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.

gcc/ChangeLog

@@ -1,3 +1,12 @@
+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.
+
 2020-04-03  Jeff Law  <law@redhat.com>
 
 	PR rtl-optimization/92264

gcc/cp/ChangeLog

@@ -1,5 +1,10 @@
 2020-04-04  Jakub Jelinek  <jakub@redhat.com>
 
+	PR debug/94441
+	* parser.c (cp_parser_omp_for_loop): Use
+	protected_set_expr_location_if_unset.
+	* cp-gimplify.c (genericize_if_stmt, genericize_cp_loop): Likewise.
+
 	PR c++/94477
 	* pt.c (tsubst_expr) <case OMP_MASTER>: Clear
 	omp_parallel_combined_clauses.

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);

gcc/cp/parser.c

@@ -39149,8 +39149,7 @@ cp_parser_omp_for_loop (cp_parser *parser, enum tree_code code, tree clauses,
 	    incr = cp_parser_omp_for_incr (parser, real_decl);
 	  else
 	    incr = cp_parser_expression (parser);
-	  if (!EXPR_HAS_LOCATION (incr))
-	    protected_set_expr_location (incr, input_location);
+	  protected_set_expr_location_if_unset (incr, input_location);
 	}
 
     parse_close_paren:

gcc/testsuite/ChangeLog

@@ -1,5 +1,8 @@
 2020-04-04  Jakub Jelinek  <jakub@redhat.com>
 
+	PR debug/94441
+	* g++.dg/opt/pr94441.C: New test.
+
 	PR c++/94477
 	* g++.dg/gomp/pr94477.C: New test.
 

gcc/testsuite/g++.dg/opt/pr94441.C

@@ -0,0 +1,16 @@
+// PR debug/94441
+// { dg-do compile }
+// { dg-options "-O3 -fno-forward-propagate --param=max-cse-insns=0 -flive-range-shrinkage -std=c++17 -fcompare-debug" }
+
+template <class,class> struct Same;
+template <class T> struct Same<T,T> {};
+
+auto f()
+{
+  if constexpr (sizeof(int)==3)
+    return 42;
+  else
+    return 42L;
+}
+
+Same<decltype(f()), long> s;

gcc/tree-iterator.c

@@ -354,4 +354,45 @@ expr_last (tree expr)
   return expr;
 }
 
+/* If EXPR is a STATEMENT_LIST containing just DEBUG_BEGIN_STMTs and
+   a single other stmt, return that other stmt (recursively).
+   If it is a STATEMENT_LIST containing no non-DEBUG_BEGIN_STMTs or
+   multiple, return NULL_TREE.
+   Otherwise return EXPR.  */
+
+tree
+expr_single (tree expr)
+{
+  if (expr == NULL_TREE)
+    return expr;
+
+  if (TREE_CODE (expr) == 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 (expr);
+      if (!n)
+	return NULL_TREE;
+      while (TREE_CODE (n->stmt) == DEBUG_BEGIN_STMT)
+	{
+	  n = n->next;
+	  if (!n)
+	    return NULL_TREE;
+	}
+      expr = n->stmt;
+      do
+	{
+	  n = n->next;
+	  if (!n)
+	    return expr_single (expr);
+	}
+      while (TREE_CODE (n->stmt) == DEBUG_BEGIN_STMT);
+      return NULL_TREE;
+    }
+
+  return expr;
+}
+
 #include "gt-tree-iterator.h"

gcc/tree-iterator.h

@@ -119,5 +119,6 @@ extern void append_to_statement_list (tree, tree *);
 extern void append_to_statement_list_force (tree, tree *);
 extern tree expr_first (tree);
 extern tree expr_last (tree);
+extern tree expr_single (tree);
 
 #endif /* GCC_TREE_ITERATOR_H  */

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

gcc/tree.h

@@ -1203,6 +1203,7 @@ get_expr_source_range (tree expr)
 }
 
 extern void protected_set_expr_location (tree, location_t);
+extern void protected_set_expr_location_if_unset (tree, location_t);
 
 extern tree maybe_wrap_with_location (tree, location_t);