[gcc r10-10671] store-merging: Fix up a -fcompare-debug bug in get_status_for_store_merging [PR104263]

[Jakub Jelinek <jakub@gcc.gnu.org>]

https://gcc.gnu.org/g:7bb29feb789dfa05297998e1c241d438307cc3d2

commit r10-10671-g7bb29feb789dfa05297998e1c241d438307cc3d2
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Jan 28 19:02:26 2022 +0100

    store-merging: Fix up a -fcompare-debug bug in get_status_for_store_merging [PR104263]
    
    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.
    
    2022-01-28  Jakub Jelinek  <jakub@redhat.com>
    
            PR tree-optimization/104263
            * gimple-ssa-store-merging.c (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.
    
    (cherry picked from commit a591c71b41e18e4ff86852a974592af4962aef57)

Diff:
---
 gcc/gimple-ssa-store-merging.c  |  5 ++++-
 gcc/testsuite/gcc.dg/pr104263.c | 25 +++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/gcc/gimple-ssa-store-merging.c b/gcc/gimple-ssa-store-merging.c
index 0623e6ec80b..4093ccaaafc 100644
--- a/gcc/gimple-ssa-store-merging.c
+++ b/gcc/gimple-ssa-store-merging.c
@@ -4963,6 +4963,7 @@ get_status_for_store_merging (basic_block bb)
   unsigned int num_statements = 0;
   gimple_stmt_iterator gsi;
   edge e;
+  gimple *last_stmt = NULL;
 
   for (gsi = gsi_after_labels (bb); !gsi_end_p (gsi); gsi_next (&gsi))
     {
@@ -4971,6 +4972,8 @@ get_status_for_store_merging (basic_block bb)
       if (is_gimple_debug (stmt))
 	continue;
 
+      last_stmt = stmt;
+
       if (store_valid_for_store_merging_p (stmt) && ++num_statements >= 2)
 	break;
     }
@@ -4979,7 +4982,7 @@ get_status_for_store_merging (basic_block bb)
     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;
diff --git a/gcc/testsuite/gcc.dg/pr104263.c b/gcc/testsuite/gcc.dg/pr104263.c
new file mode 100644
index 00000000000..79b548c629c
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr104263.c
@@ -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;
+}