Re: Is this a form of Data Flow Analysis, and what is the proper way solution to the problem?

George Neuner <gneuner2@comcast.net>
26 Jan 2006 13:32:01 -0500

          From comp.compilers

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From: George Neuner <gneuner2@comcast.net>
Newsgroups: comp.compilers
Date: 26 Jan 2006 13:32:01 -0500
Organization: Compilers Central
References: 06-01-037 06-01-069
Keywords: analysis

On 20 Jan 2006 16:12:15 -0500, "Oliver Wong" <owong@castortech.com>
wrote:


> ... after doing some more reading on DFA, ...


Be careful with your terminology ... "DFA" normally stands for
"deterministic finite automaton". Data flow analysis is not generally
abbreviated.




>Some of the articles I've read seem to imply
>that I should already have a control flow graph before starting the
>analysis, and other articles seem to imply that this the CFG should be built
>up in parallel with the analysis. Can someone clear up what the proper
>ordering is, or where I can go to read more about this?


It's easier if you construct the basic blocks and the control graph
first. Some analyses require chasing loops through the graph and
those will be complicated greatly by trying to build the graph on
demand as you perform them. Additionally some analyses are more
easily done by walking the graph backward. Everything will require
[at some point] examining the individual operations in the basic
blocks.








>In my current implementation, there is no explicit CFG being built at all.




I think the problem is that we don't really understand what you are
trying to do. You said in your original post:


"For example, in the following statement: x = y + 5 + z;
  the value written to 'x' depends on both the value of 'y'
  and the value of 'z'."


and in your last post:


"This is what I now do. The output is something like:
    LeftAssignmentOccurrence: "x":21:5:Write
        Occurences Participating in assignment value:
            Occurrence: "y":21:14:Read
            Occurrence: "z":21:19:Read "






It seems like the question you are really trying to answer is: "where
did the value of X come from". For that you are going to have to get
down and dirty with less abstract representations.


Consider the following:


      if ( ... )
            x = y + 1;
      else
            x = z - 2;
      w = x * 3;


What does the value of 'w' depend on? 'x' of course ... but which
'x'? The one that depends on 'y' or the one that depends on 'z'? Is
that important to your analysis? What if that code were part of a
loop and 'y' or 'z' depended on the redefinition of 'w'?


Conditionals and loops (among other things) are why people are
suggesting you look into value numbering and SSA. After either
transformation you will have something like:


      if ( ... )
            x{1} = y{1} + 1;
      else
            x{2} = z{1} - 2;
=> X{3} = (x{1} | x{2}) <=
      w{1} = x{3} * 3;


where each use of a symbol is subscripted. It's similar enough to the
original code that you can read and understand it but is much easier
to analyze. The additional line defining 'x{3}' is a pseudo operation
called a phi function which is used to merge conflicting definitions -
in this case it means 'x{3}' may take the value of either 'x{1}' or
'x{2}'. Phi functions are needed for analysis only and don't normally
produce any real code ... ie. there is still just one 'x' in the
program, but now we can track how it's values change and how they are
used.


In fact, given what you have said previously, a value numbered source
listing would directly answer the questions of what is read and
written provided the depiction of the symbol subscripts was clear to
the human reader.




If you can explain better what you are really trying to achieve then
we can probably help you more.


George


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