Loop-invariant code motion: Difference between revisions

Revision as of 22:06, 20 April 2005

Revision as of 15:04, 7 May 2005

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'''Loop-invariant code''' in an imperative [[Computer programming|programming]] language consists of statements which could be moved to before the loop (if the loop always terminates), or after the loop, without affecting the semantics of the program. As a result it is executed less often, providing a speedup. '''Loop-invariant code motion''' is a [[compiler optimization]] which performs this movement automatically.

==Worked Example==

If we consider the code sample, two optimization possibilites can be applied.

<code><pre>

while (i < maximum - 1)

{

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

}</pre></code>

The calculation of <pre>maximum - 1</pre> and <pre>(4+array[k])*pi+5</pre> can be moved outside the loop, and precalculated, resulting in something similar to:

<code><pre>

int maxval = maximum - 1;

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

while (i < maxval)

{

j = j + calcval;

}</pre></code>

{{compu-stub}}

[[Category: Compiler optimizations]]