[gcc r13-751] Fix profile count maintenance in vectorizer peeling.

[gcc r13-751] Fix profile count maintenance in vectorizer peeling.

[Eugene Rozenfeld]

https://gcc.gnu.org/g:5af22024f62f1f596a35d3c138d41d47d5697ca0

commit r13-751-g5af22024f62f1f596a35d3c138d41d47d5697ca0
Author: Eugene Rozenfeld <erozen@microsoft.com>
Date:   Tue Apr 26 14:28:16 2022 -0700

    Fix profile count maintenance in vectorizer peeling.
    
    This patch changes the code to save/restore profile counts for
    the epliog loop (when not using scalar loop in the epilog)
    instead of scaling them down and then back up, which may lead
    to problems if we scale down to 0.
    
    Tested on x86_64-pc-linux-gnu.
    
    gcc/ChangeLog:
    
            * tree-vect-loop-manip.cc (vect_do_peeling): Save/restore profile
            counts for the epilog loop.

Diff:
---
 gcc/tree-vect-loop-manip.cc | 35 ++++++++++++++++++++++++-----------
 1 file changed, 24 insertions(+), 11 deletions(-)

diff --git a/gcc/tree-vect-loop-manip.cc b/gcc/tree-vect-loop-manip.cc
index 11dc6cbf576..e81de45e07e 100644
--- a/gcc/tree-vect-loop-manip.cc
+++ b/gcc/tree-vect-loop-manip.cc
@@ -2810,10 +2810,21 @@ vect_do_peeling (loop_vec_info loop_vinfo, tree niters, tree nitersm1,
   if (LOOP_VINFO_PEELING_FOR_GAPS (loop_vinfo))
     skip_epilog = false;
 
+  class loop *scalar_loop = LOOP_VINFO_SCALAR_LOOP (loop_vinfo);
+  auto_vec<profile_count> original_counts;
+  basic_block *original_bbs = NULL;
+
   if (skip_vector)
     {
       split_edge (loop_preheader_edge (loop));
 
+      if (epilog_peeling && (vect_epilogues || scalar_loop == NULL))
+	{
+	  original_bbs = get_loop_body (loop);
+	  for (unsigned int i = 0; i < loop->num_nodes; i++)
+	    original_counts.safe_push(original_bbs[i]->count);
+	}
+
       /* Due to the order in which we peel prolog and epilog, we first
 	 propagate probability to the whole loop.  The purpose is to
 	 avoid adjusting probabilities of both prolog and vector loops
@@ -2828,7 +2839,6 @@ vect_do_peeling (loop_vec_info loop_vinfo, tree niters, tree nitersm1,
     }
 
   dump_user_location_t loop_loc = find_loop_location (loop);
-  class loop *scalar_loop = LOOP_VINFO_SCALAR_LOOP (loop_vinfo);
   if (vect_epilogues)
     /* Make sure to set the epilogue's epilogue scalar loop, such that we can
        use the original scalar loop as remaining epilogue if necessary.  */
@@ -2985,16 +2995,19 @@ vect_do_peeling (loop_vec_info loop_vinfo, tree niters, tree nitersm1,
 	     a merge point of control flow.  */
 	  guard_to->count = guard_bb->count;
 
-	  /* Scale probability of epilog loop back.
-	     FIXME: We should avoid scaling down and back up.  Profile may
-	     get lost if we scale down to 0.  */
-	  basic_block *bbs = get_loop_body (epilog);
-	  for (unsigned int i = 0; i < epilog->num_nodes; i++)
-	    bbs[i]->count = bbs[i]->count.apply_scale
-				 (bbs[i]->count,
-				  bbs[i]->count.apply_probability
-				    (prob_vector));
-	  free (bbs);
+	  /* Restore the counts of the epilog loop if we didn't use the scalar loop. */
+	  if (vect_epilogues || scalar_loop == NULL)
+	    {
+	      gcc_assert(epilog->num_nodes == loop->num_nodes);
+	      basic_block *bbs = get_loop_body (epilog);
+	      for (unsigned int i = 0; i < epilog->num_nodes; i++)
+		{
+		  gcc_assert(get_bb_original (bbs[i]) == original_bbs[i]);
+		  bbs[i]->count = original_counts[i];
+		}
+	      free (bbs);
+	      free (original_bbs);
+	    }
 	}
 
       basic_block bb_before_epilog = loop_preheader_edge (epilog)->src;