Prophet/Critic Hybrid Branch Prediction

Falcon, Stark, Ramirez, Lai, ValeroPresenter: Christian Wanamaker

Outline

Overview & MotivationHybrid Branch PredictionThe Prophet/Critic Branch PredictorResultsConclusions

Overview

Better Branch Prediction is a highly desirable technique because it does not require trade-offs between performance, power, and energy

Despite much research on Branch Prediction, it is by no means solved

Branch prediction is liable to become even more important as pipelines deepen and issue-widths increase

Overview (continued)

Issue width = # of uOps (micro ops) issued per clock cycle

Using the Branch Target Buffer to look ahead for branches

Perceptrons – simple neural network. Can look at a longer history than simpler counters.

Hybrid Branch Predictors

• Use two or more different branch prediction techniques

• One may override another (either based on a third selector or one may always override)

• The predictions may be combined, for instance as a majority vote

• Example: Tournament predictors (often a branch prediction buffer and a Correlating branch predictor with a third predictor choosing which of the two is used in this situation)

Prophet/Critic Branch Predictor

Basic Idea – the Prophet makes a series of predictions of future branches, the Critic critiques and if necessary alters them.

The Prophet makes predictions based on the history of the branch.

The Critic looks at the branches prediction of the Prophet after the prophet has predicted a certain number of steps ahead, then critiques the prophecy

Prophet Critic Basics

BTB looks ahead, Prophet predicts whether BTB branches are taken/not taken

Branch Outcome Register – the predictions that are in the critics “future” - the number of future bits allowed.

More future bits allow for more accurate viewing of the future

Prophet Critic Basics

Prophet Critic Basics

The branch predictions are kept in the Fetch Target Queue (FTQ)

Once the future bits are received, the critic makes it’s pronouncement

The critic overrides the prophet if it comes to a different conclusion

If so, the FTQ is purged of un-critiqued predictions, and the prophet is redirected to the path shown by the critic

Prophet/Critic Architecture (cont)

Prophet/Critic: How it works

Prophet Critic: How it works

The Prophet will mispredict AThe Critic will note that the Prophet

mispredicted in this case the first time the misprediction occurs.

In the future, when the Critic sees the misprediction, it will correct it

More future bits increase the accuracy of prediction, but reduce the history, so there is an important tradeoff here.

Prophet/Critic Filtering

The critic can be limited by multiple branches contending for the same resources

In addition, the critic is not always correctSo, easy to predict branches should be

filtered outThis is achieved with tags that are set when a

mispredict occcursIf not tagged, the critic’s critique is ignored.

Prophet/Critic Filtering

Testing and Result Testing was done on a cycle

accurate IA32 with Long Instruction Traces

The simulator had to follow bad branches, as otherwise the critic would not learn.

Branch Predictors: Gshare, 2bc-gskew, perceptron

uPC is uOps per cycle misp/kuops is misses per

thousand uops

Hardware Budgets and predictor types

Results of varying Future Bits

Results

Results

Results

Conclusions

Speedup of up to 8% with 12 future bits (using the same amount of branch prediction space)

The mispredict rate can be reduced up to 25-31%

Adding future bits helps, but more is not always better

Research suggests that the best future bits can be chosen dynamically

Thank you

Any questions?