Pipelining Hazards

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Pipelining Hazards

• Prof. Sirer
• CS 316
• Cornell University

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Pipelining Recap

• Powerful technique for masking latencies
• Logically, instructions execute one at a time
• Physically, instructions execute in parallel
• Decouples the processor model from the
  implementation
• Interface vs. implementation
• Dependencies between instructions complicate the
  implementation

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Hazards

• Data hazards
• A required operand is not ready
• Usually because a previous instruction in the
  pipeline has not committed it to the register
  file yet
• Control hazards
• The next instruction to fetch cannot be
  determined
• Usually because a jump or branch instruction has
  not determined the next PC yet
• Structural hazards
• Two instructions in the pipeline try to
  simultaneously access the same resource

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Data Hazards

• Stall
• Keep track of all instructions in the pipeline,
  and all registers they may affect
• Detect that there will be a conflict
• Inject NOPs (bubbles) into the pipeline until
  the hazard is gone
• Data Forwarding
• Keep track of all instructions in the pipeline,
  and all registers they may affect
• Add additional circuitry and wires to route data
  from a later stage back to an earlier stage

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Control Hazards

• Stall
• Inject NOPs into the pipeline when the next
  instruction is not known
• Pros simple, clean Cons slow
• Delay Slots
• Tell the programmer that the N instructions after
  a jump will always be executed, no matter what
  the outcome of the branch
• Pros The compiler may be able to fill the slots
  with useful instructions Cons breaks
  abstraction boundary
• Speculative Execution
• Insert instructions into the pipeline
• Replace instructions with NOPs if the branch
  comes out opposite of what the processor expected
• Pros Clean model, fast Cons complex

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Types of Hazards

• RAW Read After Write
• WAR Write After Read
• WAW Write After Write
• RAW indicates data dependence
• WAW and WAR hazards stem from register reuse
• Exercised by out-of-order execution
• Register renaming can eliminate them

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