https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70138
--- Comment #12 from Bernd Schmidt <bernds at gcc dot gnu.org> ---
I arrived at the same conclusion, and I was testing the following:
Index: gcc/tree-vect-loop-manip.c
===================================================================
--- gcc/tree-vect-loop-manip.c (revision 234025)
+++ gcc/tree-vect-loop-manip.c (working copy)
@@ -1692,7 +1692,8 @@ vect_update_ivs_after_vectorizer (loop_v
/* Skip reduction phis. */
stmt_info = vinfo_for_stmt (phi);
- if (STMT_VINFO_DEF_TYPE (stmt_info) == vect_reduction_def)
+ if (STMT_VINFO_DEF_TYPE (stmt_info) == vect_reduction_def
+ || STMT_VINFO_DEF_TYPE (stmt_info) == vect_double_reduction_def)
{
if (dump_enabled_p ())
dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
