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TAGE-SC-L Branch Predictors

André Seznec

INRIA/IRISA

2The TAGE-SC-L branch predictorSorry, nothing really new ..

• TAGE, JILP 2006 Considered as state-of-the-art global history

predictor

• Can be augmented with small adjunct predictors

Loop predictor: CBP-2 (2006)

Statistical Corrector + Loop Predictor,

Global history CBP-3 (2011)

Local history Micro 2011

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Optimized all parameters

• Number, size, width of the tables

• Types of the histories for the statistical components

All that for decreasing the misprediction number by 3% !!

4

(Main)

TAGE

Predictor

Stat.

Cor.

Prediction + Confidence

Loop Predictor

Loop Predictor

Global, local,

skeleton histories

Global, local,

skeleton histories

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TAGE: multiple tables, global history

predictor

The set of history lengths forms a geometric series

most of the storage for short history !!

{0, 2, 4, 8, 16, 32, 64, 128}

Capture correlation on very long histories

6TAGE:

Tagged and prediction by the longest history matching entry

pc h[0:L1]

ctr u tag

=?

ctr u tag

=?

ctr u tag

=?

prediction

pc pc h[0:L2] pc h[0:L3]

11 1 1 1 1 1

1

1

Tagless base predictor

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=?

=?

=?1

1 1 1 1 1 1

1

1

Hit

Hit

Altpred

Pred

Miss

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Prediction computation

• General case: Longest matching component provides the prediction

• Special case: Many mispredictions on newly allocated entries: weak Ctr

On many applications, Altpred more accurate than Pred Property dynamically monitored through 4-bit counters

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A tagged table entry

• Ctr: 3-bit prediction counter

• U: 2-bit counters Was the entry recently useful ?

• Tag: partial tag

Tag CtrU

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Allocate entries on mispredictions

• Allocate entries in longer history length tables On tables with U unset

• Set Ctr to Weak and U to 0

• Limited storage budget: Allocate 2 entries for 256Kbits Allocate 1 or 2 for 32Kbits

• UNLIMITED STORAGE BUDGET: multiple entries allocated in different tables

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Managing the (U)seful counter

• Increment when avoids a misprediction (Pred = taken) & (Alt ≠ taken)

• 256K: Global decrement if « difficult » to allocate

• 32K: Probabilistic decrement when conflict

• Unlimited: don’t care

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Adjunct predictors

• TAGE tracks strong correlation with the global branch history

• Small adjunct predictors to capture some missed correlation: Loop predictor Statistical Corrector

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The loop predictor

• Predict loop with constant number of iterations: 16/32 entries less than 5 bytes per entry Capture loops with long bodies and/or

irregular internal branches

S: 1.2 % M: 1 % U:0.4%

Good tradeoff for the ChampionshipImplementation: Not that great

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The Statistical Corrector predictor

• Branches with poor correlation with global history: Sometimes better predicted by a single

wide PC indexed counter than by TAGE

• More generally, track cases such that: « In this case (PC, history, prediction),

TAGE is likely (>50 %) to mispredict »

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Small predictor: very limited budget for the SC

predictor

• Just track the statistically PC biased branches « TAGE predicts this direction on this

branch, but in most cases this was wrong »

• The corrector filter:

A small partially tagged associative table 1.5 % misp.

reduction: Much simpler than a loop predictor

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Medium predictor

« Statistically » correlated branches:•Not strongly correlated with the global history, but exhibit a bias

•better predicted by averaging than tags neural tags

« Statistically » correlated branches:•Not strongly correlated with the global history, but exhibit a bias

•better predicted by averaging than tags neural tags

Branches correlated with local history,but irregular global history pattern (on other branches)

•TAGE does not learn the pattern

Branches correlated with local history,but irregular global history pattern (on other branches)

•TAGE does not learn the pattern

17MultiGehl Statistical Correlator Predictor

TAGE H

PC

Stat.Corr.

Prediction + ctr value

+

+

H + LHPCPred Gehl-like

Local hist.Local hist.

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Why does it work

• The bias table indexed with PC+TAGE output: Correct (most of the time) High counter value

Dominates, not many updates Wrong Other counters can be trained

Correlation (if it exists) can be captured

19MultiGehl Statistical Correlator Predictor for the Championship

TAGE H

PC

Stat.Corr.

Prediction + ctr value

Local hist.Local hist.

+ RAS associated history+ 2 different local histories+ simple choser 6.8 % misp reduction

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« Realistic » 256 Kbits TAGE-SC-L

« Only » •12 equal size TAGE tables +•(local hist., global hist.) 4-tables SC •+ loop predictor•No history tuning

Only 2.8 % extra mispredictions

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SC for Unlimited predictor

• GEHL based SC predictor: Use any form of history information

Very long global Mutiple local « Skeleton » global history

ignore some branches Recycle old ideas from the MAC-RHSP

predictor (2004)

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SC for unlimited predictor

• 460 predictor tables + 10 choser tables Globally about 20 % less misp. than TAGE

alone

• If one removes only : The bias: 1.6 % for a single table All global history components: 3.7 % All local history components: 3.9 % The choser: 3.2 %

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Conclusion

• TAGE-SC-L fits (nearly) all storage sizes 32Kbits ≈ 64Kbits CBP1 champion on CBP1

traces

256Kbits ≈ 512Kbits CBP3 champion on CBP4 traces

• Unlimited predictor: poTAGE-SC does better