Improve vector_vector_composition_type

We sometimes get to ask to decompose, say V2DFmode into two halves.
Currently this results in composing it from two DImode pieces
instead of the obvious two DFmode pieces.  The following adjusts
vector_vector_composition_type for this trivial case and avoids
a VIEW_CONVERT_EXPR in the initial code generation.

	* tree-vect-stmts.cc (vector_vector_composition_type):
	Handle composition of a vector from a number of elements that
	happens to match its number of lanes.

diff --git a/gcc/tree-vect-stmts.cc b/gcc/tree-vect-stmts.cc
index ae24f3e66e63d9bd9763284a47fb2c911335c4c1..9e046ced7c6e3477741f9b0f555f9b4ca4e949e9 100644 (file)
--- a/gcc/tree-vect-stmts.cc
+++ b/gcc/tree-vect-stmts.cc
@@ -2143,6 +2143,14 @@ vector_vector_composition_type (tree vtype, poly_uint64 nelts, tree *ptype)
   if (!VECTOR_MODE_P (vmode))
     return NULL_TREE;
 
+  /* When we are asked to compose the vector from its components let
+     that happen directly.  */
+  if (known_eq (TYPE_VECTOR_SUBPARTS (vtype), nelts))
+    {
+      *ptype = TREE_TYPE (vtype);
+      return vtype;
+    }
+
   poly_uint64 vbsize = GET_MODE_BITSIZE (vmode);
   unsigned int pbsize;
   if (constant_multiple_p (vbsize, nelts, &pbsize))