[gcc(refs/users/rguenth/heads/vect-force-slp)] Avoid SLP build failure for unsupported shifts

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

https://gcc.gnu.org/g:bcddead27931294a909686a38daf1df661845366

commit bcddead27931294a909686a38daf1df661845366
Author: Richard Biener <rguenther@suse.de>
Date:   Thu Oct 5 14:07:02 2023 +0200

    Avoid SLP build failure for unsupported shifts
    
    When asserting that SLP _build_ can succeed we run into the SLP
    shift discovery code trying to be clever doing vectorizable_shifts
    work and failing discovery already.  That gives a false impression
    for now, so disable that when we do single-lane builds.
    
            * tree-vect-slp.cc (vect_build_slp_tree_1): Do not fail
            fatally for shifts not supported by the target when discovering
            a single lane.

Diff:
---
 gcc/tree-vect-slp.cc | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/gcc/tree-vect-slp.cc b/gcc/tree-vect-slp.cc
index 9d7b74cb9f25..f6741433ae17 100644
--- a/gcc/tree-vect-slp.cc
+++ b/gcc/tree-vect-slp.cc
@@ -1215,7 +1215,13 @@ vect_build_slp_tree_1 (vec_info *vinfo, unsigned char *swap,
 	      if (!directly_supported_p (rhs_code, vectype, optab_vector))
 		{
 		  /* No vector/vector shift, try for a vector/scalar shift.  */
-		  if (!directly_supported_p (rhs_code, vectype, optab_scalar))
+		  if (!directly_supported_p (rhs_code, vectype, optab_scalar)
+		      /* ???  We are using this to guide operand swapping to
+			 eventually make all shift operands the same but we
+			 shouldn't fail in the end - that's be business of
+			 vectorizable_shift.
+			 Avoid spurious ICEs for single-lane discovery.  */
+		      && group_size != 1)
 		    {
 		      if (dump_enabled_p ())
 			dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,