C++ PATCH to fix ICE with vector expr folding (PR c++/83659)

[Richard Biener]

On Thu, Jan 25, 2018 at 3:45 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> On Fri, Jan 05, 2018 at 09:52:36AM +0100, Richard Biener wrote:
>> On Wed, Jan 3, 2018 at 5:31 PM, Marek Polacek <polacek@redhat.com> wrote:
>> > Here we are crashing because cxx_fold_indirect_ref got a POINTER_PLUS_EXPR
>> > with offset > signed HOST_WIDE_INT and we tried to convert it to sHWI.
>> >
>> > The matching code in fold_indirect_ref_1 uses uHWIs so I've followed suit.
>> > But that code now also uses poly_uint64 and I'm not sure if any of the
>> > constexpr.c code should use it, too.  But this patch fixes the ICE.
>>
>> POINTER_PLUS_EXPR offets are to be interpreted as signed (ptrdiff_t)
>> so using uhwi and then performing an unsigned division is wrong code.
>> See mem_ref_offset how to deal with this (ugh - poly-ints...).  Basically
>> you have to force the thing to signed.  Like just use
>>
>>   HOST_WIDE_INT offset = TREE_INT_CST_LOW (op01);
>
> Does it really matter here though?  Any negative offsets there are UB, we
> should punt on them rather than try to optimize them.
> As we known op01 is unsigned, if we check that it fits into shwi_p, it means
> it will be 0 to shwi max and then we can handle it in uhwi too.

Ah, of course.  Didn't look up enough context to see what this code
does in the end ;)

>           /* ((foo*)&vectorfoo)[1] => BIT_FIELD_REF<vectorfoo,...> */
>           if (VECTOR_TYPE_P (op00type)
>               && (same_type_ignoring_top_level_qualifiers_p
> -                (type, TREE_TYPE (op00type))))
> +                (type, TREE_TYPE (op00type)))
> +             && tree_fits_shwi_p (op01))

nevertheless this appearant "mismatch" deserves a comment (checking
shwi and using uhwi).

>             {
> -             HOST_WIDE_INT offset = tree_to_shwi (op01);
> +             unsigned HOST_WIDE_INT offset = tree_to_uhwi (op01);
>               tree part_width = TYPE_SIZE (type);
> -             unsigned HOST_WIDE_INT part_widthi = tree_to_shwi (part_width)/BITS_PER_UNIT;
> +             unsigned HOST_WIDE_INT part_widthi
> +               = tree_to_uhwi (part_width) / BITS_PER_UNIT;
>               unsigned HOST_WIDE_INT indexi = offset * BITS_PER_UNIT;
>               tree index = bitsize_int (indexi);
>
>               if (known_lt (offset / part_widthi,
>                             TYPE_VECTOR_SUBPARTS (op00type)))
>                 return fold_build3_loc (loc,
>                                         BIT_FIELD_REF, type, op00,
>                                         part_width, index);
>
>             }
>
>         Jakub