rePLay: A Hardware Framework for Dynamic Optimization

• Authors: Sanjay J. Patel;Steven S. Lumetta
• Venue: IEEE Transactions on Computers
• Year: 2001

In this paper, we propose a new processor framework that supports dynamic optimization. The rePLay Framework embeds an optimization engine atop a high-performance execution engine. The heart of the rePLay Framework is the concept of a frame. Frames are large, single-entry, single-exit optimization regions spanning many basic blocks in the program's dynamic instruction stream, yet containing only a single flow of control. This atomic property of frames increases the flexibilty in applying optimizations. To support frames, rePLay includes a hardware-based recovery mechanism that rolls back the architectural state to the beginning of a frame if, for example, an early exit condition is detected. This mechanism permits the optimizer to make speculative, aggressive optimizations upon frames. In this paper, we investigate some of the underlying phenomenon that support rePLay. Primarily, we evaluate rePLay's region formation strategy. A rePLay configuration with a 256-entry frame cache, using 74KB frame constructor and frame sequencer, achieves an average frame size of 88 Alpha AXP instructions with 68 percent coverage of the dynamic istream, an average frame completion rate of 97.81 percent, and a frame predictor accuracy of 81.26 percent. These results soundly demonstrate that the frames upon which the optimizations are performed are large and stable. Using the most frequently initiated frames from rePLay executions as samples, we also highlight possible strategies for the rePLay optimization engine. Coupled with the high coverage of frames achieved through the dynamic frame construction, the success of these optimizations demonstrates the significance of the rePLay Framework. We believe that the concept of frames, along with the mechanisms and strategies outlined in this paper, will play an important role in future processor architecture.