Loop-invariant code motion: Difference between revisions

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{{short description|Type of compiler optimization}}

{{More citations needed|date=January 2021}}

In [[computer programming]], '''[[loop-invariant code]]''' consists of statements or expressions (in an [[imperative programming|imperative]] [[programming language]]) whichthat can be moved outside the body of a loop without affecting the semantics of the program. '''Loop-invariant code motion''' (also called '''hoisting''' or '''scalar promotion''') is a [[compiler optimization]] whichthat performs this movement automatically.

==Example==

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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. 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.

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For example, if all reaching definitions for the operands of some simple expression are outside of the loop, the expression can be moved out of the loop.

Recent work usingby Moyen, Rubiano and [[Thomas Seiller|Seiller]] uses data-flow dependence analysis <ref>{{cite journalbook |last1=Moyen |first1=Jean-Yves |last2=Rubiano |first2=Thomas |last3=Seiller |first3=Thomas |titlechapter=Loop Quasi-Invariant Chunk Detection |journaltitle=Automated Technology for Verification and Analysis |series=Lecture Notes in Computer Science |date=2017 |volume=10482 |pages=91–108 |doi=10.1007/978-3-319-68167-2_7|isbn=978-3-319-68166-5 }}</ref> allows to detect not only invariant commands but larger code fragments such as an inner loop. The analysis also detects quasi-invariants of arbitrary degrees, that is commands or code fragments that become invariant after a fixed number of iterations of the loop body. This technique was later used by Aubert, Rubiano, Rusch, and [[Thomas Seiller|Seiller]] to automatically parallelise loops.<ref>{{cite book |last1=Aubert |first1=Clément |last2=Rubiano |first2=Thomas

|last3=Rusch |first3=Neea |last4=Seiller |first4=Thomas |chapter= Distributing and Parallelizing Non-canonical Loops |title= Verification, Model Checking, and Abstract Interpretation |series=Lecture Notes in Computer Science |date=2023 |volume=13881 |pages=91–108 |doi=10.1007/978-3-031-24950-1_1 }}</ref>

==Benefits==

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==See also==

* [[Code motion]]

* [[Loop invariant]]