Every so often I find myself wanting to add debug printing to the C++ interpreter for specific methods. I can never remember how I did it the last time and have to figure it out all over again, so here’s how:

diff -r 4d8381231af6 openjdk-ecj/hotspot/src/share/vm/interpreter/bytecodeInterpreter.cpp
--- a/openjdk-ecj/hotspot/src/share/vm/interpreter/bytecodeInterpreter.cpp  Tue Apr 21 09:50:43 2009 +0100
+++ b/openjdk-ecj/hotspot/src/share/vm/interpreter/bytecodeInterpreter.cpp  Wed Apr 22 11:13:35 2009 +0100
@@ -555,6 +555,15 @@
          topOfStack < istate->stack_base(),
          "Stack top out of range");
 
+  bool interesting = false;
+  if (istate->msg() != initialize) {
+    ResourceMark rm;
+    if (!strcmp(istate->method()->name_and_sig_as_C_string(),
+                "spec.benchmarks._202_jess.jess.Rete.FindDeffunction(Ljava/lang/String;)Lspec/benchmarks/_202_jess/jess/Deffunction;")) {
+      interesting = true;
+    }
+  }
+
   switch (istate->msg()) {
     case initialize: {
       if (initialized++) ShouldNotReachHere(); // Only one initialize call

The trick is getting the fully-qualified name of the method right: the method name contains dots, but the class names in its signature contain slashes. You’re there once you have that down.

I haven’t blogged for a while. I’ve been working on Shark’s performance, walking through the native code generated for critical methods and looking at what’s happening. There’s several cases where I can see that some piece of code is unnecessary, but translating that into a way that Shark can see it’s unnecessary is non-trivial. I’m thinking I may need to separate the code generation, adding an intermediate layer between the typeflow and the LLVM IR so I can add things which are maybe necessary and then remove them if not. It all seems a bit convoluted — bytecode → typeflow → new intermediate → LLVM IR → native — but the vast bulk of the Shark’s time is spent in the last step so a bit more overhead to create simpler LLVM IR should speed up compilation as well as the runtime.

None of this has been particularly bloggable, but I wanted to point out two exiting things that are happening in Shark land. Robert Schuster and Xerxes Rånby have been busy getting Shark to run on ARM, and Neale Ferguson has started porting LLVM to zSeries with the intention of getting Shark running there. I expected to see Shark on ARM sooner or later, but Shark on zSeries came completely out of the blue. I’m really looking forward to seeing that happen!