Daniele Cono D'Elia edited experim.tex  over 8 years ago

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\end{itemize}  \paragraph{Impact on code quality.}  In order to measure how much a never-firing OSR point might impact code quality, we analyzed the source-code structure of each benchmark and profiled its run-time behavior to identify performance-critical sections for OSR point insertion. The distinction between open and resolved OSR points is irrelevant in this context, as generated code is nearly identical: the only difference is that for an open OSR the call instruction triggering the transition takes an additional argument (a {\tt null} profiling value). We thus focus on open OSR only.  For iterative benchmarks, we insert an OSR point in the body of their hottest loops. We classify a loop as hottest when its body is executed for a very high cumulative number of iterations (e.g., from a few thousands up to billions) and it either calls the method with the highest {\em self} time in the program, or it performs the most computational-intensive operations for the program in its own body. These loops are natural candidates for OSR point insertion: for instance, the Jikes RVM inserts yield points on backward branches to trigger operations such as method recompilation through OSR and thread preemption for garbage collection. In the \shootout\ benchmarks, the number of such loops is typically 1 (2 for {\tt spectral-norm}).  For {\tt b-trees} - the only benchmark showing a recursive pattern - we insert an OSR point in the body of the method that accounts for the largest {\em self} execution time of the program. Such an OSR point might be useful to trigger recompilation of the code at a higher degree of optimization, or to enable some form of dynamic optimization (for instance, in a recursive search algorithm we might want to inline the comparator method provided by the user at the call).  Results for the unoptimized and optimized versions of the benchmarks are reported in \myfigure\ref{fig:code-quality-base} and \myfigure\ref{fig:code-quality-O1}, respectively. For both scenarios we observe that the overhead is very small, i.e. less than $1\%$ for most benchmarks and less than $2\%$ in the worst case. For some benchmarks, code might run slightly faster after OSR point have been inserted insertion  due to instruction cache effects. We analyzed the code produced by the x86\_64 back-end: the OSR machinery is lowered into three native instructions that load a counter in a register, compare it against a constant value and jump to the OSR block accordingly. The number of times the OSR condition is checked for each benchmark is the same as in the experiments reported in \mytable\ref{tab:sameFun}.   \paragraph{Overhead of OSR transitions.}