# Tuesday, August 1, 2006
.NET Generic Types

Recently I changed the way .NET generic types are handled by IKVM. The previous scheme was based on a naive mangling of the .NET Type.FullName property, but that turned out to be problematic for various reasons. The first obvious reason was that type names often were way too long for comfort. For example:

Type t = typeof(System.Collections.Generic.List<string>);

With IKVM 0.28 this prints out:

System.Collections.Generic.List`1[[System.String, mscorlib, Version=, Culture=neutral, PublicKeyToken=b77a5c561934e089]]
cli.System.Collections.Generic.List$$00601$$005B$$005B System$$002EString$$002C$$0020mscorlib$$002C$$0020 Version$$003D2$$002E0$$002E0$$002E0$$002C$$0020 Culture$$003Dneutral$$002C$$0020 PublicKeyToken$$003Db77a5c561934e089$$005D$$005D

(I manually inserted some spaces to make the HTML wrap.)

This obviously isn't very practical, but there is also a deeper problem. This literal translation of the name misses the fact that class loading in .NET is fundamentally different from class loading in Java. Which means that it is easy to go from Type to class name, but given a class name it is much harder to construct the corresponding type, especially if on or more of the type parameters are Java types (or remapped types).

In the next release the class name will be:


Not only more readable, but it also correctly reflects the fact that System.String is known as java.lang.String on the Java side.

I also changed the way these names are resolved to be more inline with the way Java works. The name is now parsed by the class loader that first tries to load the class which then in turn loads the component classes and constructs the generic type from that. This means that the following code will now work (if you can somehow convince your Java compiler to compile it):

import cli.System.Collections.Generic.*;

class Test
  public static void main(String[] args)
    List$$00601_$$$_LTest_$$$$_ list = new List$$00601_$$$_LTest_$$$$_();
    list.Add(new Test());

With the previous scheme there was no way to refer to the Test class in the generic class name, but now the correct class loader is used so you can refer to Java classes as well. That leaves only one question, what does list.getClass().getClassLoader() return? Well, for the time being I've decided to automatically construct a class loader that delegates to the defining class loaders of the classes used to construct the generic type, this ensures (as long as no types with the same names are used) that all types used in the definition of the generic type instance can be loaded by its class loader. It isn't actually clear to me that this is required, but at least for the time being it is, because some of the reflection code relies on this fact.

Finally, as a reminder, all this stuff is still experimental and, frankly, not intended to be practical. The main reason this support exists is to make the life of reflection/ikvmstub easier, otherwise the runtime would have to have all sorts of complicated support to deal with field types and method parameter/return types that take generic type instantiations.

Tuesday, August 1, 2006 3:11:32 PM (W. Europe Daylight Time, UTC+02:00)  #    Comments [0]
# Monday, July 24, 2006

Almann Goo released version of JaCIL on Saturday. The goal of JaCIL is basically to be a reverse ikvmc, it converts .NET assemblies into Java jars.

The binary distribution includes a couple of demos. Naturally I had to try and run them on IKVM :-) I ran into two known IKVM bugs, the first being that JaCIL generates a class and prior to Java 1.5 this wasn't a valid class name, but since Java 1.5 the class name space has been significantly enlarged, almost all checks have been removed. The second bug was that Java classes with both a Finalize() and finalize() method would be compiled incorrectly (under the covers IKVM generates a Finalize() method that overrides System.Object::Finalize and calls java.lang.Object::finalize), I've added support for this by renaming the generated Finalize method when it is necessary.

JaCIL isn't yet at the point where it can compile the ikvmc generated assembly[*], but maybe we'll eventually be able to run Java on .NET on Java on .NET on Java on .NET on Java... ;-)

[*] If you try it, you'll likely to run into an IKVM limitation. JaCIL is a .NET application and uses an ikvmc compiled version of ObjectWeb ASM and currently, IKVM.Runtime.dll gets confused if multiple versions of IKVM.GNU.Classpath.dll are loaded in the AppDomain.

Monday, July 24, 2006 2:18:39 PM (W. Europe Daylight Time, UTC+02:00)  #    Comments [0]
# Wednesday, July 12, 2006
Eclipse 3.2 x64

The Eclipse 3.2 release also includes an "early access" version for Windows x64 (among other 64 bit platforms), so I decided to try to run this version on IKVM.

While trying this I discovered a bug in the 64 bit version of the CLR. It doesn't respect MethodImpl(MethodImplOptions.NoInlining) and this means that Eclipse fails to start up, because it still does that weird stack walking to make sure that a particular method is called by the expected caller. BTW, instead of using the stack trace from an exception, they now use a subclass of SecurityManager and call getClassContext on that. That mechanism is a little better, but it is still incredibly lame.

