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, 2 insertions, 4 deletions

diff --git a/gcc/cp/cp-gimplify.c b/gcc/cp/cp-gimplify.c
index 3999695..f326d71 100644
--- a/gcc/cp/cp-gimplify.c
+++ b/gcc/cp/cp-gimplify.c
@@ -226,8 +226,7 @@ genericize_if_stmt (tree *stmt_p)
     stmt = else_;
   else
     stmt = build3 (COND_EXPR, void_type_node, cond, then_, else_);
-  if (!EXPR_HAS_LOCATION (stmt))
-    protected_set_expr_location (stmt, locus);
+  protected_set_expr_location_if_unset (stmt, locus);
   *stmt_p = stmt;
 }
 
@@ -248,8 +247,7 @@ genericize_cp_loop (tree *stmt_p, location_t start_locus, tree cond, tree body,
   tree stmt_list = NULL;
   tree debug_begin = NULL;
 
-  if (EXPR_LOCATION (incr) == UNKNOWN_LOCATION)
-    protected_set_expr_location (incr, start_locus);
+  protected_set_expr_location_if_unset (incr, start_locus);
 
   cp_walk_tree (&cond, cp_genericize_r, data, NULL);
   cp_walk_tree (&incr, cp_genericize_r, data, NULL);