Loop-invariant code motion: Difference between revisions

Revision as of 23:15, 29 April 2019

Revision as of 07:32, 11 October 2019

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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. It should also be noted that evaluatingEvaluating 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</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.

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