From sequences of dependent instructions to functions: an approach for improving performance without ILP or speculation

In this article, we present an approach for improving the performance of sequences of dependent instructions. We observe that many sequences of instructions can be interpreted as functions. Unlike sequences of instructions, functions can be translated into very fast but exponentially costly two-level combinational circuits. We present an approach that exploits this principle, speeds up programs thanks to circuit-level parallelism/redundancy, but avoids the exponential costs. We analyze the potential of this approach, and then we propose an implementation that consists of a superscalar processor with a large specific functional unit associated with specific back-end transformations. The performance of the SpecInt2000 benchmarks and selected programs from the Olden and MiBench benchmark suites improves on average from 2.4% to 12% depending on the latency of the functional units, and up to 39.6%; more precisely, the performance of optimized code sections improves on average from 3.5% to 19%, and up to 49%.