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

[Richard Biener <richard dot guenther at gmail dot com>]

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);

Richard.

> Bootstrapped/regtested on x86_64-linux, ok for trunk/7?
>
> 2018-01-03  Marek Polacek  <polacek@redhat.com>
>
>         PR c++/83659
>         * constexpr.c (cxx_fold_indirect_ref): Use unsigned HOST_WIDE_INT
>         when computing offsets.
>
>         * g++.dg/torture/pr83659.C: New test.
>
> diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
> index 1aeacd51810..cf7c994b381 100644
> --- gcc/cp/constexpr.c
> +++ gcc/cp/constexpr.c
> @@ -3109,9 +3109,10 @@ cxx_fold_indirect_ref (location_t loc, tree type, tree op0, bool *empty_base)
>               && (same_type_ignoring_top_level_qualifiers_p
>                   (type, TREE_TYPE (op00type))))
>             {
> -             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);
>
> diff --git gcc/testsuite/g++.dg/torture/pr83659.C gcc/testsuite/g++.dg/torture/pr83659.C
> index e69de29bb2d..d9f709bb520 100644
> --- gcc/testsuite/g++.dg/torture/pr83659.C
> +++ gcc/testsuite/g++.dg/torture/pr83659.C
> @@ -0,0 +1,11 @@
> +// PR c++/83659
> +// { dg-do compile }
> +
> +typedef int V __attribute__ ((__vector_size__ (16)));
> +V a;
> +
> +int
> +main ()
> +{
> +  reinterpret_cast <int *> (&a)[-1] += 1;
> +}
>
>         Marek