https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82224
--- Comment #12 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #9)
> One complication when deciding whether the downstream uses are fine is
> that we assign the same alias set to union accesses u->x and u->y.
>
> That said, the union read/write pairs we remove essentially act as
> an optimization barrier - they "union" the alias sets of all components
> of the unions (not only of those accessed). I think that's what other
> readings of the standard ask for (there's a bug about this as well).
> A mere declaration of a union needs to union the alias-sets and the
> following should be valid:
>
> union { long l; long long ll; };
>
> long foo (long *p)
> {
> *p = 0;
> *(long long *)p = 1;
> return *p;
> }
>
> this means (from an implementation perspective) that iff we do not
> want to go this far then accessing a union member via a non-union
> type isn't valid.
>
> Anyway ;) I don't see an easy way to fix the testcases in this PR
> without removing the optimization entirely. It's currently guarded
> like
>
> /* If the TBAA state isn't the same for downstream reads
> we cannot value-number the VDEFs the same. */
> alias_set_type set = get_alias_set (lhs);
> if (! vnresult
> || vnresult->set == set
> || alias_set_subset_of (set, vnresult->set))
>
> but we don't know whether vnresult is from a load or a store so we
> don't know whether it fully specifies the "TBAA state". So even
> dumbing that down to
>
> if (vnresult->set != set
> || set != get_alias_set (TREE_TYPE (lhs)))
> resultsame = false;
>
> where the latter test sees if the alias set for the access and for an
> access using a pointer is the same might keep some holes open.
>
> Some of the more interesting cases _did_ involve unions and the above
> would likely make them not optimized again (I remember sth with vector
> intrinsics and initialization / copying via unions). Oh, even more
> trivial union->x = union->x; will stop from being optimized.
>
> Index: gcc/tree-ssa-sccvn.c
> ===================================================================
> --- gcc/tree-ssa-sccvn.c (revision 254135)
> +++ gcc/tree-ssa-sccvn.c (working copy)
> @@ -3800,11 +3800,11 @@ visit_reference_op_store (tree lhs, tree
> resultsame = expressions_equal_p (result, op);
> if (resultsame)
> {
> - /* If the TBAA state isn't compatible for downstream reads
> + /* If the TBAA state isn't the same for downstream reads
> we cannot value-number the VDEFs the same. */
> alias_set_type set = get_alias_set (lhs);
> if (vnresult->set != set
> - && ! alias_set_subset_of (set, vnresult->set))
> + || set != get_alias_set (TREE_TYPE (lhs)))
> resultsame = false;
> }
> }
> @@ -5628,9 +5628,9 @@ eliminate_dom_walker::before_dom_childre
> at least all accesses the later one does or if the store
> was to readonly memory storing the same value. */
> alias_set_type set = get_alias_set (lhs);
> - if (! vnresult
> - || vnresult->set == set
> - || alias_set_subset_of (set, vnresult->set))
> + if (vnresult
> + && vnresult->set == set
> + && set == get_alias_set (TREE_TYPE (lhs)))
> {
> if (dump_file && (dump_flags & TDF_DETAILS))
> {
> Index: gcc/tree-ssa-dom.c
> ===================================================================
> --- gcc/tree-ssa-dom.c (revision 254135)
> +++ gcc/tree-ssa-dom.c (working copy)
> @@ -46,6 +46,7 @@ along with GCC; see the file COPYING3.
> #include "gimplify.h"
> #include "tree-cfgcleanup.h"
> #include "dbgcnt.h"
> +#include "alias.h"
>
> /* This file implements optimizations on the dominator tree. */
>
> @@ -1953,7 +1954,8 @@ dom_opt_dom_walker::optimize_stmt (basic
> gimple_set_vuse (new_stmt, gimple_vuse (stmt));
> cached_lhs = m_avail_exprs_stack->lookup_avail_expr (new_stmt,
> false,
> false);
> - if (cached_lhs && operand_equal_p (rhs, cached_lhs, 0))
> + if (cached_lhs && operand_equal_p (rhs, cached_lhs, 0)
> + && get_alias_set (lhs) == get_alias_set (TREE_TYPE (lhs)))
> {
> basic_block bb = gimple_bb (stmt);
> unlink_stmt_vdef (stmt);
>
> FAILs at least gcc.dg/tree-ssa/ssa-pre-26.c
And
FAIL: gnat.dg/opt39.adb scan-tree-dump-times optimized "MEM" 1 (found 9 times)
otherwise clean on x86_64-unknown-linux-gnu.
> as said I'm not 100% convinced the more strict handling avoids all of the
> cases.
