Camil Demetrescu edited overview.tex  over 8 years ago

Commit id: c598fb967c6ac2574984a91b546960345f692cb0

deletions | additions      

       

Consider the generic OSR scenario shown in \myfigure\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, which acts as a continuation function. The decision of whether an OSR should be fired at a given point \osrpoint\ of \fbase\ is based on an {\em OSR condition}. A typical example in JIT-based virtual machines is a profile counter reaching a certain hotness threshold, which indicates that \fbase\ is taking longer than expected and is worth optimizing. Another 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.   Serveral Sereral  OSR implementations adjust the stack so that execution can continue in \fvariant\ with the current frame \cite{chambers1992design, suganuma2006region}. 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~\cite{Lameed_2013,webkit14}. 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 and profile-guided compilation strategies. In both cases, \fbase\ is instrumented before its execution to incorporate the OSR machinery. We call such OSR-instrumented version \fosrfrom.