[gcc r14-2038] Improve vector_vector_composition_type

[Richard Biener <rguenth@gcc.gnu.org>]

https://gcc.gnu.org/g:1742ea94aaa758a2b0c40360f89e27770bd8ffeb

commit r14-2038-g1742ea94aaa758a2b0c40360f89e27770bd8ffeb
Author: Richard Biener <rguenther@suse.de>
Date:   Thu Jun 22 15:14:51 2023 +0200

    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:
---
 gcc/tree-vect-stmts.cc | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/gcc/tree-vect-stmts.cc b/gcc/tree-vect-stmts.cc
index ae24f3e66e6..9e046ced7c6 100644
--- 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))