Add missing target check for fully-masked fold-left reductions

The fold-left reduction code has a (rarely-used) fallback that handles
cases in which the loop is fully-masked and the target has no native
support for the reduction.  The fallback includea a VEC_COND_EXPR
between the reduction vector and a safe value, so we should check
whether that VEC_COND_EXPR is supported.

2019-12-27  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-loop.c (vectorizable_reduction): Check whether the
	target supports the required VEC_COND_EXPR operation before
	allowing the fallback handling of masked fold-left reductions.

gcc/testsuite/
	* gcc.target/aarch64/sve/mixed_size_10.c: New test.

From-SVN: r279742

gcc/ChangeLog

+2019-12-27  Richard Sandiford  <richard.sandiford@arm.com>
+
+	* tree-vect-loop.c (vectorizable_reduction): Check whether the
+	target supports the required VEC_COND_EXPR operation before
+	allowing the fallback handling of masked fold-left reductions.
+
 2019-12-24  Jiufu Guo  <guojiufu@linux.ibm.com>
 
 	* config/rs6000/rs6000.c (rs6000_option_override_internal): Enable

gcc/testsuite/ChangeLog

+2019-12-27  Richard Sandiford  <richard.sandiford@arm.com>
+
+	* gcc.target/aarch64/sve/mixed_size_10.c: New test.
+
 2019-12-26  Jakub Jelinek  <jakub@redhat.com>
 
 	PR c++/92438

gcc/testsuite/gcc.target/aarch64/sve/mixed_size_10.c

+/* { dg-options "-O3 -msve-vector-bits=256 -fno-tree-loop-distribution" } */
+
+float
+f (float *restrict x, double *restrict y)
+{
+  float res = 0.0;
+  for (int i = 0; i < 100; ++i)
+    {
+      res += x[i];
+      y[i] += y[i - 4] * 11;
+    }
+  return res;
+}

gcc/tree-vect-loop.c

@@ -6718,6 +6718,18 @@ vectorizable_reduction (stmt_vec_info stmt_info, slp_tree slp_node,
 			     " conditional operation is available.\n");
 	  LOOP_VINFO_CAN_FULLY_MASK_P (loop_vinfo) = false;
 	}
+      else if (reduction_type == FOLD_LEFT_REDUCTION
+	       && reduc_fn == IFN_LAST
+	       && !expand_vec_cond_expr_p (vectype_in,
+					   truth_type_for (vectype_in),
+					   SSA_NAME))
+	{
+	  if (dump_enabled_p ())
+	    dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
+			     "can't use a fully-masked loop because no"
+			     " conditional operation is available.\n");
+	  LOOP_VINFO_CAN_FULLY_MASK_P (loop_vinfo) = false;
+	}
       else
 	vect_record_loop_mask (loop_vinfo, masks, ncopies * vec_num,
 			       vectype_in, NULL);