Data value prediction

Bas van der Tol

Limits to ILP

Instruction Level Parallelism is limited by

• Control flow

• Data flow: true dependencies

Types of Speculative Execution

Sources of predicatable data

• Data redundancy

• Error-checking

• Program constants

• Virtual function calls

• Glue code

• Call-subgraph identities

• Register spill code

Register value locality

Value Prediction Unit

VPT Hit rate sensitivity to Size

Example use of Value Prediction

Penalties

• Misprediction penalty• Structural hazards, both on correct and mispredictions

Penalties

Configurations used for experiments

PowerPC 620 Speedup

PowerPC 620+ Speedup

Infinite Machine Model Speedup

Data cache vs. Value Prediction

Improving Prediction Accuracy

• Last Value Prediction

• Stride Prediction

• Finite Context Method Predictors (fcm)

Finite Context Models

Prediction Success

Conclusions

• Data value prediction increases performance by 5% on PowerPC 620

• A performance gain of 23% is possible

Future Developments

• More parallel execution units

• Better prediction models