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A-5 Static Single Assignment
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        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 A-5.c) and observe the dumps.

        Source file : A-5.c
        Compilation : gcc-4.7.2 -c -fdump-tree-cfg -fdump-tree-ssa A-5.c
        View result : vi -O A-5.c.*.cfg A-5.c.*.ssa
        Clear dumps : rm -f A-5.c.* A-5.o

Program
-------

        int main()
        {
            int a, b, c, d;

            d = 10;
            if (c > d) {
                a = 3;
                b = 2;
            } else {
                a = 2;
                b = 3;
            }

            c = a + b;
        }



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'  and `b' merge. You will notice  that PHI statements
            have  been  introduced.  Can  you   guess  the  syntax  of  the  PHI
            statements?

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