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\subsection{Overview}  \label{ss:overview}  Consider the generic OSR scenario shown in Figure~\ref{fi:osr-dynamics}. \ifauthorea{Figure~}{}\ref{fi:osr-dynamics}.  A base function \fbase\ is executed and it can either terminate normally (dashed lines), or an OSR event may transfer control to a variant \fvariant. The decision of whether an OSR should be fired at a given point \textsf{L} of \fbase\ is based on an {\em OSR condition}. A typical example is a guard testing whether \fbase\ has become unsafe and execution needs to fall back to a safe version \fvariant. This scenario includes deoptimization of functions generated with aggressive speculative optimizations. Another example is a profile counter reaching a certain hotness threshold, which indicates that \fbase\ is taking longer than expected and is worth optimizing. This is a frequent scenario in JIT-based virtual machines. Classical OSR implementations adjust the stack so that execution can continue in \fvariant\ with the current frame \mynote{add citations}. This requires manipulating the program state at machine code level and is highly ABI- and compiler-dependent. A simpler approach, which we follow in this article, consists of creating a new frame every time an OSR is fired, essentially regarding an OSR transition as a function call \mynote{cite WebKit and McVM}.   Our implementation targets two general scenarios: 1) {\em resolved OSR}: \fvariant\ is known before executing \fbase\ as in the deoptimization example discussed above; 2) {\em open OSR}: \fvariant\ is generated when the OSR is fired, supporting deferred compilation strategies. In both cases, \fbase\ is instrumented before its execution to incorporate the OSR machinery. We call such OSR-instrumented version \fosrfrom.  In the resolved OSR scenario (see Figure~\ref{fi:overview-osr-final}), \ifauthorea{Figure~}{}\ref{fi:overview-osr-final}),  instrumentation consists of adding a check of the OSR condition and, if it is satisfied, a tail call that fires the OSR. The called function is an instrumented version of \fvariant, which we call \fosrto. The assumption is that \fosrto\ produces the same side-effects and return value that one would obtain by \fbase\ if no OSR was performed. Differently from \fvariant, \fosrto\ takes as input all live variables of \fbase\ at \textsf{L}, executes an optional compensation code to fix the computation state ({\tt comp\_code}), and then jumps to a point \textsf{L'} from which execution can continue. The OSR practice often makes the conservative assumption that execution can always continue with the very same program state as the base function. However, this assumption may reduce the number of points where sound OSR transitions can be fired. Supporting compensation code in our framework adds flexibility, allowing OSR transitions to happen at arbitrary places in the base function. %\ref{fi:overview-osr-final} 
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