[gcc r15-1053] Relax COND_EXPR reduction vectorization SLP restriction

[Richard Biener]

https://gcc.gnu.org/g:28edeb1409a7b839407ec06031899b933390bff3

commit r15-1053-g28edeb1409a7b839407ec06031899b933390bff3
Author: Richard Biener <rguenther@suse.de>
Date:   Fri Feb 23 16:16:38 2024 +0100

    Relax COND_EXPR reduction vectorization SLP restriction
    
    Allow one-lane SLP but for the case where we need to swap the arms.
    
            * tree-vect-stmts.cc (vectorizable_condition): Allow
            single-lane SLP, but not when we need to swap then and
            else clause.

Diff:
---
 gcc/tree-vect-stmts.cc | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/gcc/tree-vect-stmts.cc b/gcc/tree-vect-stmts.cc
index b26cc74f417..c82381e799e 100644
--- a/gcc/tree-vect-stmts.cc
+++ b/gcc/tree-vect-stmts.cc
@@ -12116,7 +12116,7 @@ vectorizable_condition (vec_info *vinfo,
     = STMT_VINFO_REDUC_DEF (vect_orig_stmt (stmt_info)) != NULL;
   if (for_reduction)
     {
-      if (slp_node)
+      if (slp_node && SLP_TREE_LANES (slp_node) > 1)
 	return false;
       reduc_info = info_for_reduction (vinfo, stmt_info);
       reduction_type = STMT_VINFO_REDUC_TYPE (reduc_info);
@@ -12205,6 +12205,10 @@ vectorizable_condition (vec_info *vinfo,
 	      cond_expr = NULL_TREE;
 	    }
 	}
+      /* ???  The vectorized operand query below doesn't allow swapping
+	 this way for SLP.  */
+      if (slp_node)
+	return false;
       std::swap (then_clause, else_clause);
     }