[PATCH] store-merging: Fix up a -fcompare-debug bug in get_status_for_store_merging [PR104263]

[Jakub Jelinek via Gcc-patches]

Hi!

As mentioned in the PRthe following testcase fails, because the last
stmt of a bb with -g is a debug stmt and get_status_for_store_merging
uses gimple_seq_last_stmt (bb_seq (bb)) when testing if it is valid
for store merging.  The debug stmt isn't valid, while a stmt at that
position with -g0 is valid and so the divergence.

As we walk the whole bb already, this patch just remembers the last
non-debug stmt, so that we don't need to skip backwards debug stmts at the
end of the bb to find last real stmt.

Bootstrapped/regtested on powerpc64le-linux, ok for trunk?

2022-01-28  Jakub Jelinek  <ja...@redhat.com>

        PR tree-optimization/104263
        * gimple-ssa-store-merging.cc (get_status_for_store_merging): For
        cfun->can_throw_non_call_exceptions && cfun->eh test whether
        last non-debug stmt in the bb is store_valid_for_store_merging_p
        rather than last stmt.

        * gcc.dg/pr104263.c: New test.

--- gcc/gimple-ssa-store-merging.cc.jj  2022-01-20 11:30:45.521578942 +0100
+++ gcc/gimple-ssa-store-merging.cc     2022-01-28 11:27:25.437947561 +0100
@@ -5364,6 +5364,7 @@ get_status_for_store_merging (basic_bloc
   unsigned int num_constructors = 0;
   gimple_stmt_iterator gsi;
   edge e;
+  gimple *last_stmt = NULL;
 
   for (gsi = gsi_after_labels (bb); !gsi_end_p (gsi); gsi_next (&gsi))
     {
@@ -5372,6 +5373,8 @@ get_status_for_store_merging (basic_bloc
       if (is_gimple_debug (stmt))
        continue;
 
+      last_stmt = stmt;
+
       if (store_valid_for_store_merging_p (stmt) && ++num_statements >= 2)
        break;
 
@@ -5398,7 +5401,7 @@ get_status_for_store_merging (basic_bloc
     return BB_INVALID;
 
   if (cfun->can_throw_non_call_exceptions && cfun->eh
-      && store_valid_for_store_merging_p (gimple_seq_last_stmt (bb_seq (bb)))
+      && store_valid_for_store_merging_p (last_stmt)
       && (e = find_fallthru_edge (bb->succs))
       && e->dest == bb->next_bb)
     return BB_EXTENDED_VALID;
--- gcc/testsuite/gcc.dg/pr104263.c.jj  2022-01-28 11:32:26.718619588 +0100
+++ gcc/testsuite/gcc.dg/pr104263.c     2022-01-28 11:32:04.111944459 +0100
@@ -0,0 +1,25 @@
+/* PR tree-optimization/104263 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fcompare-debug -fnon-call-exceptions 
-fno-inline-small-functions" } */
+
+int n;
+
+int
+bar (void)
+{
+  int a;
+
+  n = 0;
+  a = 0;
+
+  return n;
+}
+
+__attribute__ ((pure, returns_twice)) int
+foo (void)
+{
+  n = bar () + 1;
+  foo ();
+
+  return 0;
+}

        Jakub