Exploring Branch Prediction

1
Exploring Branch Prediction

• Aditya Akella
• Shuchi Chawla
• Jia-Yu Pan

2
A Framework for Branch Prediction
3
Dividing Streams

• First level per branch substream
• Finer decomposition
• Branch pattern history
• Global pattern history
• Global path history

1011
1110
(b1,1)(b2,1)(b2,1)
4
Path vs Pattern

• As expected, path history provides better
  correlation

5
Predictors

• Static vs Dynamic

• Adapt to changing bias
• Damping 2 (or more) bit predictor
• Is more better?

Static is better on a large number of branches
But, dynamic far outperforms static on the rest
Adaptive schemes should exploit this
6
Implementation Issues Limited space

• Many branches map to same stream Aliasing
• Mostly a destructive effect
• More aggressive schemes may not be necessarily
  better

7
gshare
GAs
8
Implementation Issues Limited space

• Many branches map to same stream Aliasing
• Mostly a destructive effect
• More aggressive schemes may not be necessarily
  better

9
Cross-procedure Correlation

• Some static prediction schemes forget history
  on returning from a procedure call

Fun_call if (xgt0) return(1) else
return(0) Main a fun_call() if
(agt0) goto
Direction taken by the branch inside the
procedure completely specifies the direction
taken by the branch outside
10
Branch Prediction Using Data Values

• Traditional approach
• Local history information
• Global path and history information
• Reducing table interference (better indexing)
• Use PCs and branch outcomes as input
• Do not contain all the information

Misprediction!!
11
How can one improve prediction of such branches?
Use Data Values!!!!!!!!!!!!
12
How to Use Data Values

• Speculative branch execution

• Second scheme was chosen
• Lower-latency prediction
• Some branches can use combined predictions
• Solution avoids data value prediction

• Using data values directly

13
Design Problems (1)

• Large number of data values to store
• Soln store their difference
• Branch Difference Predictor (BDP)

14
Design Problems (2)

• Delay in updating data values
• Out-of-order execution, pipeline latencies

• REP replacement policy
• Replace least successful entries
• Randomly ignore a chunk of mispredictions

15
Search for a good predictor design

• Paper
• A Language for Describing Predictors and its
  Application to Automatic Synthesis

16
Designing a predictor

• Components of a predictor
• Counters and History
• More?
• Formulate as a search problem
• How?

17
Search the design space

• Representation
• Feedback Loop Model Primitives
• Pw,d(IU) gt parse tree

Feedback/ Update (U)
d
Input Index (I)
Prediction (P)
w
18
Examples

• Onebitd(PCT)
• P1,d(PC T)
• Countern,d(IT)
• Pn,d(I if T then P1 else P-1)
• Twobitd(PCT)
• MSB(Counter2,d(PCT))

19
Genetic Programming

• Process
• Population, Evaluation, Selection and populate
  next generation
• Operations on parse trees
• Replication, Crossover, Mutation, Encapsulation

20
Discussion

• Comparable good prediction accuracy
• Complex but new components
• Rediscover counters and local/global histories
• Predictors for other purposes, e.g. indirect jump
  target prediction

21
End

• Comments?
• Thank you.

22
Designing a predictor Backup

• Predictors
• Branch prediction
• Data value prediction
• Indirect jump target prediction
• Components of a predictor
• Counters and History
• More?