[Bug tree-optimization/81303] [8 Regression] 410.bwaves regression caused by r249919

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81303

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
So one useful change is the following which makes the runtime profitability
threshold 6 and thus the vector loop is never entered.  Even though that should
be quite a predictable conditional jump it turns out we mess up BB placement
so that the result isn't a big improvement (254s -> 250s).  This is probably
also due to the fact that we end up peeling the inner loop completely
(we know it iterates <= profitability threshold times).  Plus we do not
version the loop but share the non-profitable part with the peeled copy
making RAs job harder :/

Index: gcc/tree-vect-loop.c
===================================================================
--- gcc/tree-vect-loop.c        (revision 250384)
+++ gcc/tree-vect-loop.c        (working copy)
@@ -3702,8 +3702,9 @@ vect_estimate_min_profitable_iters (loop
               "  Calculated minimum iters for profitability: %d\n",
               min_profitable_iters);

-  min_profitable_iters =
-       min_profitable_iters < vf ? vf : min_profitable_iters;
+  /* We want the vectorized loop to execute at least once.  */
+  if (min_profitable_iters < (vf + peel_iters_prologue + peel_iters_epilogue))
+    min_profitable_iters = vf + peel_iters_prologue + peel_iters_epilogue;

   if (dump_enabled_p ())
     dump_printf_loc (MSG_NOTE, vect_location,