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Camil Demetrescu edited osr-llvm.tex
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%\subsection{Example}
In this section we discuss how to implement the OSR approach of \mysection\ref{se:overview} in LLVM. Our discussion is based on a simple example that illustrates a profile-driven optimization scenario. We start from a simple base function ({\tt isord}) that checks whether an array of numbers is ordered according to some criterion specified by a comparator (see \myfigure\ref{fi:isord-example}). Our goal is to instrument {\tt isord} so that, whenever the number of loop iterations exceeds a certain threshold, control is dynamically diverted to a faster version generated on the fly by inlining the comparator.
The IR code shown in this section has been generated with \clang\ and instrumented with \osrkit, a library we prototyped to help VM builders implement OSR in LLVM\footnote{Virtual register names and labels in the LLVM-produced IR code shown in this paper have been refactored to make the code more readable.}.
\osrkit\ provides a number of useful abstractions that include open and resolved OSR instrumentation of base functions, liveness analysis, generation of OSR continuation functions, and mapping of LLVM values between different versions of a program along with compensation code generation.
%To explain how the OSR approach of \mysection\ref{se:overview} can be implemented in LLVM, we consider the simple example of \myfigure\ref{fi:isord-example}. Function {\tt isord} checks whether an array of numbers is ordered according to some criterion specified by a comparator.