Criticality Driven Fetch

Abstract

Modern OoO cores achieve high levels of performance using large instruction windows. Scaling the window size improves performance by making visible more of the parallelism present in programs. However, this leads to an exponential increase in area and power. We specify Criticality Driven Fetch (CDF), a new execution paradigm that preferentially fetches, allocates, and executes instructions on the critical path of the program. By skipping over non-critical instructions, critical instructions in the ROB can span a sequential instruction window larger than the size of the ROB. This increases the amount of parallelism that can be extracted from critical instructions, thereby improving performance. In our implementation, CDF improves performance by (a) increasing the MLP for independent loads executing concurrently, (b) fetching critical path loads past hard-to-predict branches (by resolving them earlier), and (c) by initiating last level cache misses that cannot be parallelized earlier. Accelerating critical loads using CDF achieves a 6.1% IPC improvement over a baseline OoO core with prefetching. Compared to Precise Runahead, the prior state of the art work on accelerating last level cache misses on the core, we provide better performance and reduce memory traffic and energy consumption by 4.0% and 7.2% respectively.