Problem
--------

        Outline a strategy to perform dead code elimination. An assignment would
        be considered as dead code if the variable on left hand side is not used
        further. eg.  In the below  example, variable  `b' is not  used further.
        Thus the statement `b = 5' is dead code.


                Input Program             |   Dead Code Elimination
             -----------------------------------------------------------  
                void main()               |    void main()
                {                         |    {
                  int a,b,c,d;            |      int a,b,c,d;
                                          |
                  a = 2;                  |      a = 2;
                  b = 5;                  |      b = 5;   /* Dead code */
                                          |
                  c = a;                  |      c = a;
                                          |
                  fn (a, c);              |      fn (a, c);
                }                         |    }

Procedure
----------

        NOTE : Concept of SSA  - Static Single Assignment: In SSA  form each use
               of  a variable  is dominated  by  a single  definition. In  other
               words,  every  variable  in  the  SSA form  will  have  only  one
               difinition.  If  a variable  is  defined  again,  it is  given  a
               different SSA name.

        Example:
                a = 3;
                c = a;
                a = 7;
                c = a; 

        Here the variables 'a' and 'c' are  defined twice, thus have 2 SSA names
        as shown below:

                a_2 = 3;
                c_4 = a_2;
                a_5 = 7;
                c_6 = a_5;  

        a) Create a  data structure (say a  USE-table) for the SSA  variables to
           store the SSA  name and a boolean  to show if the varible  is used of
           not.
        b) You have to identify two kinds  of statements : Assignment and Return
           Identify the  assignment statements by  seeing if the gimple  code of
           the statement is GIMPLE_ASSIGN and the return statements by seeing if
           it is GIMPLE_RETURN.
        c) In case of  a return statement extract the return  variable using the
           function gimple_return_retval()
        d) In  case  of  an  assignment  statement  extract  the  LHS  of  using
           gimple_assign_lhs and store the variable  (if not already present) in
           the USE-table. Since this the  definition of the variable the boolean
           will be  false. Exctract  the RHS  (1 or 2)  of the  statements using
           gimple_assign_rhs1 and gimple_assign_rhs2 and since these are the use
           of these variables mark their corresponding boolean as true.
        e) Traverse the gimple  statements again to remove  the statements whose
           LHS  did not  have  any use  in the  entire  program using  functions
           gsi_remove () and release_defs ().