[Bug tree-optimization/60656] [4.8/4.9 regression] x86 vectorization produces wrong code

[congh at google dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60656

--- Comment #4 from Cong Hou <congh at google dot com> ---
Yes, there is a quick fix: we can check if the def with vect_used_by_reduction
is immediately used by a reduction stmt. After all, it seems that
supportable_widening_operation() is the only place that takes advantage of this
"the element order doesn't matter" feature.


diff --git a/gcc/tree-vect-stmts.c b/gcc/tree-vect-stmts.c
index 70fb411..7442d0c 100644
--- a/gcc/tree-vect-stmts.c
+++ b/gcc/tree-vect-stmts.c
@@ -7827,7 +7827,16 @@ supportable_widening_operation (enum tree_code code,
gimple stmt,
                                             stmt, vectype_out, vectype_in,
                                             code1, code2, multi_step_cvt,
                                             interm_types))
-       return true;
+        {
+          tree lhs = gimple_assign_lhs (stmt);
+          use_operand_p dummy;
+          gimple use_stmt;
+          stmt_vec_info use_stmt_info = NULL;
+          if (single_imm_use (lhs, &dummy, &use_stmt)
+              && (use_stmt_info = vinfo_for_stmt (use_stmt))
+              && STMT_VINFO_DEF_TYPE (use_stmt_info) == vect_reduction_def)
+            return true;
+        }
       c1 = VEC_WIDEN_MULT_LO_EXPR;
       c2 = VEC_WIDEN_MULT_HI_EXPR;
       break;