lecture14_DataPath_Hazards

Hazards

Structural Hazards

• 两条或更多指令在流水线需要访问同一个物理单元

Solutions

• 指令轮流使用物理资源
• 增加额外的硬件资源
• 设计指令集避免结构冒险

Data Hazards

• Some regfiles support writing a new value to a register, then reading the new value, in the same cycle.
• a register being written to is read from later.

Solutions

• Stalling

  • Wait for the first instruction to write its result before the second instruction reads the
    value

  

• Forwarding

  • Add hardware to send the result back to earlier stages before the result is written
  • Requires extra connections in the datapath

  

• Code Scheduling

  • Rearrange instructions to avoid data hazards
  • Compiler can put an unrelated instruction in the nop slot

  

Control Hazards

• If branch is not taken:

  • Instructions fetched sequentially after branch are correct

  • No control hazard

• If branch is taken, or there’s a jump:

  • The next two instructions still in the pipeline are incorrect
  • Need to convert incorrect instructions in the pipeline to nops
  • Called “flushing” the pipeline

Solutions

• Branch Prediction to Reduce Penalties（分支预测）

  • Early in the pipeline, guess which way branches will go.
  • Flush pipeline if the guess was incorrect.

• Naive branch prediction: just predict branch “not taken”, which always fetches PC+4