RE: [aspectj-users] if and loop pointcuts

[Diana Szentivanyi <diasz@xxxxxxxxxx>]

>At the time we designed the language, we could think of no compelling
>use cases for pointcuts related to if and do. So we didn't spend any
>time thinking about them.
>
>Since then I've heard a couple of examples of when these might be useful,
>most having to do with coverage problems.
>
>I'm starting to think something like this could be added to the language
>and prove useful. But I don't see it yet.  One problem is designing the
>pointcuts to be stable. Unstable pointcuts are those that are defined in
>terms that are vulnerable to small seemingly unrelated changes to the program.
>The classic example is the line number pointcut, that says something like when
>executing expressions or statements at this line number. 
>
>Could you give an example of the kind of pointcut and advice you would like 
>to write?
>

  I need to detect when a field is accessed the last time in a method. For this, 
I am given the total number of accesses (get/set) to that field; during 
execution of the method I count the number of accesses by using get and set 
pointcuts. This works fine as long as I do not have if's and loops where in some 
cases is not known beforehand how many times the field is accessed. 
  Example: I want to know when the last access to "a" happens. "a" is accessed 
in the method below, a number of times given by the formula: nr=1+1+(0|1)+(0|?) 
= 2+(0|1)+(0|?). 
  The (0 or 1) accesses is given by the "else" branch of the "if", and the (0 or 
more) accesses is given by the "while" loop (2 accesses in line 8). Of course, 
it is possible to have inner loops, or inner if statements in the if or loops.
   
  void some_method(){
    1:b=r+4;
    2:a=b+15/c;
    3:if(a<20)
    4:  c=34;
    5:else    
    6:  a=18;
    7:while(b<30){
    8: if(d<20) 
    9:  a=a-b;
   10: else
   11:  d--;
   12: b++;
   13:}
  }  
  
  What would solve my problem is the following: 
    * I am given the above formula for the number of accesses: 2+(0|1)+(0|?) 
which can be written as 2+1+1
    * The aspect language with the pointcuts helps me detect when the control 
flow left the if or the loop, when the counter of accesses is incremented with 
1; of course, after the accesses in lines 2 and 3 the counter is incremented 
with 1 in the plain get/set based pointcut advices; when the counter reaches the 
value 4 (2+1+1) it means that the last access to "a" was detected. 
    
    So, the pointcuts and advices I would write look as follows:
    
    pointcut in_if(): ...(if_statement);
    pointcut in_loop(): ...(loop);
    pointcut _a_in_if_loop(): (get(int a)||set(int a)) && 
(cflow(in_if)||cflow(in_loop));
    pointcut _a_normal(): (get(int a)||set(int a)) 
&&cflow(in_if)&&!cflow(in_loop);
    
    after(): in_if(){
    	counter_access_a++;
    }
    
    ..................
    
    after(): _a_normal(){ counter_access_a++;}
    
    
    I am aware that it is possible that the field is not at all accessed in the 
if (for example in case a<20) or in the loop (in case d>20 in every iteration) 
and then some time can pass between the field is accessed last and this fact is 
detected. 
    
    
              Thanks a lot,
                Regards,
                  Diana.