On Wed, 26 Feb 2025, Jakub Jelinek wrote:
> Hi!
>
> This PR is about ubsan error on the c - cx1 + cy1 evaluation in the first
> hunk.
>
> The following patch hopefully fixes that by doing the additions/subtractions
> in poly_uint64 rather than poly_int64. Or shall we instead perform it in
> offset_int and watch for overflows and punt somehow for those?
I think when we have the offset computation overflow ignoring such
overflow will make the memrefs_conflict_p give possibly wrong
answers. In the PR you say cselib now has those
-9223372036854775807ish offsets from sp, why does it do alias queries
with those clearly invalid offsets?
So yes, I think we need to punt on overflow. Using poly_offset_int
should work but it comes at a cost. Does poly_wide_int have the same
wide-int like overflow overloads?
> Or shall we just treat this way only the first case where it is
> adding/subtracting 3 numbers and not just 2, so there is at least a chance
> the overflow is just temporary?
>
> Bootstrapped/regtested on x86_64-linux and i686-linux (but just normal
> bootstrap, not ubsan one), ok for trunk?
>
> 2025-02-26 Jakub Jelinek <ja...@redhat.com>
>
> PR middle-end/118819
> * alias.cc (memrefs_conflict_p): Perform arithmetics on c in
> poly_uint64 type rather than poly_int64 to avoid compile time
> UB.
>
> --- gcc/alias.cc.jj 2025-01-02 11:23:24.000000000 +0100
> +++ gcc/alias.cc 2025-02-25 12:43:16.655507666 +0100
> @@ -2537,12 +2537,14 @@ memrefs_conflict_p (poly_int64 xsize, rt
> {
> if (poly_int_rtx_p (y1, &cy1))
> return memrefs_conflict_p (xsize, x0, ysize, y0,
> - c - cx1 + cy1);
> + (poly_uint64) c - cx1 + cy1);
> else
> - return memrefs_conflict_p (xsize, x0, ysize, y, c - cx1);
> + return memrefs_conflict_p (xsize, x0, ysize, y,
> + (poly_uint64) c - cx1);
> }
> else if (poly_int_rtx_p (y1, &cy1))
> - return memrefs_conflict_p (xsize, x, ysize, y0, c + cy1);
> + return memrefs_conflict_p (xsize, x, ysize, y0,
> + (poly_uint64) c + cy1);
>
> return -1;
> }
> @@ -2563,7 +2565,8 @@ memrefs_conflict_p (poly_int64 xsize, rt
>
> poly_int64 cy1;
> if (poly_int_rtx_p (y1, &cy1))
> - return memrefs_conflict_p (xsize, x, ysize, y0, c + cy1);
> + return memrefs_conflict_p (xsize, x, ysize, y0,
> + (poly_uint64) c + cy1);
> else
> return -1;
> }
> @@ -2643,7 +2646,7 @@ memrefs_conflict_p (poly_int64 xsize, rt
> poly_int64 cx, cy;
> if (poly_int_rtx_p (x, &cx) && poly_int_rtx_p (y, &cy))
> {
> - c += cy - cx;
> + c += (poly_uint64) cy - cx;
> return offset_overlap_p (c, xsize, ysize);
> }
>
>
> Jakub
>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
