Better Branch Prediction Through Prophet/Critic Hybrids A. Falcón, J. Stark, A. Ramirez, K. Lai, M. Valero Paper Presentation and Discussion

Processor Design Objectives Higher performance Higher performance Lower power Lower power Lower energy Lower energy

Better Branch Prediction Increases performance through less time spent speculating on mispredicted paths Increases performance through less time spent speculating on mispredicted paths Reduces power through lower processor frequency Reduces power through lower processor frequency Reduces energy consumption through less work wasted on misspecualtion Reduces energy consumption through less work wasted on misspecualtion

Branch Predictors = Taxi Drivers ! Taxi = Processor Driver = Branch Predictor Passenger = Pipeline Intersections = Control Branches

Branch Predictors = Taxi Drivers ! Wrong turns waste passenger time Therefore need to make less mispredictions or Lower misprediction rate

Prophet/Critic Hybrid Predictors Driver is the Prophet Driver is the Prophet Another co-driver in the backseat of the Taxi is the Critic Another co-driver in the backseat of the Taxi is the Critic Critic waits until sure that they are lost (branch misprediction) Critic waits until sure that they are lost (branch misprediction) Critic points out the mistake and they backtrack to the wrongly taken intersection (branch) Critic points out the mistake and they backtrack to the wrongly taken intersection (branch)

Related Techniques McFarling first proposed two component predictors and a selection mechanism McFarling first proposed two component predictors and a selection mechanism Jiménez et al. proposed two predictors different in their accuracy, size, and latency Jiménez et al. proposed two predictors different in their accuracy, size, and latency Grunwald et al. shows that using a confidence estimator history register improves speculation control, uses on future bit. Grunwald et al. shows that using a confidence estimator history register improves speculation control, uses on future bit.

Prophet/Critic Better ! No need for selection mechanism No need for selection mechanism Critic uses branch future bits Critic uses branch future bits Prophet and Critic operating autonomously predicting the same branch at different times, greatly improves accuracy Prophet and Critic operating autonomously predicting the same branch at different times, greatly improves accuracy

Prophet/Critic Hybrid Structure

Decoupled Front-End Architecture

Filtering the Critic Sounds a lot like a Cache !!

Theory Behind P/C Hybrids Prophet is like stream mode compressor encoders Prophet is like stream mode compressor encoders Critic uses past and future probabilities approximating a Markov model Critic uses past and future probabilities approximating a Markov model

Simulation Tools An enhanced version of Intel P4 An enhanced version of Intel P4 Gshare, 2Bc-gskew, Percepteron for Prophet Gshare, 2Bc-gskew, Percepteron for Prophet Tagged Gshare for Critic Tagged Gshare for Critic

Simulation Results (108 Benchmarks) - On average, 0-12 future bits  35% lower mispredictions - Adding just the first future bit results in 15% lower mispredictions

More results Prophet/Critic Hybrid was same size as Prophet alone, But with 25% – 31% reduced misprediction rate

Processor Performance With 4-bits  4.7 % speedup With 12-bits  8 % speedup Intel P4 with 8.6 % reduced energy

Our turn to critique Needs fast hardware to compute prediction/mispredictions and refile the FTQ before branches consumed by I-Cache Needs fast hardware to compute prediction/mispredictions and refile the FTQ before branches consumed by I-Cache Large tag  small coverage, small tag  contention; Therefore it is not universal Large tag  small coverage, small tag  contention; Therefore it is not universal How to select branches to cover in the filtered critic How to select branches to cover in the filtered critic