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B-4 Common Subexpression Elimination
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        Here  is  a  small  program   to  observe  the  optimization  of  Common
        Subexpression Elimination (CSE) or Full Redundancy Elimination (FRE). In
        the  following program  observe  that the  term "a  +  c" appears  twice
        (albeit differently)  and thus can be  calculated once and the  value is
        used next  time when  needed (the redundant  calculation second  time is
        eliminated, thus the term CSE/FRE).

        You must demand -O2 for CSE/FRE.

        Source file : B-4.c
        Compilation : gcc-4.7.2 -c -O2 -fdump-tree-all B-4.c
        View result : vi -O B-4.c.*.ssa B-4.c.*.fre1
        Clear dumps : rm -f B-4.c.* B-4.o

Program
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        int main()
        {
            int a, b, c;
            b = (a + c + b) * (c + a);
            return b;
        }


Questions
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        1   How many  times is the expression  (a + c) computed  in the original
            code (as seen in the SSA pass)? What about after the FRE pass?

        2   Have  there been  any more  optimizations  after FRE?  How will  you
            check?

        3   Why did  we have  to put a  "return b" in  this assignment?  What if
            "return b" is changed to "return 0"? Will it make any difference?