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