Do not combine PRED_LOOP_GUARD and PRED_LOOP_GUARD_WITH_RECURSION

This patch avoids both PRED_LOOP_GUARD and PRED_LOOP_GUARD_WITH_RECURSION to be
attached to one edge.  We have logic that prevents same predictor to apply to
one edge twice, but since we split LOOP_GUARD to two more specialized cases,
this no longer fires.

Double prediction happens in exchange benchmark and leads to unrealistically
low hitrates on some edges which in turn leads to bad IPA profile and misguides
ipa-cp.

Unforutnately it seems that the bad profile also leads to bit better
performance by disabling some of loop stuff, but that really ought to be done
in some meaningful way, not by an accident.

gcc/ChangeLog:

2020-08-11  Jan Hubicka  <hubicka@ucw.cz>

	* predict.c (not_loop_guard_equal_edge_p): New function.
	(maybe_predict_edge): New function.
	(predict_paths_for_bb): Use it.
	(predict_paths_leading_to_edge): Use it.

gcc/testsuite/ChangeLog:

2020-08-11  Jan Hubicka  <hubicka@ucw.cz>

	* gcc.dg/ipa/ipa-clone-2.c: Lower threshold from 500 to 400.

diff --git a/gcc/predict.c b/gcc/predict.c
index 2164a06e0837c02c3b015a6ca68fa8abbb3ea28e..4c4bba549398523f86b26684e068c130322db29b 100644 (file)
--- a/gcc/predict.c
+++ b/gcc/predict.c
@@ -3122,6 +3122,35 @@ tree_guess_outgoing_edge_probabilities (basic_block bb)
   bb_predictions = NULL;
 }
 \f
+/* Filter function predicate that returns true for a edge predicate P
+   if its edge is equal to DATA.  */
+
+static bool
+not_loop_guard_equal_edge_p (edge_prediction *p, void *data)
+{
+  return p->ep_edge != (edge)data || p->ep_predictor != PRED_LOOP_GUARD;
+}
+
+/* Predict edge E with PRED unless it is already predicted by some predictor
+   considered equivalent.  */
+
+static void
+maybe_predict_edge (edge e, enum br_predictor pred, enum prediction taken)
+{
+  if (edge_predicted_by_p (e, pred, taken))
+    return;
+  if (pred == PRED_LOOP_GUARD
+      && edge_predicted_by_p (e, PRED_LOOP_GUARD_WITH_RECURSION, taken))
+    return;
+  /* Consider PRED_LOOP_GUARD_WITH_RECURSION superrior to LOOP_GUARD.  */
+  if (pred == PRED_LOOP_GUARD_WITH_RECURSION)
+    {
+      edge_prediction **preds = bb_predictions->get (e->src);
+      if (preds)
+       filter_predictions (preds, not_loop_guard_equal_edge_p, e);
+    }
+  predict_edge_def (e, pred, taken);
+}
 /* Predict edges to successors of CUR whose sources are not postdominated by
    BB by PRED and recurse to all postdominators.  */
 
@@ -3177,10 +3206,7 @@ predict_paths_for_bb (basic_block cur, basic_block bb,
         regions that are only reachable by abnormal edges.  We simply
         prevent visiting given BB twice.  */
       if (found)
-       {
-         if (!edge_predicted_by_p (e, pred, taken))
-            predict_edge_def (e, pred, taken);
-       }
+       maybe_predict_edge (e, pred, taken);
       else if (bitmap_set_bit (visited, e->src->index))
        predict_paths_for_bb (e->src, e->src, pred, taken, visited, in_loop);
     }
@@ -3223,7 +3249,7 @@ predict_paths_leading_to_edge (edge e, enum br_predictor pred,
   if (!has_nonloop_edge)
     predict_paths_for_bb (bb, bb, pred, taken, auto_bitmap (), in_loop);
   else
-    predict_edge_def (e, pred, taken);
+    maybe_predict_edge (e, pred, taken);
 }
 \f
 /* This is used to carry information about basic blocks.  It is

diff --git a/gcc/testsuite/gcc.dg/ipa/ipa-clone-2.c b/gcc/testsuite/gcc.dg/ipa/ipa-clone-2.c
index d513020ee8bcacbfef01c63a15ac2eade77a6cc1..53ae25a1e24f48f9b0b53ad29612a75a08788ad1 100644 (file)
--- a/gcc/testsuite/gcc.dg/ipa/ipa-clone-2.c
+++ b/gcc/testsuite/gcc.dg/ipa/ipa-clone-2.c
@@ -1,5 +1,5 @@
 /* { dg-do compile } */
-/* { dg-options "-O3 -fdump-ipa-cp-details -fno-early-inlining --param ipa-cp-max-recursive-depth=8" } */
+/* { dg-options "-O3 -fdump-ipa-cp-details -fno-early-inlining --param ipa-cp-max-recursive-depth=8 --param=ipa-cp-eval-threshold=400" } */
 
 int fn();