[gcc(refs/vendors/redhat/heads/gcc-8-branch)] if-conv: Delete dead stmts backwards in ifcvt_local_dce [PR94283]

[gcc(refs/vendors/redhat/heads/gcc-8-branch)] if-conv: Delete dead stmts backwards in ifcvt_local_dce [PR94283]

[Jakub Jelinek]

https://gcc.gnu.org/g:e0f892394dbd59b411ba72113166e2396a77f563

commit e0f892394dbd59b411ba72113166e2396a77f563
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Wed Mar 25 08:08:04 2020 +0100

    if-conv: Delete dead stmts backwards in ifcvt_local_dce [PR94283]
    
    > > This patch caused:
    > >
    > > gcc /home/marxin/Programming/gcc/gcc/testsuite/gcc.c-torture/compile/990625-2.c -O3 -g -fno-tree-dce -c
    > > during GIMPLE pass: ifcvt
    > > /home/marxin/Programming/gcc/gcc/testsuite/gcc.c-torture/compile/990625-2.c: In function ‘broken030599’:
    > > /home/marxin/Programming/gcc/gcc/testsuite/gcc.c-torture/compile/990625-2.c:2:1: internal compiler error: Segmentation fault
    >
    > Likely
    >
    >   /* Delete dead statements.  */
    >   gsi = gsi_start_bb (bb);
    >   while (!gsi_end_p (gsi))
    >     {
    >
    > needs to instead work back-to-front for debug stmt adjustment to work
    
    Indeed, that seems to work.
    
    2020-03-25  Richard Biener  <rguenther@suse.de>
                Jakub Jelinek  <jakub@redhat.com>
    
            PR debug/94283
            * tree-if-conv.c (ifcvt_local_dce): Delete dead statements backwards.
    
            * gcc.dg/pr94283.c: New test.
    
    Co-authored-by: Richard Biener <rguenther@suse.de>
    (cherry picked from commit 8ea7970c4968517fb73f42bcca40d316adacf215)

Diff:
---
 gcc/testsuite/gcc.dg/pr94283.c | 16 ++++++++++++++++
 gcc/tree-if-conv.c             |  7 +++++--
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/gcc/testsuite/gcc.dg/pr94283.c b/gcc/testsuite/gcc.dg/pr94283.c
new file mode 100644
index 00000000000..ac162d6ac7b
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr94283.c
@@ -0,0 +1,16 @@
+/* PR debug/94283 */
+/* { dg-do compile } */
+/* { dg-options "-O3 -fno-tree-dce -fcompare-debug" } */
+
+void
+foo (int *n)
+{
+  for (int i = 0; i < 32; i++)
+    {
+      int x = 0;
+      x++;
+      if (i & 4)
+	x++;
+      x++;
+    }
+}
diff --git a/gcc/tree-if-conv.c b/gcc/tree-if-conv.c
index dd9bf469730..db38b1328ff 100644
--- a/gcc/tree-if-conv.c
+++ b/gcc/tree-if-conv.c
@@ -2811,13 +2811,15 @@ ifcvt_local_dce (basic_block bb)
 	}
     }
   /* Delete dead statements.  */
-  gsi = gsi_start_bb (bb);
+  gsi = gsi_last_bb (bb);
   while (!gsi_end_p (gsi))
     {
+      gimple_stmt_iterator gsiprev = gsi;
+      gsi_prev (&gsiprev);
       stmt = gsi_stmt (gsi);
       if (gimple_plf (stmt, GF_PLF_2))
 	{
-	  gsi_next (&gsi);
+	  gsi = gsiprev;
 	  continue;
 	}
       if (dump_file && (dump_flags & TDF_DETAILS))
@@ -2827,6 +2829,7 @@ ifcvt_local_dce (basic_block bb)
 	}
       gsi_remove (&gsi, true);
       release_defs (stmt);
+      gsi = gsiprev;
     }
 }