Revisiting Stack Caches for Energy Efficiency

With the growing focus on energy efficiency, it is important to find ways to reduce energy without sacrificing performance. The L1 data cache is a significant contributor to processor energy consumption. We advocate treating data from the program’s stack differently from non-stack data to reduce energy. We characterize stack accesses to determine how they differ from general memory accesses in terms of footprint, frequency, and ratio of loads to stores. We then propose two ways to optimize for these characteristics. First, the implicit stack cache limits stack data to residing in designated ways of the data cache, reducing the energy required per stack access. We show that it can reduce data cache dynamic energy by 37% with no reduction in performance. Second, the explicit stack cache stores stack data in a separate L1 cache. In addition to reducing the energy per access, it also has additional benefits over the implicit policy in that it can be virtually tagged and have a different writeback policy. We show that this approach can lead to additional energy savings, with no performance impact. These optimizations are implemented purely in the hardware and thus require no changes to existing code.