ECE6130: Computer Architecture: Instruction Level Parallelism ILP

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ECE6130 Computer ArchitectureInstruction Level
Parallelism (ILP)

• Dr. Xubin He
• http//iweb.tntech.edu/hexb
• Email hexb_at_tntech.edu
• Tel [phone], Brown Hall 319

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Dynamic Hardware Branch Prediction

• Dynamic Hardware Branch Prediction control
  dependences rapidly become the limiting factor as
  the amount of ILP to be exploited increases,
  which is particularly true when multiple
  instructions are to be issued per cycle.
• Basic Branch Prediction and Branch-Prediction
  Buffers
• A small memory indexed by the lower portion of
  the address of the branch instruction, containing
  a bit that says whether the branch was recently
  taken or not simple, and useful only when the
  branch delay is longer than the time to calculate
  the target address
• The prediction bit is inverted each time there is
  a wrong prediction an accuracy problem
  (mispredict twice) a remedy

Taken
Not taken
Predict taken
Predict taken
11
10
Taken
Taken
Not taken
Not taken
Predict not taken
Predict not taken
01
00
Taken
Not taken
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Correlating Branch Predictors

• Dynamic Hardware Branch Prediction
• Correlating Branch Predictors
• The behavior of branch b3 is correlated with the
  behavior of branches b1 and b2 (b1 b2 both not
  taken ? b3 will be taken) A predictor that uses
  only the behavior of a single branch to predict
  the outcome of that branch can never capture this
  behavior.
• Branch predictors that use the behavior of other
  branches to make prediction are called
  correlating predictors.

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• Dynamic Hardware Branch Prediction
• Correlating Branch Predictors

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• Dynamic Hardware Branch Prediction
• Correlating Branch Predictors
• The standard predictor mispredicted all branches!
• A 1-bit correlation predictor uses two bits, one
  bit for last branch being not taken and the other
  for taken (in general the last branch executed is
  not the same instruction as the branch being
  predicted).

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• Dynamic Hardware Branch Prediction
• Correlating Branch Predictors
• With the 1-bit correlation predictor, also called
  a (1,1) predictor, the only misprediction is on
  the first iteration!
• In general case an (m,n) predictor uses the
  behavior of the last m branches to choose from 2m
  branch predictors, each of which is an n-bit
  predictor for a single branch.

Branch address
2-bit per-branch predictors
4
xx prediction
xx

• The number of bits in an (m,n) predictor is
• 2mn (number of prediction entries selected
  by the branch address)

2-bit global branch history