This paper presents a novel hardware-based approach for identifying, profiling, and monitoring hot spots in order to support runtime optimization of general purpose programs. The proposed approach consists of a set of tightly coupled hardware tables and control logic modules that are placed in the retirement stage of a processor pipeline removed from the critical path. The features of the proposed design include rapid detection of program hot spots after changes in execution behavior, runtime-tunable selection criteria for hot spot detection, and negligible overhead during application execution. Experiments using several SPEC95 benchmarks, as well as several large WindowsNT applications, demonstrate the promise of the proposed design.
This paper presents a new mechanism for collecting and deploying runtime optimized code. The code-collecting component resides in the instruction retirement stage and lays out hot execution paths to improve instruction fetch rate as well as enable further code optimization. The code deployment component uses an extension to the Branch Target Buffer to migrate execution into the new code without modifying the original code. No significant delay is added to the total execution of the program due to these components. The code collection scheme enables safe runtime optimization along paths that span function boundaries. This technique provides a better platform for runtime optimization than trace caches, because the traces are longer and persist in main memory across context switches. Additionally, these traces are not as susceptible to transient behavior because they are restricted to frequently executed code. Empirical results show that on average this mechanism can achieve better instruction fetch rates using only 12KB of hardware than a trace cache requiring 15KB of hardware, while producing long, persistent traces more suited to optimization.
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