An Adiabatic Framework for a Low Energy µ-Architecture & Compiler

Adiabatic process in thermodynamics transfers energy across zero temperature difference. The adiabatic CMOS design style attempts to switch a transistor to transfer energy across its source and drain while the voltage difference is zero. We define an adiabatic microarchitecture that pushes instructions across zero IPC gradient. The IPC gradient can be zero across time: for the same stage IPC over time does not vary, or across space: adjacent pipeline stages have zero variance. The reason to consider adiabatic microarchitectures is that the energy for a given computation can be shown to be minimum for an adiabatic microarchitecture. An adiabatic compiler, really a backend, is defined to be a compiler to support an adiabatic microarchitecture achieve its goals. The minimal support provided by an adiabatic compiler includes a static estimation of program ILP. We add new passes to the MachSUIF compiler, to flag instruction groups that can potentially walk through a superscalar pipeline as a group. Hence, these instruction groups offer a fairly robust model of superscalar microarchitecture ILP. A compile time scheduling analysis can also generate instruction slack values. The slack indicates the program region within which an instruction can be scheduled. We also present a dispatch stage dynamic scheduling algorithm that utilizes the compiler annotated slacks to reschedule instructions with the explicit objective of minimizing the dispatch stage IPC variance. In other words, the proposed dispatch stage is adiabatic. Preliminary experimental results demonstrate an average reduction of 4.16% in IPC variance over SPEC2000 benchmarks with the adiabatic compiler and microarchitecture. The preliminary evaluation also shows the average processor dispatch stage energy reduction of 3.9% over the same SPEC2000 benchmarks. We expect to add similar IPC smoothening control knobs at instruction fetch and issue stages as well in the future, which should result in a more significant energy reduction.