The good news is that after adding a hack to IKVM to insert some extra junk in that particular method that prevents it from being inlined, Eclipse 3.2 x64 seems to run fine on IKVM.

Coincidentally, Eclipse 3.2 doesn't work on IKVM, because my implementation of getClassContext returned one stack frame too few (the GNU Classpath VMStackWalker interface is somewhat inconsistent, the other two methods in it both skip two frames, but getClassContext is apparantly only supposed to skip one frame).

Update: I got an e-mail from someone on the CLR team and he explained that the observed behaviour is by design. The method is not inlined, but the JIT optimizes the call into a tail call, thereby removing the stack frame from the call stack. Strictly speaking this is indeed a legal optimization, but I have to say I don't really see the point, if I wanted a tail call I would have used the tail call instruction.

Wednesday, July 12, 2006 4:59:45 PM (W. Europe Daylight Time, UTC+02:00)  #    Comments [0]
# Wednesday, July 5, 2006
Eclipse Java Compiler

The recent Eclipse 3.2 release includes a release of the stand alone Eclipse Java Compiler (they call it the JDT Core Batch Compiler). It is available as ecj.jar.

I switched to using this version for building GNU Classpath. If you want to use the same version, simply download ecj.jar and run it through ikvmc.

Wednesday, July 5, 2006 5:23:41 PM (W. Europe Daylight Time, UTC+02:00)  #    Comments [0]
# Thursday, June 15, 2006
Custom Attributes as Annotations
I implemented partial and experimental support for applying .NET Custom Attributes in Java code as annotations. The following example now works:

import cli.System.Runtime.InteropServices.DllImportAttribute;

class beep
  private static native int Beep(int dwFreq, int dwDuration);

  public static void main(String[] args)
    for (int i = 1000; i < 3000; i += 200)
      Beep(i, 50);

I realise that the name of the annotation type is a bit ugly, but it's consistent with the way delegates are handled and it enables efficient handling of custom attributes at runtime (when compiling code in dynamic mode) and doesn't require access to the ikvmstub generated stub classes.

At the moment only attributes that have a zero arg or single string arg constructor are supported and attribute fields or properties are not exposed.

The Annotation inner interface cannot be used for anything except annotating. If you try to implement it or declare variables or parameters of its type, you will get internal errors in IKVM.Runtime.dll (because there is no corresponding .NET type). I will probably fix this at some point, but for now simply don't use that type except for annotating.

Finally, even though the Annotation annotation type is marked as Retention(RetentionPolicy.RUNTIME), it won't be visible through Java reflection. It's complicated to make this work and the value of it is limited. As an alternative, you can use .NET reflection to access the custom attributes.

Thursday, June 15, 2006 2:31:25 PM (W. Europe Daylight Time, UTC+02:00)  #    Comments [0]
# Thursday, June 8, 2006
More AtomicInteger Performance

In my previous entry on AtomicInteger I showed timings on my single CPU ThinkPad which has a 1.7 GHz Pentium M. Today I investigated the performance on my iMac which has a 1.83 GHz Core Duo and like the ThinkPad runs Windows XP SP2.

I've also modified AtomicInteger to take advantage of Interlocked.Increment, Decrement and Exchange. Instead of following the reference implementation which builds everything on top of the compareAndSet operation.

Running the same test produces the following numbers:

  Time (ms)
JDK 1.5 HotSpot Server VM


JDK 1.5 HotSpot Client VM






Note that the test is significantly slower than on the Pentium M. This is most likely because of the bus locking overhead.

It gets even more interesting when we modify the test to have two threads that concurrently increment the same field:

  Time (ms)
JDK 1.5 HotSpot Server VM


JDK 1.5 HotSpot Client VM






The first thing to note is the fact that the test now takes an order of magnitude more time. Presumably, this is caused by the communication overhead between the two cores.

The second is that here IKVM significantly outperforms HotSpot Server. The reason for this is simple: IKVM uses Interlocked.Increment which ultimately uses the XADD x86 instruction that atomically performs the load/increment/store. By contrast, the AtomicInteger reference implementation uses the following loop to increment the value:

public final int incrementAndGet() {
    for (;;) {
        int current = get();
        int next = current + 1;
        if (compareAndSet(current, next))
            return next;

When another thread updates the field between the get and compareAndSet, the compareAndSet will fail and the loop will have to run for another iteration. When multiple CPUs are continuously trying to increment the value this is likely to happen relatively frequently.

