Cap niter_for_unrolled_loop to upper bound

For the reasons explained in PR77536, niter_for_unrolled_loop assumes 5
iterations in the absence of profiling information, although it doesn't
increase beyond the estimate for the original loop.  This left a hole in
which the new estimate could be less than the old one but still greater
than the limit imposed by CEIL (nb_iterations_upper_bound, unroll factor).

2017-05-04  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-ssa-loop-manip.c (niter_for_unrolled_loop): Add commentary
	to explain the use of truncating division.  Cap the number of
	iterations to the maximum given by nb_iterations_upper_bound,
	if defined.

gcc/testsuite/
	* gcc.dg/vect/vect-profile-1.c: New test.

From-SVN: r247591

1 files changed, 12 insertions, 0 deletions

diff --git a/gcc/tree-ssa-loop-manip.c b/gcc/tree-ssa-loop-manip.c
index 70d1f844..d3a57f1 100644
--- a/gcc/tree-ssa-loop-manip.c
+++ b/gcc/tree-ssa-loop-manip.c
@@ -1104,6 +1104,9 @@ niter_for_unrolled_loop (struct loop *loop, unsigned factor)
   gcc_assert (factor != 0);
   bool profile_p = false;
   gcov_type est_niter = expected_loop_iterations_unbounded (loop, &profile_p);
+  /* Note that this is really CEIL (est_niter + 1, factor) - 1, where the
+     "+ 1" converts latch iterations to loop iterations and the "- 1"
+     converts back.  */
   gcov_type new_est_niter = est_niter / factor;
 
   /* Without profile feedback, loops for which we do not know a better estimate
@@ -1120,6 +1123,15 @@ niter_for_unrolled_loop (struct loop *loop, unsigned factor)
 	new_est_niter = 5;
     }
 
+  if (loop->any_upper_bound)
+    {
+      /* As above, this is really CEIL (upper_bound + 1, factor) - 1.  */
+      widest_int bound = wi::udiv_floor (loop->nb_iterations_upper_bound,
+					 factor);
+      if (wi::ltu_p (bound, new_est_niter))
+	new_est_niter = bound.to_uhwi ();
+    }
+
   return new_est_niter;
 }