[Bug tree-optimization/116573] [15 Regression] Recent SLP work appears to generate significantly worse code on RISC-V

[rguenth at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116573

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
Btw, I've written up a patch as well:

diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
index 62c7f90779f..199d79029e4 100644
--- a/gcc/tree-vect-loop.cc
+++ b/gcc/tree-vect-loop.cc
@@ -3078,10 +3078,23 @@ start_over:
       if (direct_internal_fn_supported_p (IFN_SELECT_VL, iv_type,
                                          OPTIMIZE_FOR_SPEED)
          && LOOP_VINFO_LENS (loop_vinfo).length () == 1
-         && LOOP_VINFO_LENS (loop_vinfo)[0].factor == 1 && !slp
+         && LOOP_VINFO_LENS (loop_vinfo)[0].factor == 1
          && (!LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo)
              || !LOOP_VINFO_VECT_FACTOR (loop_vinfo).is_constant ()))
        LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo) = true;
+
+      /* If any of the SLP instances cover more than a single lane
+        we cannot use .SELECT_VL at the moment, even if the number
+        of lanes is uniform throughout the SLP graph.  */
+      if (LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo))
+       for (slp_instance inst : LOOP_VINFO_SLP_INSTANCES (loop_vinfo))
+         if (SLP_TREE_LANES (SLP_INSTANCE_TREE (inst)) != 1
+             && !(SLP_INSTANCE_KIND (inst) == slp_inst_kind_store
+                  && SLP_INSTANCE_TREE (inst)->ldst_lanes))
+           {
+             LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo) = false;
+             break;
+           }
     }

   /* Decide whether this loop_vinfo should use partial vectors or peeling,