Add a zero-copy path for Collections.sort() on ArrayList.

Given that ArrayLists are array backed, we can sort the underlying
array in place. A similar optimisation can be applied to Vector and
CopyOnWriteArrayList but vector isn't used often enough to slow
everything else down with an instanceof check and nobody should be
daft enough to try sorting a CopyOnWriteArrayList.

Note that it's possible to apply the same optimisation to
Collections.sort(List<T>, Comparator<? super T>) but it involves a
larger refactoring to allow us to use Object[] instead of T[] and is
accompanied by a general loss of readability.

ArrayList performance improves for all array sizes :

BEFORE

Scenario{vm=app_process, trial=0, benchmark=Sort_arrayList, arrayListLength=4} 39021.26 ns; σ=206.27 ns @ 3 trials
Scenario{vm=app_process, trial=0, benchmark=Sort_arrayList, arrayListLength=16} 120903.59 ns; σ=1188.88 ns @ 5 trials
Scenario{vm=app_process, trial=0, benchmark=Sort_arrayList, arrayListLength=64} 435484.98 ns; σ=19141.88 ns @ 10 trials
Scenario{vm=app_process, trial=0, benchmark=Sort_arrayList, arrayListLength=256} 1772876.81 ns; σ=5542.74 ns @ 3 trials
Scenario{vm=app_process, trial=0, benchmark=Sort_arrayList, arrayListLength=1024} 7289431.83 ns; σ=110869.13 ns @ 10 trials

AFTER

Scenario{vm=app_process, trial=0, benchmark=Sort_arrayList, arrayListLength=4} 9161.79 ns; σ=90.04 ns @ 6 trials
Scenario{vm=app_process, trial=0, benchmark=Sort_arrayList, arrayListLength=16} 29988.90 ns; σ=74.05 ns @ 3 trials
Scenario{vm=app_process, trial=0, benchmark=Sort_arrayList, arrayListLength=64} 118142.86 ns; σ=1133.52 ns @ 7 trials
Scenario{vm=app_process, trial=0, benchmark=Sort_arrayList, arrayListLength=256} 456091.11 ns; σ=9110.18 ns @ 10 trials
Scenario{vm=app_process, trial=0, benchmark=Sort_arrayList, arrayListLength=1024} 1851939.39 ns; σ=69542.41 ns @ 10 trials

For all other lists, there's no measurable difference for
most sizes. length == 4 is harder to be sure about, given the
large standard deviations and we'd expect the time spent in
instanceof to be larger relative to the rest of what's going on.

BEFORE

Scenario{vm=app_process, trial=0, benchmark=Sort_vector, arrayListLength=4} 49002.02 ns; σ=1392.58 ns @ 10 trials
Scenario{vm=app_process, trial=0, benchmark=Sort_vector, arrayListLength=16} 154967.88 ns; σ=332.29 ns @ 3 trials
Scenario{vm=app_process, trial=0, benchmark=Sort_vector, arrayListLength=64} 588207.14 ns; σ=1863.60 ns @ 3 trials
Scenario{vm=app_process, trial=0, benchmark=Sort_vector, arrayListLength=256} 2292967.79 ns; σ=4578.14 ns @ 3 trials
Scenario{vm=app_process, trial=0, benchmark=Sort_vector, arrayListLength=1024} 9299582.60 ns; σ=179459.24 ns @ 10 trials

AFTER

Scenario{vm=app_process, trial=0, benchmark=Sort_vector, arrayListLength=4} 49324.98 ns; σ=240.86 ns @ 3 trials
Scenario{vm=app_process, trial=0, benchmark=Sort_vector, arrayListLength=16} 155639.91 ns; σ=215.12 ns @ 3 trials
Scenario{vm=app_process, trial=0, benchmark=Sort_vector, arrayListLength=64} 581950.81 ns; σ=938.11 ns @ 3 trials
Scenario{vm=app_process, trial=0, benchmark=Sort_vector, arrayListLength=256} 2285933.64 ns; σ=13265.42 ns @ 3 trials
Scenario{vm=app_process, trial=0, benchmark=Sort_vector, arrayListLength=1024} 8981475.84 ns; σ=431676.48 ns @ 10 trials

bug: 18821961

Change-Id: I30a3eaf89ff478663f555c93556167107da10873

1 files changed, 55 insertions, 0 deletions

diff --git a/benchmarks/src/benchmarks/regression/CollectionsBenchmark.java b/benchmarks/src/benchmarks/regression/CollectionsBenchmark.java
new file mode 100644
index 0000000..43d6d59
--- /dev/null
+++ b/benchmarks/src/benchmarks/regression/CollectionsBenchmark.java
@@ -0,0 +1,55 @@
+/*
+ * Copyright (C) 2014 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package benchmarks.regression;
+
+import com.google.caliper.Param;
+import com.google.caliper.SimpleBenchmark;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.Random;
+import java.util.Vector;
+
+public class CollectionsBenchmark extends SimpleBenchmark {
+    @Param({"4", "16", "64", "256", "1024"})
+    private int arrayListLength;
+
+    public void timeSort_arrayList(int nreps) {
+        List<Integer> input = buildList(arrayListLength, true /* use array list */);
+        for (int i = 0; i < nreps; ++i) {
+            Collections.sort(input);
+        }
+    }
+
+    public void timeSort_vector(int nreps) {
+        List<Integer> input = buildList(arrayListLength, false /* use array list */);
+        for (int i = 0; i < nreps; ++i) {
+            Collections.sort(input);
+        }
+    }
+
+    private static List<Integer> buildList(int arrayListLength, boolean useArrayList) {
+        Random random = new Random();
+        random.setSeed(0);
+        List<Integer> list = useArrayList ? new ArrayList<Integer>(arrayListLength) :
+                new Vector<Integer>(arrayListLength);
+        for (int i = 0; i < arrayListLength; ++i) {
+            list.add(random.nextInt());
+        }
+        return list;
+    }
+}