poly_int: vect_get_constant_vectors

For now, vect_get_constant_vectors can only cope with constant-length
vectors, although a patch after the main SVE submission relaxes this.
This patch adds an appropriate guard for variable-length vectors.
The TYPE_VECTOR_SUBPARTS use in vect_get_constant_vectors will then
have a to_constant call when TYPE_VECTOR_SUBPARTS becomes a poly_int.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree-vect-slp.c (vect_get_and_check_slp_defs): Reject
	constant and extern definitions for variable-length vectors.
	(vect_get_constant_vectors): Note that the number of units
	is known to be constant.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r256140

gcc/ChangeLog

@@ -2,6 +2,15 @@
 	    Alan Hayward  <alan.hayward@arm.com>
 	    David Sherwood  <david.sherwood@arm.com>
 
+	* tree-vect-slp.c (vect_get_and_check_slp_defs): Reject
+	constant and extern definitions for variable-length vectors.
+	(vect_get_constant_vectors): Note that the number of units
+	is known to be constant.
+
+2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>
+	    Alan Hayward  <alan.hayward@arm.com>
+	    David Sherwood  <david.sherwood@arm.com>
+
 	* tree-vect-stmts.c (vectorizable_conversion): Treat the number
 	of units as polynomial.  Choose between WIDE and NARROW based
 	on multiple_p.

gcc/tree-vect-slp.c

@@ -405,6 +405,20 @@ again:
 	{
 	case vect_constant_def:
 	case vect_external_def:
+	  /* We must already have set a vector size by now.  */
+	  gcc_checking_assert (maybe_ne (current_vector_size, 0U));
+	  if (!current_vector_size.is_constant ())
+	    {
+	      if (dump_enabled_p ())
+		{
+		  dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
+				   "Build SLP failed: invalid type of def "
+				   "for variable-length SLP ");
+		  dump_generic_expr (MSG_MISSED_OPTIMIZATION, TDF_SLIM, oprnd);
+		  dump_printf (MSG_MISSED_OPTIMIZATION, "\n");
+		}
+	      return -1;
+	    }
 	  break;
 
 	case vect_reduction_def:
@@ -3233,6 +3247,7 @@ vect_get_constant_vectors (tree op, slp_tree slp_node,
       = build_same_sized_truth_vector_type (STMT_VINFO_VECTYPE (stmt_vinfo));
   else
     vector_type = get_vectype_for_scalar_type (TREE_TYPE (op));
+  /* Enforced by vect_get_and_check_slp_defs.  */
   nunits = TYPE_VECTOR_SUBPARTS (vector_type);
 
   if (STMT_VINFO_DATA_REF (stmt_vinfo))