# Saturday, March 20, 2004
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A Less Broken Snapshot?
In Tuesday's snapshot, ikvmc was completely broken. Sorry about that. The CoreClasses cache introduced an incorrect dependency between Object, Throwable and String. This caused Throwable or String to be loaded while it was being loaded and that resulted in an exception: System.ArgumentException: Item has already been added. Key in dictionary: "java.lang.Throwable" Key being added: "java.lang.Throwable"

Hopefully this snapshot will be a little better quality, but don't hold your breath, because the main change in this version is the addition of local variable liveness analysis to the verifier. This required some tricky code and made it clear to me (again) that the verifier desperately needs to be rewritten.

The trigger for the local variable liveness analysis was to be able to emit debugging information for local variables, but it also has the nice side effect of allowing a little better code generation. Previously, if a local variable slot was shared between two different reference types, the .NET local would have the type of the common base type, even if the uses were in fact totally distinct. The compiler had to emit downcasts whenever it emitted a load from one of those locals. In classpath.dll there were 1288 such downcasts. With the new liveness information, it is now possible to split those Java locals in multiple .NET locals, so these downcasts are now gone. Another optimization, which doesn't seem all that exciting, is the elimination of dead stores to local variables. In itself this is a fairly pointless optimization, because the CLR/Mono JIT will probably do it anyway. However, there is one very important optimization that can be done because of dead store elimination, in exception handling. Whenever an exception handler discards the exception object and the IKVM bytecode compiler can detect this, it can skip the (expensive) stack trace capturing that is normally required. I had already hacked some support to recognize these exception handlers (in classpath.dll there were 313 optimized exception handlers), but now it works much better (there are now 444 optimized exception handlers).

What's new?

  • Decorated the various ByteCodeHelper methods with the [DebuggerStepThroughAttribute] attribute to make stepping through the source code in the debugger less disruptive.
  • Restored the signature decoding methods in ClassFile.cs that I removed in the previous version. I had failed to realise that they're different from the ones in ClassLoaderWrapper, because they deal with unloadable classes.
  • Fixed CoreClasses to decouple the different classes (accessing one no longer triggers loading the others).
  • Changed handling of package accessible final fields (they're no longer turned into a property).
  • Fixed System.setOut (copy & paste mistake, it tried to set "in").
  • The debugging information is now classified as Java/Text. Not sure if this affects anything, but it seemed like the right thing to do.
  • Fixed debugging line number information to make sure the firt CIL instruction has a corresponding line number. Previously, Visual Studio .NET refused to step into an ikvmc compiled method.
  • Optimized dead stores to local variables and use new dead store information to optimize exception handling.
  • Local variables are now properly typed and have their names attached in debugging information (when ikvmc with the -debug option is used). NOTE: if the same variable name is reused in a method, the debugging information for those variables is not yet emitted correctly.
  • Fixed mapping of System.IntPtr to gnu.classpath.RawData. The mapping is now private to classpath.dll.
  • Changed JVM.CriticalFailure to write to always write to stderr on Unix instead of try to display a message box.
  • Fixed race condition between returning from Thread.join and the thread being removed from the thread pool / marked as dead.
  • Fixed Thread.yield to not consume thread interrupted status. (Note that Thread.sleep(0) behaves as Thread.yield() and also does not consume the interrupted status).

New snapshots: just the binaries and source plus binaries.

Saturday, March 20, 2004 2:26:49 PM (W. Europe Standard Time, UTC+01:00)  #    Comments [0]
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