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Modifying the 'middle end' of a popular compiler yields more-efficient parallel programs

Compilers are programs that convert computer code written in high-level languages intelligible to humans into low-level instructions executable by machines. But there's more than one way to implement a given computation, and modern compilers extensively analyze the code they process, trying to deduce the implementations that will maximize the efficiency of the resulting software. Code explicitly written to take advantage of parallel computing, however, usually loses the benefit of compilers' optimization strategies. That's because managing parallel execution requires a lot of extra code, and existing compilers add it before the optimizations occur. The optimizers aren't sure how to interpret the new code, so they don't try to improve its performance. At the Association for Computing Machinery's Symposium on Principles and Practice of Parallel Programming next week, researchers from MIT's Computer Science and Artificial Intelligence Laboratory will present a new variation on a popular open-source compiler that optimizes before adding the code necessary for parallel execution. As a consequence, says Charles E. Leiserson, the Edwin Sibley Webster Professor in Electrical Engineering and Computer Science at MIT, the compiler "now optimizes parallel code better than any commercial or open-source compiler, and it also compiles where some of these other compilers don't." That improvement comes purely from optimization strategies that were already part of the compiler the researchers modified, which was designed to compile conventional, serial programs. The researchers' approach should also make it much more straightforward to add optimizations specifically tailored to parallel programs. And that will be crucial as computer chips add more and more "cores," or parallel processing units, in the years ahead. Read more at

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