rs6000.c (rs6000_builtin_vectorization_cost): Correct costs for vec_construct.

2016-08-12  Bill Schmidt  <wschmidt@linux.vnet.ibm.com>

	* config/rs6000/rs6000.c (rs6000_builtin_vectorization_cost):
	Correct costs for vec_construct.

From-SVN: r239417

gcc/ChangeLog

+2016-08-12  Bill Schmidt  <wschmidt@linux.vnet.ibm.com>
+
+	* config/rs6000/rs6000.c (rs6000_builtin_vectorization_cost):
+	Correct costs for vec_construct.
+
 2016-08-12  Bin Cheng  <bin.cheng@arm.com>
 
 	PR tree-optimization/69848

gcc/config/rs6000/rs6000.c

@@ -5266,16 +5266,20 @@ rs6000_builtin_vectorization_cost (enum vect_cost_for_stmt type_of_cost,
         return 2;
 
       case vec_construct:
-	elements = TYPE_VECTOR_SUBPARTS (vectype);
+	/* This is a rough approximation assuming non-constant elements
+	   constructed into a vector via element insertion.  FIXME:
+	   vec_construct is not granular enough for uniformly good
+	   decisions.  If the initialization is a splat, this is
+	   cheaper than we estimate.  Improve this someday.  */
 	elem_type = TREE_TYPE (vectype);
 	/* 32-bit vectors loaded into registers are stored as double
-	   precision, so we need n/2 converts in addition to the usual
-	   n/2 merges to construct a vector of short floats from them.  */
+	   precision, so we need 2 permutes, 2 converts, and 1 merge
+	   to construct a vector of short floats from them.  */
 	if (SCALAR_FLOAT_TYPE_P (elem_type)
 	    && TYPE_PRECISION (elem_type) == 32)
-	  return elements + 1;
+	  return 5;
 	else
-	  return elements / 2 + 1;
+	  return max (2, TYPE_VECTOR_SUBPARTS (vectype) - 1);
 
       default:
         gcc_unreachable ();