Improving Processor Efficiency Through Enhanced Instruction Fetch

Instruction fetch is an important pipeline stage for embedded processors, as it can consume a significant fraction of the total processor energy. This dissertation describes the design and implementation of two new fetch enhancements that seek to improve overall energy efficiency without any performance tradeoff. Instruction packing is a combination architectural/compiler technique that leverages code redundancy to reduce energy consumption, code size, and execution time. Frequently occurring instructions are placed into a small instruction register file (IRF), which requires less energy to access than an L1 instruction cache. Multiple instruction register references are placed in a single packed instruction, leading to reduced cache accesses and static code size. Hardware register windows and compiler optimizations tailored for instruction packing yield greater reductions in fetch energy consumption and static code size. The Lookahead Instruction Fetch Engine (LIFE) is a microarchitectural technique designed to exploit the regularity present in instruction fetch. The nucleus of LIFE is the Tagless Hit Instruction Cache (TH-IC), a small cache that assists the instruction fetch pipeline stage as it efficiently captures information about both sequential and non-sequential transitions between instructions. TH-IC provides a considerable savings in fetch energy without incurring the performance penalty normally associated with small filter instruction caches. Furthermore, TH-IC makes the common case (cache hit) more energy efficient by making the tag check unnecessary. LIFE extends TH-IC by making use of advanced control flow metadata to further improve utilization of fetch-associated structures such as the branch predictor, branch target buffer, and return address stack. LIFE enables significant reductions in total processor energy consumption with no impact on application execution times even for the most aggressive power-saving configuration. Both IRF and LIFE (including TH-IC) improve overall processor efficiency by actively recognizing and exploiting the common properties of instruction fetch.