Section: Software

Parwiz: a dynamic dependency analyser

Participant : Alain Ketterlin.

We have developed a dynamic dependence analyzer. Such a tool consumes the trace of memory (or object) accesses, and uses the program structure to list all the data dependences appearing during execution. Data dependences in turn are central to the search for parallel sections of code, with the search for parallel loops being only a particular case of the general problem. Most current works of these questions are either specific to a particular analysis (e.g., computing dependence densities to select code portions for thread-level speculation), or restricted to particular forms of parallelism (e.g., typically to fully parallel loops). Our tool tries to generalize existing approaches, and focuses on the program structures to provide helpful feedback either to a user (as some kind of “smart profiler”), or to a compiler (for feedback-directed compilation). For example, the tool is able to produce a dependence schema for a complete loop nest (instead of just a loop). It also targets irregular parallelism, for example analyzing a loop execution to estimate the expected gain of parallelization strategies like inspector-executor.

We have developed this tool in relation to our minimal profiling research project. However, the tool itself has been kept independent of our profiling infrastructure, getting data from it via a well-defined trace format. This intentional design decision has been motivated by our work on distinct execution environments: first on our usual x86-64 benchmark programs, and second on less regular, more often written in Java, real-world applications. The latter type of applications is likely the one that will most benefit from such tools, because their intrinsic execution environment does not offer enough structure to allow effective static analysis techniques. Parallelization efforts in this context will most likely rely on code annotations, or specific programming language constructs. Programmers will therefore need tools to help them choose between various constructs. Our tool has this ambition. We already have a working tool-chain for C/C++/Fortran programs (or any binary program). We are in the process of developing the necessary infrastructure to connect the dynamic dependence profiler to instrumented Java programs. Other managed execution environments could be targeted as well, e.g., Microsoft's .Net architecture, but we have no time and/or workforce to devote to such time-consuming engineering efforts.