Editing Loop-invariant code motion (section)

==Example==
In the following code sample, two optimizations can be applied.

<syntaxhighlight lang="C">
int i = 0;
while (i < n) {
    x = y + z;
    a[i] = 6 * i + x * x;
    ++i;
}
</syntaxhighlight>

Although the calculation <code>x = y + z</code> and <code>x * x</code> is loop-invariant, precautions must be taken before moving the code outside the loop. It is possible that the loop condition is <code>false</code> (for example, if <code>n</code> holds a negative value), and in such case, the loop body should not be executed at all. One way of guaranteeing correct behaviour is using a conditional branch outside of the loop. Evaluating the loop condition can have [[Side effect (computer science)|side effects]], so an additional evaluation by the <code>if</code> construct should be compensated by replacing the <code>while</code> loop with a <code>[[Do while loop|do {} while]]</code>. If the code used <code>do {} while</code> in the first place, the whole guarding process is not needed, as the loop body is guaranteed to execute at least once.

<syntaxhighlight lang="C">
int i = 0;
if (i < n) {
    x = y + z;
    int const t1 = x * x;
    do {
        a[i] = 6 * i + t1;
        ++i;
    } while (i < n);
}
</syntaxhighlight>

This code can be optimized further. For example, [[strength reduction]] could remove the two multiplications inside the loop (<code>6*i</code> and <code>a[i]</code>), and [[induction variable]] elimination could then elide <code>i</code> completely. Since <code>6 * i</code> must be in lock step with <code>i</code> itself, there is no need to have both.