Disable loop distribution for loops with estimated iterations 0

[Jan Hubicka via Gcc-patches]

Hi,
this prevents useless loop distribiton produced in hmmer.  With FDO we now
correctly work out that the loop created for last iteraiton is not going to
iterate however loop distribution still produces a verioned loop that has no
chance to survive loop vectorizer since we only keep distributed loops
when loop vectorization suceeds and it requires number of (header) iterations
to exceed the vectorization factor.

Bootstrapped/regtested x86_64-linux, OK?

gcc/ChangeLog:

        * tree-loop-distribution.cc (loop_distribution::execute): Disable
        distribution for loops with estimated iterations 0.

diff --git a/gcc/tree-loop-distribution.cc b/gcc/tree-loop-distribution.cc
index cf7c197aaf7..8ff2108f284 100644
--- a/gcc/tree-loop-distribution.cc
+++ b/gcc/tree-loop-distribution.cc
@@ -3871,10 +3871,20 @@ loop_distribution::execute (function *fun)
 
          bool destroy_p;
          int nb_generated_loops, nb_generated_calls;
+         bool only_patterns = !optimize_loop_for_speed_p (loop)
+                              || !flag_tree_loop_distribution;
+         /* do not try to distribute loops that are not expected to iterate.  
*/
+         if (!only_patterns)
+           {
+             HOST_WIDE_INT iterations = estimated_loop_iterations_int (loop);
+             if (iterations < 0)
+               iterations = likely_max_loop_iterations_int (loop);
+             if (!iterations)
+               only_patterns = true;
+           }
          nb_generated_loops
            = distribute_loop (loop, work_list, cd, &nb_generated_calls,
-                              &destroy_p, (!optimize_loop_for_speed_p (loop)
-                                           || !flag_tree_loop_distribution));
+                              &destroy_p, only_patterns);
          if (destroy_p)
            loops_to_be_destroyed.safe_push (loop);