On December 12, 2021 11:49:12 AM GMT+01:00, Jan Hubicka via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>Hi,
>As discussed in the PR, we miss some optimization becuase
>gimple-ssa-isolate-paths turns NULL memory accesses to volatile and adds
>__builtin_trap after them. This is seen as a side-effect by IPA analysis
>and additionally the (fully unreachable) builtin_trap is believed to load
>all global memory.
>
>I think we should think of less intrusive gimple representation of this, but
>it is also easy enough to special case that in IPA analysers as done in
>this patch. This is a win even if we improve the representation since
>gimple-ssa-isolate-paths is run late and this way we improve optimization
>early.
>
>This affects 1623 functions during cc1plus link.
>Current cc1plus disambiguation stats are:
>
>Alias oracle query stats:
>
> refs_may_alias_p: 76712244 disambiguations, 90400285 queries
>
> ref_maybe_used_by_call_p: 581770 disambiguations, 77716066 queries
>
> call_may_clobber_ref_p: 322533 disambiguations, 325280 queries
>
> stmt_kills_ref_p: 106434 kills, 5728738 queries
>
> nonoverlapping_component_refs_p: 0 disambiguations, 8849 queries
>
> nonoverlapping_refs_since_match_p: 29947 disambiguations, 66277 must
> overlaps, 97248 queries
> aliasing_component_refs_p: 55737 disambiguations, 3055719 queries
>
> TBAA oracle: 27336487 disambiguations 67140201 queries
>
> 16214932 are in alias set 0
>
> 9713534 queries asked about the same object
>
> 92 queries asked about the same alias set
>
> 0 access volatile
>
> 12049850 are dependent in the DAG
>
> 1825306 are aritificially in conflict with void *
>
>
>
>Modref stats:
>
> modref kill: 70 kills, 8279 queries
>
> modref use: 29557 disambiguations, 701616 queries
>
> modref clobber: 1612655 disambiguations, 21688020 queries
>
> 4925889 tbaa queries (0.227125 per modref query)
>
> 864389 base compares (0.039856 per modref query)
>
>
>
>PTA query stats:
>
> pt_solution_includes: 13512509 disambiguations, 27571176 queries
>
> pt_solutions_intersect: 1594296 disambiguations, 15943975 queries
>
>
>
>Bootstrapped/regtested x86_64-linux, comitted.
>
>gcc/ChangeLog:
>
>2021-12-12 Jan Hubicka <hubi...@ucw.cz>
>
> PR ipa/103665
> * ipa-modref.c (modref_access_analysis::analyze): Terminate BB
> analysis on NULL memory access.
> * ipa-pure-const.c (analyze_function): Likewise.
>
>diff --git a/gcc/ipa-modref.c b/gcc/ipa-modref.c
>index 55fa873e1f0..2c89c63baf6 100644
>--- a/gcc/ipa-modref.c
>+++ b/gcc/ipa-modref.c
>@@ -1750,6 +1750,19 @@ modref_access_analysis::analyze ()
> for (si = gsi_start_nondebug_after_labels_bb (bb);
> !gsi_end_p (si); gsi_next_nondebug (&si))
> {
>+ /* NULL memory accesses terminates BB. These accesses are known
>+ to trip undefined behaviour. gimple-ssa-isolate-paths turns them
>+ to volatile accesses and adds builtin_trap call which would
>+ confuse us otherwise. */
>+ if (infer_nonnull_range_by_dereference (gsi_stmt (si),
>+ null_pointer_node))
Does that correctly handle flag_delete_null_pointer_checks and address spaces
with null?
>+ {
>+ if (dump_file)
>+ fprintf (dump_file, " - NULL memory access; terminating BB\n");
>+ if (flag_non_call_exceptions)
>+ set_side_effects ();
>+ break;
>+ }
> analyze_stmt (gsi_stmt (si), always_executed);
>
> /* Avoid doing useles work. */
>diff --git a/gcc/ipa-pure-const.c b/gcc/ipa-pure-const.c
>index fea8b08c4eb..25503f360e6 100644
>--- a/gcc/ipa-pure-const.c
>+++ b/gcc/ipa-pure-const.c
>@@ -1097,6 +1097,28 @@ analyze_function (struct cgraph_node *fn, bool ipa)
> !gsi_end_p (gsi);
> gsi_next (&gsi))
> {
>+ /* NULL memory accesses terminates BB. These accesses are known
>+ to trip undefined behaviour. gimple-ssa-isolate-paths turns them
>+ to volatile accesses and adds builtin_trap call which would
>+ confuse us otherwise. */
>+ if (infer_nonnull_range_by_dereference (gsi_stmt (gsi),
>+ null_pointer_node))
>+ {
>+ if (dump_file)
>+ fprintf (dump_file, " NULL memory access; terminating BB%s\n",
>+ flag_non_call_exceptions ? "; looping" : "");
>+ if (flag_non_call_exceptions)
>+ {
>+ l->looping = true;
>+ if (stmt_can_throw_external (cfun, gsi_stmt (gsi)))
>+ {
>+ if (dump_file)
>+ fprintf (dump_file, " can throw externally\n");
>+ l->can_throw = true;
>+ }
>+ }
>+ break;
>+ }
> check_stmt (&gsi, l, ipa);
> if (l->pure_const_state == IPA_NEITHER
> && l->looping
