I am doing some tests with escape analysis in Java 7 in order to better understand what objects are eligible to stack allocation.
Here is the code I wrote to test stack allocation:
import java.util.ArrayList;
import java.util.Iterator;
public class EscapeAnalysis {
private static final long TIME_TO_TEST = 10L * 1000L; // 10s
static class Timestamp {
private long millis;
public Timestamp(long millis) {
this.millis = millis;
}
public long getTime() {
return millis;
}
public void setTime(long time) {
millis = time;
}
}
public static void main(String[] args) {
long r = 0;
System.out.println("test1");
r += test1();
System.out.println("test2");
r += test2();
System.out.println("test3");
r += test3();
System.out.println("test4");
r += test4();
System.out.println("test5");
r += test5();
System.out.println("test6");
r += test6();
System.out.println(r);
}
public static long test1() {
long r = 0;
long start = System.currentTimeMillis();
while (System.currentTimeMillis() - start < TIME_TO_TEST) {
r += new Timestamp(System.currentTimeMillis()).getTime();
}
return r;
}
public static long test2() {
ArrayList<Integer> l = new ArrayList<Integer>(1000);
for (int i = 0; i < 1000; ++i) {
l.add(i);
}
long r = 0;
long start = System.currentTimeMillis();
while (System.currentTimeMillis() - start < TIME_TO_TEST) {
for (Iterator<Integer> it = l.iterator(); it.hasNext(); ) {
r += it.next().longValue();
}
}
return r;
}
public static long test3() {
long r = 0;
long start = System.currentTimeMillis();
while (System.currentTimeMillis() - start < TIME_TO_TEST) {
Timestamp ts = new Timestamp(System.currentTimeMillis());
ts.setTime(42);
r += ts.getTime();
}
return r;
}
public static long test4() {
ArrayList<Integer> l = new ArrayList<Integer>(1000);
for (int i = 0; i < 1000; ++i) {
l.add(i);
}
long r = 0;
long start = System.currentTimeMillis();
while (System.currentTimeMillis() - start < TIME_TO_TEST) {
Iterator<Integer> it = l.iterator();
r += it.next().longValue();
r += it.next().longValue();
r += it.next().longValue();
r += it.next().longValue();
}
return r;
}
public static long test5() {
ArrayList<Integer> l = new ArrayList<Integer>(1000);
for (int i = 0; i < 1000; ++i) {
l.add(i);
}
long r = 0;
long start = System.currentTimeMillis();
while (System.currentTimeMillis() - start < TIME_TO_TEST) {
Iterator<Integer> it = l.iterator();
for (int i = 0; i < l.size(); ++i) {
r += it.next().longValue();
}
}
return r;
}
public static long test6() {
long r = 0;
long start = System.currentTimeMillis();
while (System.currentTimeMillis() - start < TIME_TO_TEST) {
for (Timestamp ts = new Timestamp(System.currentTimeMillis());
ts.getTime() > 0;
ts.setTime(ts.getTime() + System.currentTimeMillis())) {
r += ts.getTime();
}
}
return r;
}
}
And here is what it outputs with Java 7 on Linux
java -server -version
java version "1.7.0_02"
Java(TM) SE Runtime Environment (build 1.7.0_02-b13)
Java HotSpot(TM) 64-Bit Server VM (build 22.0-b10, mixed mode)
java -server -verbose:gc -XX:CompileThreshold=1 -cp bin EscapeAnalysis
test1
test2
[GC 15616K->352K(59776K), 0,0014270 secs]
[GC 15968K->288K(59776K), 0,0011790 secs]
[GC 15904K->288K(59776K), 0,0018170 secs]
[GC 15904K->288K(59776K), 0,0011100 secs]
[GC 15904K->288K(57152K), 0,0019790 secs]
[GC 15520K->320K(56896K), 0,0011670 secs]
[GC 15232K->284K(56256K), 0,0011440 secs]
test3
test4
test5
[GC 14876K->348K(55936K), 0,0005340 secs]
[GC 14620K->348K(56000K), 0,0004560 secs]
[GC 14300K->316K(55296K), 0,0004680 secs]
[GC 13948K->316K(55488K), 0,0003590 secs]
[GC 13692K->316K(54784K), 0,0004580 secs]
[GC 13436K->316K(54976K), 0,0005430 secs]
[GC 13180K->316K(54272K), 0,0004500 secs]
[GC 12924K->316K(54464K), 0,0005090 secs]
[GC 12668K->316K(53760K), 0,0004490 secs]
[GC 12412K->316K(53888K), 0,0004350 secs]
[GC 12156K->316K(53312K), 0,0005060 secs]
test6
6737499643744733086
I am using GC logs to known whether objects were allocated on the stack (idea from Escape analysis in Java) which might not be 100% reliable but seems to give good hints.
Baed on the output, stack allocation works for test1, test3, test4 and test6 and doesn’t work for test2 and test5. I don’t understand why this doesn’t work with an iterator in for-loop although it works
- with an iterator outside a for-loop (see test4),
- with another object inside a for-loop (see test6).
I have read the code for the ArrayList iterator and I don’t understand why it would not be eligible for stack allocation in tests 2 and 5 since it does neither escape the current method nor the current thread.
Any idea?
EA is something the C2 compiler analyses based on the IR it generates therefore you need it to compile the method before enjoying the benefits. Each test is called once only so there is no chance for it to compile. Details on EA and the C2 IR in the hotspot internals wiki (https://wiki.openjdk.java.net/display/HotSpot/Overview+of+Ideal,+C2%27s+high+level+intermediate+representation and https://wiki.openjdk.java.net/display/HotSpot/EscapeAnalysis)
Here’s a version that attempts to show the impact
which generates the following output when run with
-server -XX:CompileThreshold=1one danger here is that the compilation of this method has changed it more fundamentally, I haven’t attempted to guard against this so some use of
LogCompilationorPrintCompilationmight be required to check.