One of the key parts of Smalltalk is the 'live' debugger. Unlike the general dynamic language features which are well supported by the additions from JSR292 the debugger requires support which may not have been considered as necessary to support dynamic languages. So we were not sure we would be able to provide that portion of the Smalltalk experience on the jvm.
The good news is that we were able to implement almost all of the Smalltalk debugger features using only the services provided in the released jdk7. I thought I would take a moment to describe how we did it to both demonstrate the approach and to solicit suggestions for improvements. The Smalltalk debugger is 'live' in that it exists as a separate thread within the same process/memory space as the thread being debugged. This allows one to manipulate and inspect all objects from the same viewpoint as the debugged thread. Smalltalk offers the ability to inspect all instances of a class(type), all references to a specific object, the variables on all levels of the stack, senders and implementers of methods, and the ability to single step through method sends. There is also the ability to restart a thread from any level of the stack but we opted to wait on the coro patch before implementing this ( I also don't use it as it can have quite a few side effects ). The approach we took has two facets, we ( mainly oscar ) coded a C++ jvmti agent with a JNI interface which allowed us to call some JVMTI apis from within the jvm being debugged and we added some logic to the callsite to handle the stepping. Implementers and senders of methods is handled via reflection on the classes and methods present so that was easy. To support all instances and all references requires heap inspection which we get from using the jvmti heap functions. This had some issues with some of the support classes for invoke dynamic but we were able to use a two pass tagging approach to make sure we found all of the references to our objects. This has to find objects both in arrays and in instance vars. Inspecting the stack was straight forward once we filtered the stack trace to only have our method sends present. As an option one can also inspect the full jvm trace. Using the jvmti variable access api allows the locating the variable which is then placed into a static field of out debugger support class. This field is then access by Smallltalk via a primitive (in Smalltalk a primitive is the way we share with the underlying environment). Once we have this object we can manipulate its instance vars from the Smalltalk side as well. When an error is thrown the thread is suspended ( we added some jvmti thread management apis just to get away from the deprecated methods) and a new thread is launched with an instance of the debugger and a pointer to the thread to debug. At this point one can only inspect the thread locals and anything else in the object memory. The thread is not restartable so we kill it ( by sending ThreadDeath ). But it the error is a halt or breakpoint we can then step the thread along. We tried this with jvmti but is was broken and seems to add quite a bit of delay to everything. Plus its a callback approach which looked like a lot of work. So instead we tweaked the call site logic to add a debug check. I liked the way this worked a lot. For a dynamic look up we already have a callsite with a target of one or more GWTs to select the implementation which matches the receiver class. What we have to do to implement a debugger is to place before the first GWT a test which determines if this is the time to suspend. Unfortunately GWTs are added to the end so we need a way to keep the test at the beginning (thanks to John and Réme suggestion) we can simply have a callsite have a target which is another callsite. The first callsite points at the test logic and the second gets the GWT chain. One nice thing is that we can revert to the single site version using a debug flag. The code to get the initial target for the bootstrap callsite looks like: private void setBootstrapTarget(MethodHandle mh){ // get the appropriate initial call site sequence if( RtDebugger._debugEnable){ // for debugging we need to have a sequence of methodHandles that is always the first // code executed when a callsite is invoked. This checks to see if we should // hold here for a debug step or continue. _realSite = new MutableCallSite(mh); // this is the extra call site to hold the gwts MethodHandle invoker = _realSite.dynamicInvoker(); MethodHandles.Lookup lookup=MethodHandles.lookup(); MethodHandle debugEntry= null; MethodType mt=MethodType.methodType(void.class, RtObject.class); try { debugEntry = lookup.findStatic(RtDebugger.class, "debugEntry", mt); } catch (Throwable e) { e.printStackTrace(); } invoker = MethodHandles.foldArguments(invoker , debugEntry); // does nothing except suspend if necessary _realSite.setTarget(mh); this.setTarget(invoker); }else{ // normal behavior is to just use the RtCallSite to hold the GWT chain as its target this.setTarget(mh); } } The debug test code tests for the correct thread and for the stack depth ( jvmti api) and looks like: public static void debugEntry(RtObject arg){ if(_debugThread == Thread.currentThread()){ if(getStackSize(_debugThread) <= _debugDepth){ // send debugger update notice notifyDebugStep(); suspendThreadPrim(_debugThread); } } } As you can see it did not take much code ( lots of thinking though ) to implement a stepping debugger. regards mark
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