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.