Camil Demetrescu edited approach.tex  over 8 years ago

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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}), instrumentation consists of adding a check of the OSR condition and, if it is satisfied, a function call that fires the OSR, followed by a return statement that prevents the base function from performing further computations after an OSR. The invoked 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 were was  performed. Differently from \fvariant, \fosrto\ takes as input all live variables of \fbase\ at \textsf{L}, performs an optional compensation code ({\tt comp_code}) to fix the computation state, 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}