On Sun, Dec 12, 2021 at 4:34 AM Richard Biener via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> 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?
Yes.
infer_nonnull_range_by_dereference has the following check:
/* We can only assume that a pointer dereference will yield
non-NULL if -fdelete-null-pointer-checks is enabled. */
if (!flag_delete_null_pointer_checks
|| !POINTER_TYPE_P (TREE_TYPE (op))
|| gimple_code (stmt) == GIMPLE_ASM
|| gimple_clobber_p (stmt))
return false;
And then calls walk_stmt_load_store_ops with check_loadstore as the
callback function which does:
/* Some address spaces may legitimately dereference zero. */
addr_space_t as = TYPE_ADDR_SPACE (TREE_TYPE (op));
if (targetm.addr_space.zero_address_valid (as))
return false;
return operand_equal_p (TREE_OPERAND (op, 0), (tree)data, 0);
Thanks,
Andrew Pinski
>
> >+ {
> >+ 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
>
