On Fri, 5 Jul 2019, Richard Biener wrote:
>
> There's a path I added where get_alias_set () != 0 was meant to
> cover -fno-strict-aliasing but it doesn't since quite some time
> (oops). Fixed as follows - preprartory for PR91091 where we'd
> otherwise break the testcase below.
>
> Bootstrap / regtest running on x86_64-unknown-linux-gnu.
>
> I'll also backport this to the GCC 8/9 branches where the issue
> is latent.
Applied with adjusting gcc.dg/tree-ssa/ssa-fre-61.c to use
-fstrict-aliasing. The testcase could be handled without
if alignment were considered, I'll leave that or a followup.
Richard.
> Richard.
>
> 2019-07-05 Richard Biener <rguent...@suse.de>
>
> PR tree-optimization/91091
> * tree-ssa-sccvn.c (vn_reference_lookup_3): Overlap of
> accesses can happen with -fno-strict-aliasing.
>
> * gcc.dg/tree-ssa/pr91091-1.c: New testcase.
>
> Index: gcc/tree-ssa-sccvn.c
> ===================================================================
> --- gcc/tree-ssa-sccvn.c (revision 273133)
> +++ gcc/tree-ssa-sccvn.c (working copy)
> @@ -1996,7 +1996,8 @@ vn_reference_lookup_3 (ao_ref *ref, tree
> activation of a union member via a store makes the
> values of untouched bytes unspecified. */
> && (known_eq (ref->size, BITS_PER_UNIT)
> - || (get_alias_set (lhs) != 0
> + || (flag_strict_aliasing
> + && get_alias_set (lhs) != 0
> && ao_ref_alias_set (ref) != 0)))
> {
> tree *saved_last_vuse_ptr = data->last_vuse_ptr;
> Index: gcc/testsuite/gcc.dg/tree-ssa/pr91091-1.c
> ===================================================================
> --- gcc/testsuite/gcc.dg/tree-ssa/pr91091-1.c (nonexistent)
> +++ gcc/testsuite/gcc.dg/tree-ssa/pr91091-1.c (working copy)
> @@ -0,0 +1,23 @@
> +/* { dg-do run } */
> +/* { dg-options "-O3 -fno-strict-aliasing" } */
> +
> +struct s { int x; } __attribute__((packed));
> +struct t { int x; };
> +
> +void __attribute__((noinline,noipa))
> +swap(struct s* p, struct t* q)
> +{
> + p->x = q->x;
> + q->x = p->x;
> +}
> +
> +int main()
> +{
> + struct t a[2];
> + a[0].x = 0x12345678;
> + a[1].x = 0x98765432;
> + swap ((struct s *)((char *)a + 1), a);
> + if (a[0].x != 0x12345678)
> + __builtin_abort ();
> + return 0;
> +}
>
--
Richard Biener <rguent...@suse.de>
SUSE Linux GmbH, Maxfeldstrasse 5, 90409 Nuernberg, Germany;
GF: Felix Imendörffer, Mary Higgins, Sri Rasiah; HRB 21284 (AG Nürnberg)
