8350758: G1: Use actual last prediction in accumulated survivor rate prediction too

Reviewed-by: ayang, iwalulya

Author: Thomas Schatzl

src/hotspot/share/gc/g1/g1SurvRateGroup.cpp

@@ -68,26 +68,23 @@ void G1SurvRateGroup::stop_adding_regions() {
     _accum_surv_rate_pred = REALLOC_C_HEAP_ARRAY(double, _accum_surv_rate_pred, _num_added_regions, mtGC);
     _surv_rate_predictors = REALLOC_C_HEAP_ARRAY(TruncatedSeq*, _surv_rate_predictors, _num_added_regions, mtGC);
 
-    // Assume that the prediction for the newly added regions is the same as the
-    // ones at the (current) end of the array. Particularly predictions at the end
-    // of this array fairly seldom get updated, so having a better initial value
-    // that is at least somewhat related to the actual application is preferable.
-    double new_pred = _stats_arrays_length > 1
-                    ? _accum_surv_rate_pred[_stats_arrays_length - 1] - _accum_surv_rate_pred[_stats_arrays_length - 2]
-                    : InitialSurvivorRate;
-
     for (uint i = _stats_arrays_length; i < _num_added_regions; ++i) {
       // Initialize predictors and accumulated survivor rate predictions.
       _surv_rate_predictors[i] = new TruncatedSeq(10);
       if (i == 0) {
         _surv_rate_predictors[i]->add(InitialSurvivorRate);
-        _accum_surv_rate_pred[i] = 0.0;
+        _accum_surv_rate_pred[i] = InitialSurvivorRate;
       } else {
-        _surv_rate_predictors[i]->add(_surv_rate_predictors[i-1]->last());
-        _accum_surv_rate_pred[i] = _accum_surv_rate_pred[i-1] + new_pred;
+        // Assume that the prediction for the newly added regions is the same as the
+        // ones at the (current) end of the array. Particularly predictions at the end
+        // of this array fairly seldom get updated, so having a better initial value
+        // that is at least somewhat related to the actual application is preferable.
+        double next_pred = _surv_rate_predictors[i-1]->last();
+        _surv_rate_predictors[i]->add(next_pred);
+        _accum_surv_rate_pred[i] = _accum_surv_rate_pred[i-1] + next_pred;
       }
     }
-    _last_pred = new_pred;
+    _last_pred = _surv_rate_predictors[_num_added_regions-1]->last();
 
     _stats_arrays_length = _num_added_regions;
   }