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b1cs 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 : b1cs.c
        Compilation : gcc -c -O2 -fdump-tree-all b1cs.c
        View result : vi -O b1cs.c.*.ssa b1cs.c.*.fre; rm -f b1cs.c.* b1cs.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?