[Bug target/126516] [17 Regression] 9% slowdown of calculix on Zen3 since r17-2723-g3f2b3eced0f827

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126516

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
I'll note the change will aggressively prefer SSE vectorization width,
irrespective of the cost of the operations leading up to the fold-left
reduction.

Also (but unlikely here), for slp_inst_kind_reduc_group we only
actually implement fold-left reductions for SLP_TREE_LANES == 1
unless there's no actual fold-left reduction to be performed
(VF == 1).  Meaning, slp_inst_kind_reduc_group with VF == 1
is free, which isn't reflected in the change.  But unlikely to be
the issue here.

So you might want to check whether the following fixes the regression:

diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
index dcfe4531f11..b2a96820ad3 100644
--- a/gcc/config/i386/i386.cc
+++ b/gcc/config/i386/i386.cc
@@ -27212,7 +27212,7 @@ ix86_vector_costs::finish_cost (const vector_costs
*scalar_costs)
     {
       unsigned int vf = vect_vf_for_cost (loop_vinfo);
       for (auto inst : LOOP_VINFO_SLP_INSTANCES (loop_vinfo))
-       if ((SLP_INSTANCE_KIND (inst) == slp_inst_kind_reduc_group
+       if (((SLP_INSTANCE_KIND (inst) == slp_inst_kind_reduc_group && vf != 1)
             || SLP_INSTANCE_KIND (inst) == slp_inst_kind_reduc_chain)
            && (vect_reduc_type (loop_vinfo, SLP_INSTANCE_TREE (inst))
                == FOLD_LEFT_REDUCTION))