RE: Query regarding index of local variables

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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