-----------------------------
a5ss Static Single Assignment
-----------------------------
    
        Now, we are going to take a look at the Static Single Assignment
        form that GCC uses later  during optimization. In SSA form, each
        variable  may  be  assigned  at most  once.  Also,  exactly  one
        assignment of each variable dominates every use of the variable.

        In order to  implement SSA, local variables are  suffixed with a
        unique number which represents  the assignment. For example, two
        assignments to the variable `a' may look like `a_0' and `a_1'.

        Compile  the following  program  (file a5ss.c)  and observe  the
        GIMPLE dumps.

        Source file : a5ss.c
        Compilation : gcc -c -fdump-tree-cfg -fdump-tree-ssa a5ss.c
        View result : vi -O a5ss.c.*.cfg a5ss.c.*.ssa
        Clear dumps : rm -f a5ss.c.* a5ss.o

Program (similar to the loops assignment)
-------

        int main()
        {
            int a, b, n, i;
            for(i=0; i < n; i++)
            {
                if(a > b)
                    a = b + n;
                else
                    a = b * n;
            };
        }


Questions
---------

        1   Do  you  notice  that  each   usage  of  local  variable  as
            an  operand  uses a  suffix  that  can  be tracked  to  it's
            assignment? Why do some variables have a suffix `(D)'?

        2   Find the  point in  the CFG  in which the  paths of  the two
            assignments to variable  `a' ("a = b  + n" and "a =  b * n")
            merge. You will  notice that a PHI  statement is introduced.
            Can you guess the syntax of the PHI statement?

        3   Can you think of an example  in which a PHI node merges more
            than two versions of a variable?