Thursday, August 01, 2002

This weblog has moved to http://weblog.ikvm.net/

Stuart commented: On most aspects, you've convinced me. Just a few little comments: wrt "throws Exception" vs "throws Throwable" - there's no difference. Any Java method can throw any Throwable subclass that isn't an Exception subclass, just as any Java method can throw any RuntimeException subclass. Thus "throws Exception" really does let you throw anything. Actually, no. Try to compile the following: class test extends Throwable { public static void main(String[] args) throws Exception { throw new test(); } } It doesn't compile. One of the inellegancies of Java's checked exceptions model is that there are two classes that removed the checkedness, both Error and RuntimeException. Btw, are you arranging that (most? any?) .NET exceptions end up as subclasses of j.l.Exception, or just direct Throwable subclasses? Throwable, the most important reason is that I don't want to mess with the class hierarchy, if at all possible. I agree also that remapping things like InputStream is probably too hard. If you have a generic class/interface/method remapping system, though, I can imagine actually experimenting with Collection vs IEnumerable and indexers on List. One of the goals of XML remapping system, is to enable exactly these types of experiments. It probably isn't flexible enough to do everything you'd want to do, but in the end we'll get there. When you get IK.VM working on Mono I might even have a try at that myself, since it shouldn't need any deep magic in the VM itself. I think it'll still be a while before Reflection.Emit in Mono is at the level where it is complete enough. Oh, and I like the idea of custom attributes to do "throws" expressions on glue code. I wonder if we could persuade the Mono people to get mcs to actually honor the "throws" attributes - ie, compile C# code with Java's 'checked exceptions' semantics. That would be pretty cool :) It would be cool, but I don't really like checked exceptions. After programming in Java (almost exclusively) for about five years, I finally decided that the cons outnumber the pros. The idea is compelling, but in practice there are too many problems, but I don't really want to start that war, as it has been waged many times already ;-) BTW, I just changed the idiv, ldiv, irem & lrem implementations to explicity check for -1, and for a lame little test I wrote, performance seems to be OK (same as JDK 1.1 and slightly faster than JDK 1.4, HotSpot doesn't like microbenchmarks ;-)). 4:14:02 PM    Comments

This weblog has moved to http://weblog.ikvm.net/

Zoltan commented on the div issue: I believe the reason the CLR throws an exception in this case is that the div CIL opcode is compiled down to the idiv x86 opcode, which raises a hardware exception in case of overflow. A div opcode which does not throw an exception would be much slower, since it would have to be implemented by a series of x86 opcodes. You're correct. I mistakenly believed that the x86 idiv instruction didn't throw an exception on overflow, but it does (it doesn't in real-mode, which is what I remembered). Still, it would be trivial in the x86 overflow exception handler to detect that it is handling this case and then resume execution after the idiv instruction with the correct results in the registers. This wouldn't make any performance difference for the common case, only the exceptional case would be a little slower. Stuart commented: Horrible hacky proposed solution: implement idiv using the broken div, but add an exception handler to every java method that uses idiv that tests for the pathological case and somehow resumes in the right place. Two problems with this approach: 1) OverflowException can also be caused by trying to allocate a new array with a negative size, so I would somehow have to detect this. 2) Resuming after the exception is very hard (you cannot jump into a try block in .NET, so I'd have to generate lots of code, which would also make to common case slower). Or just wrap map java (x/y) to try{x/y} catch(OverflowException e) {x}. How much execution overhead is there for a try-catch in the non-exception case? This is an interesting suggestion. *writing some C# microbenchmarks* Here is the code I cooked up:  public static void Main(string[] args)  {   int d = args.Length + 1;   int n = (args.Length - 1) * -1;   int start = Environment.TickCount;   for(int i = 0; i < 10000000; i++)   {    try    {     n = n / d;    }    catch(OverflowException)    {     n = int.MinValue;    }   }   int end = Environment.TickCount;   Console.WriteLine(end - start);   start = Environment.TickCount;   for(int i = 0; i < 10000000; i++)   {    if(d == -1)    {     n = n * d;    }    else    {     n = n / d;    }   }   end = Environment.TickCount;   Console.WriteLine(end - start);  } On my system there is almost no difference between these two methods, so I think I'll go for the explicit test, because of the additional complexity of moving the stack into the try block. 9:43:40 AM    Comments

This weblog has moved to http://weblog.ikvm.net/

More very useful comments from Stuart: I'm not sure what I think about your choice to not remap things like IOException, and to handle it in the pseudo-native code. The principle of "least surprise" applies here. When Java code specifically calls .NET I/O code, it expect System.IO.IOException (because that is what the documentation says), and vice versa. With regard to "throws Exception" in the generated JAR files, I'm not sure how I feel about it either. On the one hand, yes, it's certainly awkward for the Java programmer. On the other hand, it's exactly what the CLR really *does*, by implication - any method *can* throw any exception. Here's a problem case: I could define a Java exception, use IK.VM to compile it to CIL, and then write a C# class that throws it and call that C# class from Java - how would you handle that case with your proposed solution? You can either preserve the "real" Java mapping of the compiled Java exception, OR you can make it a subclass of RuntimeException, but you can't do both. But you really do want to be able to preserve the semantics of a custom-compiled Java exception. Assuming "throws Exception" for all C# code seems to be the only way you *can* deal with this case, because you there's no way you can detect it to specialcase it. You've conviced me, making all .NET methods declare "throws Throwable" (not Exception), is the only viable solution to this. I'm probably going to support a custom attribute to explicitly declare a throws clause in .NET code for newly written code, to make writing glue code a little less painful. Yet another couple of semantics-impedance-mismatch issues that I just thought of: Do you plan to allow indexers and foreach to work on java.util.List and java.util.Collection, respectively? Will Collection implement(/extend) IEnumerable with Iterator remapping to IEnumerator? How about the converse ( for (Iterator i = new System.ArrayList().iterator(); i.hasNext(); ) {...} )? Or even, more complicatedly, {Input,Output}Stream and Reader/Writer mapping to .NET equivalents? No, none of these. It's much easier to writer wrapper classes to handle these conversions than to have the VM handle it. Will JavaBeans-compatible getFoo()/setFoo() methods remap to a property called foo? Or even to a property called Foo? (even the naming conventions are incompatible...) Java "properties" are too semantically weak to be useful. There are many methods getXxx() where Xxx isn't a read-only property (Component.getGraphics(), for example) so I'm not going to do anything with them. You'll notice that I'm posing ever more complicated scenarios - sorry! :) The feedback is very helpful, thanks! The reason I'm deliberately making your life difficult is that you seem to have all the easy cases well covered already... I disagree ;-) The hardest part was figuring out how to do the basic object model mapping (like Sam Ruby said, Object, String, and Exception). and besides, I'm highly interested to see just how deeply integrated into the .NET platform Java can really get without losing it's soul... Me too. The priorities are the hard part, in many cases there is a trade off between performance and 100% compatibility, but a compatible JVM is a higher priority than interop with other .NET code (which isn't too say that isn't important). 9:15:03 AM    Comments