Double Inspection for Run-Time Loop Parallelization

The Inspector/Executor is well-known for parallelizing loops with irregular access patterns that cannot be analyzed statically. The downsides of existing inspectors are that it is hard to amortize their high run-time overheads by actually executing the loop in parallel, that they can only be applied to loops with dependencies that do not change during their execution and that they are often specifically designed for array codes and are in general not applicable in object oriented just-intime compilation. In this paper we present an inspector that inspects a loop twice to detect if it is fully parallelizable. It works for arbitrary memory access patterns, is conservative as it notices if changing data dependencies would cause errors in a potential parallel execution, and most importantly, as it is designed for current multicore architectures it is fast – despite of its double inspection effort: it pays off at its first use. On benchmarks we can amortize the inspection overhead and outperform the sequential version from 2 or 3 cores onward.