Pipelining Hazards

• Instruction dependence limits achievable parallelism
  • Control and data dependence (aka hazards)
• Finite amount of hardware limits achievable parallelism
  • Structural hazards
• Control dependence
  • On average, every fifth instruction is a branch (coming from if-else, for, do-while,…)
  • Branches execute in the third phase
    • Introduces bubbles unless you are smart

Control Dependence

What do you fetch in X and Y slots?

Options: Nothing, fall-through, learn past history and predict (today best predictors achieve on average 97% accuracy for SPEC2000)

Data Dependence