Isothermality: making speculative optimizations affordable
Date
2007-12-22T22:05:38Z
Authors
Pereira, David John
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Abstract
Partial Redundancy Elimination (PRE) is a ubiquitous optimization used by compilers to remove
repeated computations from programs. Speculative PRE (SPRE), which uses program profiles
(statistics obtained from running a program), is more cognizant of trends in run time behaviour
and therefore produces better optimized programs. Unfortunately, the optimal version of SPRE is
a very expensive algorithm, of high-order polynomial time complexity, and unlike most compiler
optimizations, which run effectively in linear time complexity over the size of the program that they
are optimizing.
This dissertation uses the concept of “isothermality”—the division of a program into a hot region
and a cold region—to create the Isothermal SPRE (ISPRE) optimization, an approximation to
optimal SPRE. Unlike SPRE, which creates and solves a flow network for each program expression
being optimized—a very expensive operation—ISPRE uses two simple bit-vector analyses, optimizing
all expressions simultaneously. We show, experimentally, that the ISPRE algorithm works, on
average, nine times faster than the SPRE algorithm, while producing programs that are optimized
competitively.
This dissertation also harnesses the power of isothermality to empower another kind of ubiquitous
compiler optimization, Partial Dead Code Elimination (PDCE), which removes computations
whose values are not used. Isothermal Speculative PDCE (ISPDCE) is a new, simple, and efficient
optimization which requires only three bit-vector analyses. We show, experimentally, that ISPDCE
produces superior optimization than PDCE, while keeping a competitive running time.
On account of their small analysis costs, ISPRE and ISPDCE are especially appropriate for use in
Just-In-Time (JIT) compilers.
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Keywords
compiler, optimizations, code motion, partial redudancy elimination, partial dead code elimination