GCC questions

[Radu Cornea]

>>On Tue, 3 Aug 1999, Mike Stump wrote:

> > Date: Tue, 3 Aug 1999 16:57:50 -0700 (PDT)
> > From: Radu Cornea <radu@ics.uci.edu>
> 

...

> > 5. I am asking these questions because I need to solve this problem in gcc:
> > in some cases, gcc transforms initial structured code into unstructured
> > code. This happens for example for if statements that have as condition
> > logical OR (||) or AND (&&) expressions. In order to generate structured
> > code, the then-statement have to be duplicated (as many times as the number
> > of ORs or ANDs). For this, i need to save somehow the then-statement tree
> > when it is generated and duplicate it (and of course, the jumps need to be
> > changed).
> 
> Ok, too bad you didn't say exactly what you wanted to do.  Sounds like
> it is mostly outside the scope of compilation though.  We can give
> better answers to your question, if the question is the right
> question.  Tell us what you really want to do...
> 
> I think what you want is an full function ast, that preserves
> constructs like if and while.  If that is it, then if you can use just
> C++, then you might consider turning on the template code everywhere,
> and munching on it instead.  In that style of code we preserve things
> like:
> 
> DEFTREECODE (DECL_STMT, "decl_stmt", 'e', 3)
> DEFTREECODE (IF_STMT, "if_stmt", 'e', 3)
> DEFTREECODE (FOR_STMT, "for_stmt", 'e', 4)
> DEFTREECODE (WHILE_STMT, "while_stmt", 'e', 2)
> DEFTREECODE (DO_STMT, "do_stmt", 'e', 2)
> DEFTREECODE (RETURN_STMT, "return_stmt", 'e', 1)
> DEFTREECODE (BREAK_STMT, "break_stmt", 'e', 0)
> DEFTREECODE (CONTINUE_STMT, "continue_stmt", 'e', 0)
> DEFTREECODE (SWITCH_STMT, "switch_stmt", 'e', 2)
> DEFTREECODE (GOTO_STMT, "goto_stmt", 'e', 1)
> 
> directly, and you can the walk that code and do what you want.  But
> since I don't know what you want to do, I don't know if this is a
> solution to what you want to do.
> 

I will try to explain what I am trying to do.
We are working here at a compiler that uses gcc (a modified version for
generating three-address code). Some optimizations and scheduling
techniques that we use work only if the code is what we call "structured".
In some situations, gcc transforms "structured code" into "unstructured".
I will explain below what we mean by that.
For e.g., for the C code:

...

if (a||b)
  <x>;
else
  <y>;

<z>;

...


(x, y and z are statements, a and b are expressions)

gcc generates something like this:

         |
        <a>
        /  \
       /   <b>      This is what we call
       |  /   \     "unstructured code"
       [x]    [y]
         \   /
          \ /
	   |
          [z]
           |


We want to get a code like this:

         |
        <a>
        /  \
       /   <b>
      /    / \     This is what we call
    [x]  [x] [y]   "structured code"
      \   |  /     i.e. can be viewed
       \  | /      as an if inside 
        \ |/       another if.
	 \|
          |       
         [z]
          |

So, the [x] statement must be duplicated.

This problem happens only when the if condition is a OR or AND logical
expression (it is an optimization made by gcc).

If you have any idea how could I modify gcc to get that kind of code, I
would appreciate very much.

Thanks,

Radu.


--
Radu Cornea
Graduate Student
Information & Computer Science
University of California - Irvine
E-mail: radu@ics.uci.edu