Back to the Future: The Case for Reversible Computing

Abstract

There is one, and only one way, consistent with fundamental physics, that the efficiency of general digital computation can continue increasing indefinitely, and that is to apply the principles of reversible computing. We need to begin intensive development work on this technology soon if we want to maintain advances in computing and the attendant economic growth NOTE: This paper is an extended author's preprint of the feature article titled "Throwing Computing Into Reverse" 1 (print) or "The Future of Computing Depends on Making it Reversible" 2 (online), published by IEEE Spectrum in Aug.-Sep. 2017. This preprint is based on the original draft manuscript that the author submitted to Spectrum, prior to IEEE edits and feedback from external readers.Since the dawn of the transistor, technologists, and the world at large, have grown accustomed to a steady trend of exponentially-improving performance for information technologies at any given cost level. This performance growth has been enabled by the underlying trend, described by Moore's Law, 3 of the exponentially-increasing number of electronic devices (such as transistors) that can be fabricated on an integrated circuit. According to the classic rules of semiconductor scaling, 4 as transistors were made smaller, they became simultaneously cheaper, faster, and more energy-efficient, a massive win-win-win scenario, which resulted in concordantly massive investments in the ongoing push to advance semiconductor fabrication technology to ever-smaller length scales.Unfortunately, there is today a growing consensus within industry, academia, and government labs that semiconductor scaling has not very much life left; maybe 10 years or so, at best. Multiple issues that come into play as we dive deeper into the nanoscale mean that the classic scaling trends are losing steam. Already, the decreasing logic voltages required due to various short-channel effects resulted in the plateauing of clock speeds more than a decade ago, driving the shift towards today's multi-core architectures. But now, even multi-core architectures face the looming threat of increasing amounts of "dark silicon," 5,6 as heat dissipation constraints prevent us from being able to cram any more operations per second into each unit of chip area, due to the energy that is converted to heat in each operation. Fundamentally, achieving higher performance within a system of any given size, cost, and power budget requires that individual