Loop-invariant code motion: Difference between revisions

Revision as of 22:07, 9 March 2009

Revision as of 01:29, 10 March 2009

Line 13:

<source lang="C">

whilefor (ji <= maximum0; -i 1< n; i++) {

x = y + z;

{

ja[i] = j6*i + (4+array[k])x*pi+5x;

}

</source>

The calculation ofcalculations <code>maximumx -= 1y + z</code> and <code>(4+array[k])x*pi+5x</code> can be moved outside the loop, and precalculated, resulting in something similar to:

<source lang="C">

int maxvalx = maximumy -+ 1z;

t1 = x*x;

int calcval = (4+array[k])*pi+5;

whilefor (ji = 0; i < maxvaln; i++) {

a[i] = 6*i + t1;

{

j += calcval;

}

</source>

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.

It might then occur to us that the since the loop exit value of <code>j</code> must fall between <code> maxval - calcval</code> and <code>maxval + calcval</code>, or if not it was already beyond that range, then there is no need for the loop at all.

==See also==

* [[Loop optimization]]

* [[Compiler optimization]]

[[Category:Compiler optimizations]]