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Re: Popping from operand stack Arrin Daley Mon Nov 17 16:01:03 2008

Hi Pranav

Perhaps a better way might be to insert DUP (or some of the other DUP variants) after the iload? instructions so that you copy the contents of the stack then you are free to do what you like with these as long as you return the stack to the state it was in before your instrumentation.

eg
   BYTECODES                                             STACK

  0:   iload_1                                  i1
  1:   iload_2                                  i1,i2
        DUP2                                    i1,12,i1,i2
        ....Some code to instrument the values of i1 and i2...
                                                i1,i2
  2:   if_icmpeq       16
  5:   getstatic       #2; //Field              
java/lang/System.out:Ljava/io/PrintStream;
  8:   ldc     #3; //String TRUE
............ as so on.


Note in this case copying the values was easy because they were single word values doubles require more care and you may have to use other DUP bytecodes and perhaps something a bit more complicated, shouldn't be too bad though.

Notice the stack returns to i1,i2 if you don't return the stack to the same state you'll have problems.

Remember to call MethodGen.setMaxStack() when you're finished because you've possibly changed the maximum stack height.

Hope this helps

Bye Arrin

Pranav Kuber wrote:
Hi,

I'm new to using BCEL and have a query.

I'm trying to instrument all IF instructions. So I just need to find out a
way where I could access the operand stack constants (iload_1 and iload_2).

For example, -
if I have some code in this format

if(a!=b)
    do Something...
else
   do something...

The corresponding bytecode generated would be -

   0:   iload_1
   1:   iload_2
   2:   if_icmpeq       16
   5:   getstatic       #2; //Field
java/lang/System.out:Ljava/io/PrintStream;
   8:   ldc     #3; //String TRUE
............ as so on.

I need to pop iload_1 and iload_2 from the stack and store them somewhere.

Is there any way I could do that?

Thanks for the help.

Regards,
Pranav


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