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>

gcc/ChangeLog

+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.
+
 2020-03-24  Christophe Lyon  <christophe.lyon@linaro.org>
 
 	* doc/sourcebuild.texi (ARM-specific attributes): Add

gcc/testsuite/ChangeLog

+2020-03-25  Jakub Jelinek  <jakub@redhat.com>
+
+	PR debug/94283
+	* gcc.dg/pr94283.c: New test.
+
 2020-03-24  Sandra Loosemore  <sandra@codesourcery.com>
 
 	* gcc.dg/analyzer/sigsetjmp-5.c: Require sigsetjmp support.

gcc/testsuite/gcc.dg/pr94283.c

+/* 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++;
+    }
+}

gcc/tree-if-conv.c

@@ -2973,9 +2973,11 @@ ifcvt_local_dce (class loop *loop)
 	}
     }
   /* 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_store_p (stmt))
 	{
@@ -2986,14 +2988,13 @@ ifcvt_local_dce (class loop *loop)
 	  if (dse_classify_store (&write, stmt, false, NULL, NULL, latch_vdef)
 	      == DSE_STORE_DEAD)
 	    delete_dead_or_redundant_assignment (&gsi, "dead");
-	  else
-	    gsi_next (&gsi);
+	  gsi = gsiprev;
 	  continue;
 	}
 
       if (gimple_plf (stmt, GF_PLF_2))
 	{
-	  gsi_next (&gsi);
+	  gsi = gsiprev;
 	  continue;
 	}
       if (dump_file && (dump_flags & TDF_DETAILS))
@@ -3003,6 +3004,7 @@ ifcvt_local_dce (class loop *loop)
 	}
       gsi_remove (&gsi, true);
       release_defs (stmt);
+      gsi = gsiprev;
     }
 }