Fix updating of probabilities in update_bb_profile_for_threading

Fix updating of probabilities in update_bb_profile_for_threading

[Jan Hubicka]

Hi,
this is another small fix to improve upatting fater profile.
update_bb_profile_for_threading was buggy in years by computing
probability before updating frequency.
Moreover for bugus profiles it is better to avoid dropping count
to complete 0 because that implies optimization for size.


Bootstrapped/regtested x86_64-linux,c omitted.

Honza

Index: ChangeLog
===================================================================
--- ChangeLog	(revision 244748)
+++ ChangeLog	(working copy)
@@ -1,3 +1,8 @@
+2017-01-21  Jan Hubicka  <hubicka@ucw.cz>
+
+	PR lto/78407
+	* cfg.c (update_bb_profile_for_threading): Fix updating of probablity.
+
 2017-01-21  Bernd Schmidt  <bschmidt@redhat.com>
 
 	rtl-optimization/79125
Index: cfg.c
===================================================================
--- cfg.c	(revision 244732)
+++ cfg.c	(working copy)
@@ -869,6 +869,10 @@ update_bb_profile_for_threading (basic_b
       bb->count = 0;
     }
 
+  bb->frequency -= edge_frequency;
+  if (bb->frequency < 0)
+    bb->frequency = 0;
+
   /* Compute the probability of TAKEN_EDGE being reached via threaded edge.
      Watch for overflows.  */
   if (bb->frequency)
@@ -882,15 +886,12 @@ update_bb_profile_for_threading (basic_b
 		 "%i->%i too small (it is %i, should be %i).\n",
 		 taken_edge->src->index, taken_edge->dest->index,
 		 taken_edge->probability, prob);
-      prob = taken_edge->probability;
+      prob = taken_edge->probability * 6 / 8;
     }
 
   /* Now rescale the probabilities.  */
   taken_edge->probability -= prob;
   prob = REG_BR_PROB_BASE - prob;
-  bb->frequency -= edge_frequency;
-  if (bb->frequency < 0)
-    bb->frequency = 0;
   if (prob <= 0)
     {
       if (dump_file)