-----------------------------
A-5 Static Single Assignment
-----------------------------

                                             |
int main()                                   |main ()
{                                            |{
    int a, b, c, d, n;                       |  int n;
                                             |  int d;
                                             |  int c;
                                             |  int b;
                                             |  int a;
                                             |
                                             |<bb 2>:
    d = 10;                                  |  d_3 = 10;
    if (c > d) {                             |  if (c_4(D) > d_3)
        a = 2;                               |    goto <bb 3>;
        b = 2;                               |  else
    } else {                                 |    goto <bb 4>;
        a = 2;                               |
        b = 3;                               |<bb 3>:
    }                                        |  a_5 = 2;
                                             |  b_6 = 2;
                                             |  goto <bb 5>;
                                             |
                                             |<bb 4>:
                                             |  a_7 = 2;
                                             |  b_8 = 3;
                                             |
                                             |<bb 5>:
                                             |  # a_1 = PHI <a_5(3), a_7(4)>
                                             |  # b_2 = PHI <b_6(3), b_8(4)>
    c = a + b;                               |  c_9 = a_1 + b_2;
                                             |  return;
                                             |
}                                            |}

Solutions
---------

        1   Local variables are  suffixed with a unique  number which represents
            the assignment. For  example, `d_3' refers to the  usage of variable
            `d'  and  the  only  assignment  which reaches  this  point  is  the
            assignment `d_3'.  Variables that are  likely to be used  before any
            assignment are suffixed  with `(D)' to indicate  that the assignment
            was implicit in the declaration.  This happens to both uninitialized
            local variables and formal parameters.  For example, variable `c' is
            used as `c_4(D)' in the `if' condition.

        2   A PHI node merges assignments from different paths and creates a new
            assignment. For example:

                # a_1 = PHI <a_5(3), a_7(4)> 

            indicates that if  the control reaches here from basic  block 3 then
            use the  value of a_5;  otherwise if  the control reaches  here from
            basic block 4 then use the value of a_7.

        3   A switch statement can result in a merge of more than two paths (and
            hence possibly more than two assignments). Consider:

            int main()
            {
                int a, b;
                switch (a)
                {
                    case 1:
                        b = 2;
                        break;
                    case 2:
                        b = 3;
                        break;
                    case 3:
                        b = 4;
                    case 4:
                        b = 5;
                        break;
                    case 5:
                    case 6:
                        b = 6;
                        break;
                    default:
                        b = 7;
                }
                return b;
            }
            
            In this case  the PHI node at  the end of the  switch statement will
            merge multiple assignments such as:

            # b_1 = PHI <b_4(3), b_5(4), b_7(6), b_8(7), b_9(8)>