Hi Andrew,
I have gone thru this concept of assigning a slot to local variables
by observing the local variable tables of the class files, yep each
slot is being taken by many local var. so I used getMaxLocals() and it
worked fine (removed my own way of fidning max index, which sometimes is
causing an error due to incomplete lv table).
Please find a comment inline regarding the comment made by you "stack
height mismatch verification error".
>Koduru, Rajendra Kumar Reddy wrote:
>> Hi Andrew
>>
>> Thank you for recommending me the VM spec...
>
>Reading the VM spec is not as stupid an idea as it might
>sound. I'm not
>aware of any more concise or helpful reference for bytecode
>manipulators. Can anyone suggest a better one?
>
>>
>> But my prob is a little different,
>
>It seems that your problem can be summarized as, "How do i
>choose a free
>slot?" That's the question i answered in my previous email.
>see below
>for further explanation.
>
>>
>> as you have said I can add local variables using
>>
>> LocalVar var = localvargen.addLocalvar()
>> int index = var.getIndex();
>> llcalvargen.setstart( ilist.append ( new ASTORE ( index ) ) )
>> .
>> .
>> .
>>
>> as many as required
>>
>> and at the calling setMaxLocals();
>>
>> That is OK.
>
>none of this is actually required. the local variable table
>is just for
>debugging purposes, and as i said before, may be inaccurate,
>incomplete,
>obfuscated, or missing. feel free to ignore LocalVar objects
>completely
>and just choose some slots yourself and use them.
>
>The usual way of choosing slots, as far as I'm aware, is to call
>getMaxLocals:
>
>MethodGen mg = ...
>int freeSlot = mg.getMaxLocals();
>int freeSlot2 = freeSlot + 1;
>mg.getInstructionList().append(new ASTORE(freeSlot));
>...
>
>>
>> but now consider a scenario like this
>>
>> public int getI(){
>> return 2;
>> }
>>
>> Now I am trying to insert try-finally to this method by
>> instrumenting in bytecode
>>
>> which should look like
>>
>> public int getI(){
>> try{
>> return 2;
>> }finally {
>> System.out.println("sample");
>> }
>>
>> }
>>
>> To create this, I should create a empty catch block which throws
>> the exception that is not handled and redirect all the calls
>to finally
>> before the
>> return statement
>>
>> now comes the problem with local variables.
>>
>>
>> I will get the instructionlist
>> I will find the instance of return, and insert a JSR instr
>> before this return and redirect all the branches
>
>btw, when you get past your other problems, it seems like you
>might have
>a stack height mismatch verification error at your subroutine. ask
>about that if it happens. if i'm right, it'll happen in any non-void
>method you instrument.
Andrew, I did not get that type of error, however could you
please let me know , why were you expecting this??
in non void methods, I was using
1. new ASTORE(x) // storing the result
2. jsr call // calling jsr
3. new ALOAD(x) // loading back the result
4. return // returning the result
and all the branches were reidrected to 1 (i.e store statement)
in case of non void methods
so could you please let me know , why were you expecting an
error?? So that I could rectify it
am I right in doing like this???
However just to mention, I was also using methodgen.setMaxStack
before returning the isntrumented method
>
>> Next step is creation of catch block, where I have a problem
>> here I should store the exception, make a jsr
>> call, load the exception which I stored before and rethrow it
>>
>> so I use ilist.append ( new ASTORE ( ? ) )
>> ilist.append ( new JSR() );
>> ilist.load ( new ALOAD ( ? ) )
>>
>> so what should I use in the place of "?" (2,3,4......)
>
>i suggest mg.getMaxLocals()
>
>>
>> same while creating finally block, I should store the return
>> address
>>
>> ilist.append ( new ASTORE ( ?? ) )
>> // performs the instrumentation of system.out.....
>> ret ??
>>
>> so what should I use in place of "?" (2,3,4......)
>
>i suggest mg.getMaxLocals() + 1
>
>>
>> so what I am doing is
>>
>> getting the max_index (maximum index) available in the
>> localvargen
>> and assigning max_index+1 to the first ?
>> and assigning max_index+2 to the second ??
>>
>> am I wrong in doing like these?????
>
>If the local variable table were complete and accurate, this method
>would be correct, but you should not assume that.
>getMaxLocals() always
>returns a free slot (assuming you call setMaxLocals()
>appropriately) and
>frees you from writing your own code to find max_index.
>
>>
>> is there some other efficient way for doing this???
>
>getMaxLocals()
>
>>
>> Please help me in this regard.
>>
>> Thank you
>> Reddy
>>
>
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