c++: Fix further protected_set_expr_location related -fcompare-debug issues [PR94441]

My recent protected_set_expr_location changes work well when
that function is called unconditionally, but as the testcase shows, the C++
FE has a few spots that do:
  if (!EXPR_HAS_LOCATION (stmt))
    protected_set_expr_location (stmt, locus);
or similar.  Now, if we have for -g0 stmt of some expression that can
have location and has != UNKNOWN_LOCATION, while -g instead has
a STATEMENT_LIST containing some DEBUG_BEGIN_STMTs + that expression with
that location, we don't call protected_set_expr_location in the -g0 case,
but do call it in the -g case, because on the STATEMENT_LIST
!EXPR_HAS_LOCATION.
The following patch introduces a helper function which digs up the single
expression of a STATEMENT_LIST and uses that expression in the
EXPR_HAS_LOCATION check (plus changes protected_set_expr_location to
also use that helper).

Or do we want a further wrapper, perhaps C++ FE only, that would do this
protected_set_expr_location_if_unset (stmt, locus)?

2020-04-04  Jakub Jelinek  <jakub@redhat.com>

	PR debug/94441
	* tree-iterator.h (expr_single): Declare.
	* tree-iterator.c (expr_single): New function.
	* tree.h (protected_set_expr_location_if_unset): Declare.
	* tree.c (protected_set_expr_location): Use expr_single.
	(protected_set_expr_location_if_unset): New function.

	* parser.c (cp_parser_omp_for_loop): Use
	protected_set_expr_location_if_unset.
	* cp-gimplify.c (genericize_if_stmt, genericize_cp_loop): Likewise.

	* g++.dg/opt/pr94441.C: New test.

1 files changed, 14 insertions, 24 deletions

diff --git a/gcc/tree.c b/gcc/tree.c
index 0fe3afc..fa956da 100644
--- a/gcc/tree.c
+++ b/gcc/tree.c
@@ -5148,33 +5148,23 @@ protected_set_expr_location (tree t, location_t loc)
     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);
+      t = expr_single (t);
+      if (t && CAN_HAVE_LOCATION_P (t))
+	SET_EXPR_LOCATION (t, loc);
     }
 }
 
+/* Like PROTECTED_SET_EXPR_LOCATION, but only do that if T has
+   UNKNOWN_LOCATION.  */
+
+void
+protected_set_expr_location_if_unset (tree t, location_t loc)
+{
+  t = expr_single (t);
+  if (t && !EXPR_HAS_LOCATION (t))
+    protected_set_expr_location (t, loc);
+}
+
 /* Data used when collecting DECLs and TYPEs for language data removal.  */
 
 class free_lang_data_d