Disturb profile less in tree-tailcall

[Jan Hubicka <hubicka at ucw dot cz>]

Hi,
with tail recursion and accumulation it is quite common case that the profile
is unrealistic and the recursive call is triggered more often then the
entry block.  This patch prevents tailcall from dropping entry block profile
to 0 (and making it very cold) in this case.

Bootstrapped/regtested x86_64-linux, comitted.

	* tree-tailcall.c (eliminate_tail_call): Be more careful about not
	disturbin profile of entry block.

Index: tree-tailcall.c
===================================================================
--- tree-tailcall.c	(revision 254884)
+++ tree-tailcall.c	(working copy)
@@ -889,10 +889,18 @@ eliminate_tail_call (struct tailcall *t)
 
   /* Number of executions of function has reduced by the tailcall.  */
   e = single_succ_edge (gsi_bb (t->call_gsi));
-  decrease_profile (EXIT_BLOCK_PTR_FOR_FN (cfun), e->count ());
-  decrease_profile (ENTRY_BLOCK_PTR_FOR_FN (cfun), e->count ());
+
+  profile_count count = e->count ();
+
+  /* When profile is inconsistent and the recursion edge is more frequent
+     than number of executions of functions, scale it down, so we do not end
+     up with 0 executions of entry block.  */
+  if (count >= ENTRY_BLOCK_PTR_FOR_FN (cfun)->count)
+    count = ENTRY_BLOCK_PTR_FOR_FN (cfun)->count.apply_scale (7, 8);
+  decrease_profile (EXIT_BLOCK_PTR_FOR_FN (cfun), count);
+  decrease_profile (ENTRY_BLOCK_PTR_FOR_FN (cfun), count);
   if (e->dest != EXIT_BLOCK_PTR_FOR_FN (cfun))
-    decrease_profile (e->dest, e->count ());
+    decrease_profile (e->dest, count);
 
   /* Replace the call by a jump to the start of function.  */
   e = redirect_edge_and_branch (single_succ_edge (gsi_bb (t->call_gsi)),