Speculative Code Compaction: Eliminating Dead Code via Speculative Microcode Transformations

The computing landscape has been increasingly characterized by processor architectures with increasing core counts, while a majority of the software applications remain inherently sequential. Although state-of-the-art compilers feature sophisticated optimizations, a significant chunk of wasteful computation persists due to the presence of data-dependent operations and irregular control-flow patterns that are unpredictable at compile-time. This work presents speculative code compaction (SCC), a novel microarchitectural technique that significantly enhances the capabilities of the microcode engine to aggressively and speculatively eliminate dead code from hot code regions resident in the micro-op cache, and further generate a compact stream of micro-ops, based on dynamically predicted machine code invariants. SCC also extends existing micro-op cache designs to co-host multiple versions of unoptimized and speculatively optimized micro-op sequences, providing the fetch engine with significant flexibility to dynamically choose from and stream the appropriate set of micro-ops, as and when deemed profitable.SCC is a minimally-invasive technique that can be implemented at the processor front-end using a simple ALU and a register context table, and is yet able to substantially accelerate the performance of already compile-time optimized and machine-tuned code by an average of 6% (and as much as 30%), with an average of 12% (and as much as 24%) savings in energy consumption, while eliminating the need for profiling and offering increased adaptability to changing datasets and workload patterns.