Advancements in branch predictors have allowed modern processors to aggressively speculate and gain significant performance with every generation of increasing out-of-order depth and width. Unfortunately, there are branches that are still hard-to-predict (H2P) and mis-speculation on these branches is severely limiting the performance scalability of future processors. One potential solution to mitigate this problem is to predicate branches by substituting control dependencies with data dependencies. Predication is very costly for performance as it inhibits instruction level parallelism. To overcome this limitation, prior works selectively applied predication at run-time on H2P branches that have low confidence of branch prediction. However, these schemes do not fully comprehend the delicate trade-offs involved in suppressing speculation and can suffer from performance degradation on certain workloads. Additionally, they need significant changes not just to the hardware but also to the compiler and the instruction set architecture, rendering their implementation complex and challenging.In this paper, by analyzing the fundamental trade-offs between branch prediction and predication, we propose Auto-Predication of Critical Branches (ACB) - an end-to-end hardware-based solution that intelligently disables speculation only on branches that are critical for performance. Unlike existing approaches, ACB uses a sophisticated performance monitoring mechanism to gauge the effectiveness of dynamic predication, and hence does not suffer from performance inversions. Our simulation results show that, with just 386 bytes of additional hardware and no software support, ACB delivers 8% performance gain over a baseline similar to the Skylake processor. We also show that ACB reduces pipeline flushes because of mis-speculations by 22%, thus effectively helping both power and performance.

Branch prediction accuracy plays a dominant role in the performance provided by modern Out-of-Order(OOO) superscalar processors. While global history-based branch predictors are more popular, local history-based predictors offer an additional dimension towards enhancing the overall branch prediction accuracy. Integrating the local predictors in modern cores, though, comes with non-trivial challenges associated with managing the local predictor's state and repairing this state on any branch misprediction is essential for the local predictor to operate effectively. Using a highly accurate, industry standard simulator modeling a Skylake-like OOO core and workloads spanning diverse categories including Server, High Performance Computing (HPC) and personal computing suites, besides SPEC, we methodically highlight the issues that need to be tackled, why local predictor repair is non-trivial and the performance opportunity that is lost when the local predictor repair is not handled efficiently. We discuss the issues with prior techniques and quantify their limitations when using them in current OOO cores. Further, we propose three practical, implementable and efficient repair techniques with minimal storage requirements that provide significant performance gains for local predictors. Unlike prior repair techniques that can only attain 50% of the oracular gains, our realistic repair techniques retain about 80% of the oracular gains resulting in significantly better application performance.