Debugging

Debugging JBurg-Generated BURMS

Basic Principles

JBurg generated code can fail in two ways that are particular to it:

1. The pattern matcher may fail to find a valid sequence of reductions to cover the entire tree.
2. The sequence of reductions selected may not be what the programmer thought it would be.

Failure to Find a Valid Cover

The first problem, failing to find a valid sequence of reductions to cover the tree, is often caused by one of these problems:

• The specification may not have the tree shape correctly specified. This is often the case when the specification is under development.
• There may be a typo in the specification; unless the specification uses an enumeration to provide nonterminal names, JBurg will create nonterminals as it sees them, and typing statment instead of statement will cause a pattern match to fail.
• The front end may have loose error handling that allows badly formed trees into the code generator.

Compiler-Compile Time Options

Specify -g on the command line to build debugging code into your BURM.

Compile Time Techniques

Intercepting BURM errors

[Java target specific] When the BURM cannot find a valid cover for a tree, it throws an IllegalStateException. Your driver can catch this exception and dump an annotated AST for analysis in JBurg's debugger, and/or otherwise fix up the error:

private static Object generateCode(CommonTree tree)
throws Exception
{
    SecondTreeParser burm = new SecondTreeParser();
    try
    {
        burm.burm(tree);
        return burm.getResult();
    }
    catch ( IllegalStateException ex )
    {
        burm.dump(tree, new PrintWriter("/tmp/failedBurm.xml"));
        throw ex;
    }
}

Using the Debugger

JBurg's debugger is part of the source distribution; build it via ant ErrorAnalyzer.jar and invoke it... TBD

Advanced Error Analysis in the BURM

Use the DefaultErrorHandler directive to add code that gets control when the BURM cannot generate a valid cover of the input AST:


/
* Error recovery routine: deduce what we can from the problem
* tree, then abort this BURM with an exception that the caller
* can catch and ignore.
/
DefaultErrorHandler
{
new UnknownTreeHandler(reducer.getProblems()).analyze(p);
BURMAbortException.abort();
}


The UnknownTreeHandler is an Apache Falcon subsystem that explores the annotated AST and attempts to diagnose the problem. Because failure to match a subtree causes the parent tree to also fail to match, the UnknownTreeHandler recursively explores the tree and attempts to identify the most deeply nested subtree that failed to match.