Loop-invariant code motion: Difference between revisions

Revision as of 00:44, 27 November 2008

Revision as of 00:50, 27 November 2008

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In [[computer programming]], '''loop-invariant code''' consists of statements (in an [[imperative programming|imperative]] [[programming language]]) which can be moved outside the body of a loop without affecting the semantics of the program. As a result it is executed less often, providing a speedup. Another effect of this transformation is allowing constants to storebe constantsstored in registers and not having to calculate the address and access the memory/ (or cache line) at each iteration.

'''Loop-invariant code motion''' (also called '''hoisting''' or '''scalar promotion''') is a [[compiler optimization]] which performs this movement automatically. However, if too many variables are created, there will be high [[register pressure]], especially on processors with few registers, like the 32-bit [[x86]]. If the compiler runs out of registers, some variables will be [[register spilling|spilled]]. To counteract this, the “opposite” optimization can be performed, [[rematerialization]].