Introduction to Compiler Construction in a Java World

Translating JVM Code to MIPS Code 225

loop, the expression a[i] is invariant; that is, it is a loop invariant expression. We deal with

these in the next section.

And as we shall see below, there is still much to be gained from common sub-expression

elimination when it comes to dealing with Java arrays.

Lifting Loop Invariant Code

Loop invariant expressions can be lifted out of the loop and computed in the predecessor

block to the loop header.

For example, consider the j-- code for summing the two matrices from the previous

section

5

.

int i = 0;

while (i <= 999) {

int j = 0;

while (j <= 999) {

c[ i ][ j ] = a [ i ][j ] + b [ i ][ j ];

j = j + 1;;

}

i = i + 1