tree: Fix -fcompare-debug issues due to protected_set_expr_location [PR94323]

The following testcase FAILs since recently when the C++ FE started calling
protected_set_expr_location more often.
With -g, it is called on a STATEMENT_LIST that contains a DEBUG_BEGIN_STMT
and CLEANUP_POINT_EXPR, and as STATEMENT_LISTs have !CAN_HAVE_LOCATION_P,
nothing is set.  Without -g, it is called instead on the CLEANUP_POINT_EXPR
directly and changes its location.

The following patch recurses on the single non-DEBUG_BEGIN_STMT statement
of a STATEMENT_LIST if any to make the two behave the same.

2020-03-26  Jakub Jelinek  <jakub@redhat.com>

	PR debug/94323
	* tree.c (protected_set_expr_location): Recurse on STATEMENT_LIST
	that contains exactly one non-DEBUG_BEGIN_STMT statement.

	* g++.dg/debug/pr94323.C: New test.

gcc/ChangeLog

 2020-03-26  Jakub Jelinek  <jakub@redhat.com>
 
+	PR debug/94323
+	* tree.c (protected_set_expr_location): Recurse on STATEMENT_LIST
+	that contains exactly one non-DEBUG_BEGIN_STMT statement.
+
 	PR debug/94281
 	* gimple.h (gimple_seq_first_nondebug_stmt): New function.
 	(gimple_seq_last_nondebug_stmt): Don't return NULL if seq contains

gcc/testsuite/ChangeLog

+2020-03-26  Jakub Jelinek  <jakub@redhat.com>
+
+	PR debug/94323
+	* g++.dg/debug/pr94323.C: New test.
+
 2020-03-26  Martin Liska  <mliska@suse.cz>
 
 	PR testsuite/94334

gcc/testsuite/g++.dg/debug/pr94323.C

+// PR debug/94323
+// { dg-do compile }
+// { dg-options "-O2 -fcompare-debug" }
+
+volatile int a;
+
+void
+foo ()
+{
+  ({
+     a;
+   });
+}

gcc/tree.c

@@ -5146,6 +5146,33 @@ protected_set_expr_location (tree t, location_t loc)
 {
   if (CAN_HAVE_LOCATION_P (t))
     SET_EXPR_LOCATION (t, loc);
+  else if (t && TREE_CODE (t) == STATEMENT_LIST)
+    {
+      /* With -gstatement-frontiers we could have a STATEMENT_LIST with
+	 DEBUG_BEGIN_STMT(s) and only a single other stmt, which with
+	 -g wouldn't be present and we'd have that single other stmt
+	 directly instead.  */
+      struct tree_statement_list_node *n = STATEMENT_LIST_HEAD (t);
+      if (!n)
+	return;
+      while (TREE_CODE (n->stmt) == DEBUG_BEGIN_STMT)
+	{
+	  n = n->next;
+	  if (!n)
+	    return;
+	}
+      tree t2 = n->stmt;
+      do
+	{
+	  n = n->next;
+	  if (!n)
+	    {
+	      protected_set_expr_location (t2, loc);
+	      return;
+	    }
+	}
+      while (TREE_CODE (n->stmt) == DEBUG_BEGIN_STMT);
+    }
 }
 
 /* Data used when collecting DECLs and TYPEs for language data removal.  */