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