Filtering techniques to improve trace-cache efficiency

Abstract

The trace cache is becoming an important building block of modern, wide-issue, processors. The paper has three main contributions: it indicates that trace cache optimizations directed to reducing power consumption are do not necessarily coincide with optimisations directed to increasing fetch bandwidth; it extends our understanding on how well the trace cache utilizes its resources and introduces a new trace-cache organization based on filtering techniques. We observe that: (1) the majority of traces that are inserted into the trace-cache are rarely used again before being replaced; (2) the majority of the instructions delivered for execution originate from the fewer traces that are heavily and repeatedly used; and that (3) techniques that aim to improve instruction fetch bandwidth may increase the number of traces built during program execution. Based on these observations, we propose splitting the trace cache into two components: the filter trace-cache (FTC) and the main trace-cache (MTC). The FTC/MTC organization exhibits an important benefit: it decreases the number of traces built, thus reducing power consumption while improving overall performance. An extension of the filtering concept involves adding a second level (L2) trace-cache that stores less frequent traces that are replaced in the FTC or the MTC. The extra level of caching allows for order-of-magnitude reduction in the number of trace builds. Second level trace cache proves particularly useful for applications with large instruction footprints.