Room 206 (2nd floor, badged access)
2 March 2023 - 14h00
Exploring Instruction Fusion Opportunities in General Purpose Processors
by Arthur Perais from TIMA
Abstract: The Complex Instruction Set Computer (CISC) paradigm has led to the introduction of instruction cracking in which an architectural instruction is divided into multiple microarchitectural instructions (µ-ops). However, the dual concept, instruction fusion is also prevalent in modern microarchitectures to maximize resource utilization. In essence, some architectural instructions are too complex to be executed as a unit, so they should be cracked, while others are too simple to waste resources on executing them as a unit, so they should be fused with others.
In this paper, we focus on instruction fusion and explore opportunities for fusing additional instructions in a high-performance general purpose pipeline. We show that enabling fusion for common RISC-V idioms improves performance by 7%. Then, we determine experimentally that enabling fusion only for memory instructions achieves 86% of the potential of fusion in this particular case. Finally, we propose the Helios microarchitecture, able to fuse non-consecutive and non-contiguous memory instructions, and discuss microarchitectural changes required to do so efficiently while preserving correctness. Helios allows to fuse an additional 5.5% of dynamic instructions, yielding a 14.2% performance uplift over no fusion (8.2% over baseline fusion).