10.1.3 Local Common Subexpression Elimination

Constants are produced, not only by the programmer, but also by the compiler. The programmer may initialize a variable to be 1, using this as both the loop counter and as an array index. Then, in allocating space for the array, the compiler may multiply such elements by 4 to indicate that they are to use four bytes of storage.

EXAMPLE 3 Local constant propagation

        I := 2                     I := 2
        ...                        ...

        T1 := 4 * I                T1 := 4 * 2

A related optimization computes constant expressions at compile-time rather than generating code to perform the optimization.

EXAMPLE 4 Local constant folding



        I := 2                     I := 2
          ...                     ...

          T1 := 4 * I              T1 := 8

Constant folding is sometimes called constant computation. It can require a little more alertness on the part of the compiler, as shown in Example 5.

EXAMPLE 5 Local constant folding

   I := 2 * I            I := 8 * I
   (no reference to I)
   I := 4 * I

Here, I has been modified twice.

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