Re: Popping from operand stack

Well, in general it's not going to be easy to do what you want in the 
general case, but perhaps you only care about simple cases, where
there is only one comparison in an if, or some other restriction.

In theory, the IF_CMPEQ can simply be replaced with a call to
ALOAD x
INVOKEVIRTUAL

where x is the object that you are calling your method on,
since in this case, the two parameters are on the top of the stack.
in the same way that parameters would be pushed.

----- Original Message ----- 
From: "Pranav Kuber" <[email protected]>
To: "BCEL Users List" <[email protected]>
Sent: Monday, November 17, 2008 7:09 PM
Subject: Re: Popping from operand stack


> Thanks for the help.
>
> Actually, I want to completely replace the if instruction with my code. So
> at runtime, instead of calling the if instruction if_icmpeq, my code shall
> get executed. So I just need the values from the stack to make a method 
> call
> passing these two values as arguments, thus, bypassing the if instruction.
>
> -Pranav
>
> On Mon, Nov 17, 2008 at 6:56 PM, Arrin Daley <[email protected]> 
> wrote:
>
>> 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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>