As a step torward a viable, single-issue out-of-order soft core, this work presents Copy-Free Checkpointing (CFC), an FPGA-friendly register renaming design. CFC supports speculative execution by implementing checkpoint recovery. Compared against the best conventional register renaming implementation CFC requires 7.5x to 6.4x fewer LUTs and is at least 10% faster.

Soft processors often use data caches to reduce the gap between processor and main memory speeds. To achieve high efficiency, simple, blocking caches are used. Such caches are not appropriate for processor designs such as Runahead and out-of-order execution that require nonblocking caches to tolerate main memory latencies. Instead, these processors use non-blocking caches to extract memory level parallelism and improve performance. However, conventional non-blocking cache designs are expensive and slow on FPGAs as they use content-addressable memories (CAMs). This work proposes NCOR, an FPGA-friendly non-blocking cache that exploits the key properties of Runahead execution. NCOR does not require CAMs and utilizes smart cache controllers. A 4 KB NCOR operates at 329 MHz on Stratix III FPGAs while it uses only 270 logic elements. A 32 KB NCOR operates at 278 Mhz and uses 269 logic elements.

This work introduces a new branch predictor design that increases the perceived predictor capacity without increasing its delay by using a large virtual second-level table allocated in the second-level caches. Virtualization is applied to a state-of-theart multi-table branch predictor. We evaluate the design using instruction count as proxy for timing on a set of commercial workloads. For a predictor whose size is determined by access delay constraints, accuracy can be improved by 8.7%. Alternatively, the design can be used to achieve the same accuracy as a non-virtualized design while using 25% less dedicated storage.

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Abstract-Soft processors often use data caches to reduce the gap between processor and main memory speeds. To achieve high efficiency, simple, blocking caches are used. Such caches are not appropriate for processor designs such as runahead and out-of-order execution that require non-blocking caches to tolerate main memory latencies. Conventional nonblocking caches are expensive and slow on FPGAs as they use content-addressable memories (CAMs). This work exploits key properties of runahead execution and demonstrates an FPGA-friendly non-blocking cache design that does not require CAMs. A non-blocking 4KB cache operates at 329MHz on Stratix III FPGAs while it uses only 270 logic elements. A 32KB non-blocking cache operates at 278Mhz and uses 269 logic elements.