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\fi  %\subsection{Example}  In this section we discuss one possible embodiment of the OSR approach of \mysection\ref{se:overview} in LLVM. Our discussion is based on a simple running 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 on-the-fly  by inlining the comparator. The IR code shown in this section\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.} has been generated with \clang\ and instrumented with \osrkit, a library we prototyped to help VM builders implement deploy  OSR in LLVM. \osrkit\ provides a number of useful abstractions that include open and resolved OSR instrumentation of IR base functions without breaking the SSA form, liveness analysis, generation of OSR continuation functions, and mapping of LLVM values between different versions of a program along with compensation code generation\footnote{The accompanying artifact allows the interested reader to get acquainted with \osrkit\ and repeat the sample scenario described in this section.}. %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. 
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\paragraph{OSR Instrumentation in IR.}  To defer the compilation of the continuation function until the comparator is known at run time, we used \osrkit\ to instrument {\tt isord} with an open OSR point at the beginning of the loop body, as shown in \myfigure\ref{fig:isordfrom}. Portions added to the original code by OSR istrumentation are highlighted in grey.  %The figure illustrates how the original {\tt isord} code is instrumented by \tinyvm, highlighting in grey the added portions.   A new basic block is New instructions are  placed at the beginning of the loop body, which body to  increments a hotness counter {\tt p.osr} and jumps jump  to an OSR-firing block if the counter reaches the threshold (1000 iterations in this example). The OSR block contains a tail call to the target generation stub, which receives as parameters the four live variables at the OSR point ({\tt v}, {\tt n}, {\tt i}, {\tt c}). \osrkit\ allows the stub to receive the run-time value {\tt val} of an IR object that can be used to produce the continuation function -- in our example, the pointer to the comparator function to be inlined.Notice that maintaining the SSA form requires \osrkit\ to adjust $\phi$-nodes.  The stub (see \myfigure\ref{fig:isordstub}) calls a code generator that: 1) builds an optimized version of {\tt isord} by inlining the comparator, and 2) uses it to create the continuation function {\tt isordto} shown in \myfigure\ref{fig:isordascto}. The stub passes to the code generator four parameters that are hard-coded by \osrkit: parameters:  1) a pointer to the {\tt isord} IR code, 2) a pointer to the basic block in {\tt isord} from which the OSR is fired, 3)the stub's {\tt val} parameter, and 4)  a user-defined object to support code generation in MCJIT. MCJIT, and 4) the stub's {\tt val} parameter (the first three are hard-wired by \osrkit).  The stub terminates with a tail call to {\tt isordto}. To generate the continuation function from the optimized version created by the inliner, \osrkit\ replaces the function entry point, removes dead code, replaces live variables with the function parameters, and fixes $\phi$-nodes accordingly. Additions resulting from the IR instrumentation are in grey, while removals are struck-through. \ifdefined\noauthorea  \begin{figure}[t]