode;
              if (int_mode_for_size (ilen * 8, 0).exists (&mode)
                  && GET_MODE_SIZE (mode) * BITS_PER_UNIT == ilen * 8
                  && have_insn_for (SET, mode)

thus specifically only have_insn_for (SET, mode), which I guess is
good enough for this case as well?

>         Jakub
>