ProphetCritic Hybrid Branch Prediction

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Prophet/Critic Hybrid Branch Prediction

• Falcon, Stark, Ramirez, Lai, Valero
• Presenter Christian Wanamaker

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Outline

• Overview Motivation
• Hybrid Branch Prediction
• The Prophet/Critic Branch Predictor
• Results
• Conclusions

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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

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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.

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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)

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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

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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

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Prophet Critic Basics
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Prophet Critic Basics

• The branch predictions are kept in the Fetch
  Target Queue (FTQ)
• Once the future bits are received, the critic
  makes its 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

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Prophet/Critic Architecture (cont)
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Prophet/Critic How it works
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Prophet Critic How it works

• The Prophet will mispredict A
• The 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.

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Prophet/Critic Filtering

• The critic can be limited by multiple branches
  contending for the same resources
• In addition, the critic is not always correct
• So, easy to predict branches should be filtered
  out
• This is achieved with tags that are set when a
  mispredict occcurs
• If not tagged, the critics critique is ignored.

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Prophet/Critic Filtering
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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

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• Hardware Budgets and predictor types

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Results of varying Future Bits
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Results
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Results
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Results
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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

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Thank you

• Any questions?