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From Stuart Monteith <stuk...@stoo.me.uk>
Subject Re: JavaStackFrame/JavaLocation local variable support
Date Fri, 26 Jun 2009 10:41:50 GMT
Hi,
    I was wondering what peoples thoughts were regarding program 
counters, line number table and variable tables.
There is a tension between most users of the Kato API and the JDI 
connector and its obligations towards supplying the information JDWP 
requires.
JDWP, for the most part, would like to know the location of a stack 
frame, i.e. a program counter normally, and using that to look up the 
variable tables and line number tables.

Some changes have been made to the API to supply the local variables 
(through JavaStackFrame.getVariable(int)) and their locations/types 
through JavaMethod.getVariables().
However, we haven't resolved the issue of the program counter or the 
line numbers.

Just now the line numbers are available through 
JavaLocation.getLineNumber(), where available. However, JDWP never asks 
for a stack frame's line number, it maps from a stack frames
location to a line number using the line number table from a stack 
frame's method.

So, should we forgo having the simple JavaLocation.getLineNumber() and 
only supply the line number table (where appropriate)?
Of course, having a "getProgramCounter()" method would be useful, but 
what should we do for compiled methods? There is a strong requirement 
for us to return the contents
of local variables in compiled methods as well as interpreted methods. 
However, that requires synthesizing a bytecode program counter to 
retrieve the correct variables, which implies
that line numbers could be generated too. However, as with C, etc, the 
debugging information derived from optimized code is usually inaccurate.
For line numbers, I imagine we'd either have the line numbers or not if 
they are inaccurate. But for local variables, it would be sensible to 
alter the variable table information to suit the
optimized code, to give a consistent picture.

Regards,
    Stuart

Stuart Monteith wrote:
> Hi,
>    I've been looking at local variables in relation to the JDI 
> connector. For the BOF at JavaOne we'd like for there to be a 
> prototype of  local variable support in the API. I've been looking at 
> what JDWP requires as we would have to be able to satisfy its queries 
> using the Kato API. This has made me lean towards exposing the 
> variable table and have us retrieve the local variables from the stack 
> frames by slot number.
>
> So my suggestion for the API is this:
>
> ---------------------------------
>
> JavaMethod
> -------------
>
> // returns all local variables
> // empty if there are no variables.
> Iterator<JavaVariable> getVariable() throws DataUnavailable;
>
> JavaVariable
> -------------
>
> // Local variable's name
> // throws DataUnavailable if the variable was derived from bytecode 
> and so the name is unknown. Caller is free to make a name up.
> String getName() throws DataUnavailable;
>
> // The local variable's signature in JNI format.
> String getSignature();
>
> // The start of the local variable's scope within the bytecode.
> int getStart();
>
> // The number of bytes this variables scope covers over the bytecode.
> int getLength();
>
> // The slot this variable occupies. Passed to 
> JavaStackFrame.getVariable() to retrieve the contents.
> int getSlot();
>
>
> JavaStackFrame
> ------------------
>
> // Gets the value of a variable from a stack frame.
> // Returns a JavaObject for an object reference, null for a null 
> object reference. Primitives are returned as boxed primitives.
> // throws CorruptDataException if object reference is incorrect, or if 
> the float or double are set to invalid values.
> // throws DataUnavailable if this method is not supported or if stack 
> not in correct state to return variables.
> // throws IndexOutOfBoundsException if an invalid slot number if passed.
> Object getVariable(int slot) throws CorruptDataException, 
> DataUnavailable, IndexOutOfBoundsException;
>
>
> ---------------------------------
>
> The bytecode offset can be calculated with:
>    JavaLocation.getAddress() - ( 
> JavaMethod.getBytecodeSections().next().getBase().getAddress())
> but I think that might be a little too tedious, and doesn't allow 
> cleverness with JITted frames. So we will probably have to add:
>
> // Return program counter in bytecode.
> int JavaLocation.getBytecodePC();
>
> alternatively the JavaVariable.getStart() would use absolute 
> addresses, which could conceivably work with JITed frames, if the 
> tables are maintained during compilation.
>
> We should also expose the line number table too as that will aid class 
> file reproduction and queries for line numbers based on bytecode 
> program counters.
>
> A slightly different scheme would have the 
> JavaStackFrame.getVariable(int slot) method look like:
>    Object getVariable(JavaVariable var);
> but I don't think it gains us much.
>
> Retrieving all of the variables would therefore look something like this:
>
> void dumpVariables(JavaThread thread) throws Exception {
>    Iterator frames = thread.getStackFrames();
>    while (frames.hasNext()) {
>       JavaStackFrame frame = (JavaStackFrame) frames.next();
>       JavaLocation location = frame.getLocation();
>       JavaMethod method = location.getMethod();
>       int pc = location.getBytecodePC();
>            System.out.println(location.toString()+":");
>
>       Iterator variables = method.getVariables();
>       while (variables.hasNext()) {
>          JavaVariable variable = (JavaVariable) variables.next();
>
>          if (pc >= variable.getStart() && pc <= 
> variable.getStart()+variables.getLength()) {
>             Object value = frame.getVariable( variable.getSlot());
>                        System.out.println("\t"+ 
> variable.getSignature()+" "+variable.getName()+" = "+ value.toString());
>          }
>       }
>    }
> }
>
> Let me know what you think,
>    Stuart
>
> Stuart Monteith wrote:
>> Hello,
>>    With Steve's work on JVMTI/python coming along, the issue of what 
>> to do about local methods is coming up. Currently there is no means 
>> to determine the names and values of local variables through the 
>> current API.
>>
>> The most obvious way of implementing this is to have the API do all 
>> of the processing by exposing the variables as name and value pairs.
>>
>> For example:
>> interface JavaStackFrame {
>>    List<LocalVariable> getLocalVariables();
>> }
>>
>> where:
>>
>> interface LocalVariable {
>>    String getName();
>>    Object getValue();
>> }
>>
>> Where the value is a JavaObject, or an boxed primitive.
>>
>> The other extreme is for the necessary information to be made 
>> available for the callers of the API to generate this information 
>> themselves.
>> This would mean properly exposing:
>>    Program Counter - currently we have JavaLocation.getAddress(), 
>> which is an address in memory, rather than a bytecode program 
>> counter. For JITted frames we'd still need the bytecode program counter.
>>    Local variable table - this is to determine which variables there 
>> are, their types and their indexes into the local variable array
>>    Local variable array - the contents of the local variables need to 
>> be exposed, and their proper types should be returnable (JavaObject, 
>> int, etc).
>>
>> Doing it that way might be beneficial for more user stories, there is 
>> more information available to reconstruct the class file, for instance.
>> There is also the small matter of what to do when the local variable 
>> table is not available. When the API exposes all that it knows the 
>> values might still be retrievable, although I have my doubts as to 
>> how useful that would be if you don't know the types.
>>
>> Thoughts?
>>    Stuart
>>

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