Standard disclaimer: As always these microbenchmarks are designed to magnify a particular effect. Be careful about drawing overly large conclusions from them.

Thursday, June 8, 2006 9:08:01 PM (W. Europe Daylight Time, UTC+02:00)  #    Comments [2]
# Thursday, June 1, 2006
IKVM 0.28 bug fix release

A new 0.28 release to fix two bugs reported by users. The first bug is a regression that caused native method resolution not to work for statically compiled code and the second bug is that instanceof didn't work with ghost interface arrays (e.g. obj instanceof Serializable[] would cause the runtime to abort). No other changes.

Files are available here: ikvm- (sources + binaries), ikvmbin- (binaries), ikvmbin-generics- (binaries built from generics branch)

Thursday, June 1, 2006 9:48:05 AM (W. Europe Daylight Time, UTC+02:00)  #    Comments [0]
# Tuesday, May 30, 2006

Last week Tom Tromey posted a proposal for a patch to the Classpath patches mailing list for JSR-166 support. Triggered by this I did some preliminary investigation to see how much work it will be to support this in IKVM.

The reference implementation uses the sun.misc.Unsafe class to calculate the offset of a field in a class and to directly access this field using the appropriate memory model semantics. HotSpot has intrinsic support for the performance cricitical methods in sun.misc.Unsafe. Unfortunately, this model isn't very suitable for IKVM, because the CLI does not specify a way to portably obtain the offset of a field in a class (and it even allows the offset to change over time). I've implemented all the required methods in sun.misc.Unsafe based on reflection, but obviously this is very slow (preliminary measurements show a 10x to 100x slowdown compared to JDK 1.5).

The good news is that some important classes can easily be ported to a more CLI friendly approach. As an example I've modified the reference implementation of AtomicInteger to use System.Threading.Interlocked.CompareExchange() instead of sun.misc.Unsafe.compareAndSwapInt(). This is a small change and allows for good performance. Here is the benchmark I used:

import java.util.concurrent.atomic.*;

public class test {
static AtomicInteger foo = new AtomicInteger(0);
public static void main(String[] args) {
long start = System.currentTimeMillis();
for (int i = 0; i < 100000000; i++) {
long end = System.currentTimeMillis();
System.out.println(end - start);

Here are the timings (IKVM and C# timings on Microsoft .NET Framework 1.1):

  Time (ms)
JDK 1.5 HotSpot Server VM


JDK 1.5 HotSpot Client VM






I included the C# version for comparision. Here is the source for it:

using System;
using System.Threading;

public class test {
static int foo;
static void Main() {
int start = Environment.TickCount;
for (int i = 0; i < 100000000; i++) {
Interlocked.Increment(ref foo);
int end = Environment.TickCount;
Console.WriteLine(end - start);
} }

Note how much easier the C# programming model is. Given support for managed by ref arguments, there is no need for an extra indirection or hacks like sun.misc.Unsafe.

Tuesday, May 30, 2006 12:27:17 PM (W. Europe Daylight Time, UTC+02:00)  #    Comments [1]
# Tuesday, May 23, 2006
IKVM 0.28 Released

I released 0.28 (same bits as last week's release candidate) to SourceForge.

Tuesday, May 23, 2006 1:30:32 PM (W. Europe Daylight Time, UTC+02:00)  #    Comments [1]
# Tuesday, May 16, 2006
Using foreach to enumerate Java collections in C# 3.0
A long standing feature request for IKVM has been to make Java collections implement System.IEnumerable and I've always resisted doing that for various reasons.

I while ago -- while discussing this with Stuart Ballard -- I realised that with the proposed C# 3.0 language extensions, in particular "extension methods" it should be possible to make Java collections foreach-able with little effort:

namespace java.util
  public static class CollectionExtensionMethods
    public static IEnumerator GetEnumerator(this java.util.Collection col)
       return new CollectionEnumerator(col);

For those who don't know how extension methods work, basically the above code (note the this keyword before the argument type) adds a method to java.util.Collection that feels like an instance method.

Given the way foreach works, I had expected that it too would recognize this GetEnumerator method and therefore work on all Java collections, but alas that is not the case.
If you want to see this changed before C# 3.0 is released, please vote. Alternatively, if you're Miguel and on the ECMA C# committee, it should be obvious what to do ;-)

Tuesday, May 16, 2006 3:03:28 PM (W. Europe Daylight Time, UTC+02:00)  #    Comments [4]