Fix profile update in scale_profile_for_vect_loop

When vectorizing 4 times, we sometimes do
  for
    <4x vectorized body>
  for
    <2x vectorized body>
  for
    <1x vectorized body>

Here the second two fors handling epilogue never iterates.
Currently vecotrizer thinks that the middle for itrates twice.
This turns out to be scale_profile_for_vect_loop that uses
niter_for_unrolled_loop.

At that time we know epilogue will iterate at most 2 times
but niter_for_unrolled_loop does not know that the last iteration
will be taken by the epilogue-of-epilogue and thus it think
that the loop may iterate once and exit in middle of second
iteration.

We already do correct job updating niter bounds and this is
just ordering issue.  This patch makes us to first update
the bounds and then do updating of the loop.  I re-implemented
the function more correctly and precisely.

The loop reducing iteration factor for overly flat profiles is bit funny, but
only other method I can think of is to compute sreal scale that would have
similar overhead I think.

Bootstrapped/regtested x86_64-linux, will commit it shortly.

gcc/ChangeLog:

	PR middle-end/110649
	* tree-vect-loop.cc (scale_profile_for_vect_loop): Rewrite.
	(vect_transform_loop): Move scale_profile_for_vect_loop after
	upper bound updates.

diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
index 61343eadee55104efeb751808fb8fcc492daffd7..b44fb9c77126b5096ef94a2c8341d83476fa953f 100644 (file)
--- a/gcc/tree-vect-loop.cc
+++ b/gcc/tree-vect-loop.cc
@@ -10842,31 +10842,30 @@ vect_get_loop_len (loop_vec_info loop_vinfo, gimple_stmt_iterator *gsi,
 static void
 scale_profile_for_vect_loop (class loop *loop, unsigned vf)
 {
-  edge preheader = loop_preheader_edge (loop);
-  /* Reduce loop iterations by the vectorization factor.  */
-  gcov_type new_est_niter = niter_for_unrolled_loop (loop, vf);
-  profile_count freq_h = loop->header->count, freq_e = preheader->count ();
-
-  if (freq_h.nonzero_p ())
-    {
-      profile_probability p;
-
-      /* Avoid dropping loop body profile counter to 0 because of zero count
-        in loop's preheader.  */
-      if (!(freq_e == profile_count::zero ()))
-        freq_e = freq_e.force_nonzero ();
-      p = (freq_e * (new_est_niter + 1)).probability_in (freq_h);
-      scale_loop_frequencies (loop, p);
-    }
-
+  /* Loop body executes VF fewer times and exit increases VF times.  */
   edge exit_e = single_exit (loop);
-  exit_e->probability = profile_probability::always () / (new_est_niter + 1);
-
-  edge exit_l = single_pred_edge (loop->latch);
-  profile_probability prob = exit_l->probability;
-  exit_l->probability = exit_e->probability.invert ();
-  if (prob.initialized_p () && exit_l->probability.initialized_p ())
-    scale_bbs_frequencies (&loop->latch, 1, exit_l->probability / prob);
+  profile_count entry_count = loop_preheader_edge (loop)->count ();
+
+  /* If we have unreliable loop profile avoid dropping entry
+     count bellow header count.  This can happen since loops
+     has unrealistically low trip counts.  */
+  while (vf > 1
+        && loop->header->count > entry_count
+        && loop->header->count < entry_count * vf)
+    vf /= 2;
+
+  if (entry_count.nonzero_p ())
+    set_edge_probability_and_rescale_others
+           (exit_e,
+            entry_count.probability_in (loop->header->count / vf));
+  /* Avoid producing very large exit probability when we do not have
+     sensible profile.  */
+  else if (exit_e->probability < profile_probability::always () / (vf * 2))
+    set_edge_probability_and_rescale_others (exit_e, exit_e->probability * vf);
+  loop->latch->count = single_pred_edge (loop->latch)->count ();
+
+  scale_loop_profile (loop, profile_probability::always () / vf,
+                     get_likely_max_loop_iterations_int (loop));
 }
 
 /* For a vectorized stmt DEF_STMT_INFO adjust all vectorized PHI
@@ -11476,7 +11475,6 @@ vect_transform_loop (loop_vec_info loop_vinfo, gimple *loop_vectorized_call)
                           niters_vector_mult_vf, !niters_no_overflow);
 
   unsigned int assumed_vf = vect_vf_for_cost (loop_vinfo);
-  scale_profile_for_vect_loop (loop, assumed_vf);
 
   /* True if the final iteration might not handle a full vector's
      worth of scalar iterations.  */
@@ -11547,6 +11545,7 @@ vect_transform_loop (loop_vec_info loop_vinfo, gimple *loop_vectorized_call)
                          assumed_vf) - 1
         : wi::udiv_floor (loop->nb_iterations_estimate + bias_for_assumed,
                           assumed_vf) - 1);
+  scale_profile_for_vect_loop (loop, assumed_vf);
 
   if (dump_enabled_p ())
     {