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CSE 2021: Computer Organization Lecture-11 CPU Design : Pipelining-2 Review, Hazards Shakil M. Khan IF for Load (Review) CSE-2021 July-19-2012 2 ID for Load (Review) CSE-2021 July-19-2012 3 EX for Load (Review) CSE-2021 July-19-2012 4

  1. CSE 2021: Computer Organization Lecture-11 CPU Design : Pipelining-2 Review, Hazards Shakil M. Khan

  2. IF for Load (Review) CSE-2021 July-19-2012 2

  3. ID for Load (Review) CSE-2021 July-19-2012 3

  4. EX for Load (Review) CSE-2021 July-19-2012 4

  5. MEM for Load (Review) CSE-2021 July-19-2012 5

  6. WB for Load (Review) Wrong register number CSE-2021 July-19-2012 6

  7. Corrected Datapath for Load (Review) CSE-2021 July-19-2012 7

  8. Pipelined Control (Review) CSE-2021 July-19-2012 8

  9. Data Hazards in ALU Instructions • Consider this sequence: sub $2, $1,$3 and $12,$2,$5 or $13,$6,$2 add $14,$2,$2 sw $15,100($2) • We can resolve hazards with forwarding – how do we detect when to forward? CSE-2021 July-19-2012 9

  10. Dependencies & Forwarding CSE-2021 July-19-2012 10

  11. Detecting the Need to Forward • Pass register numbers along pipeline – e.g., ID/EX.RegisterRs = register number for Rs sitting in ID/EX pipeline register • ALU operand register numbers in EX stage are given by – ID/EX.RegisterRs, ID/EX.RegisterRt • Data hazards when Fwd from 1a. EX/MEM.RegisterRd = ID/EX.RegisterRs EX/MEM 1b. EX/MEM.RegisterRd = ID/EX.RegisterRt pipeline reg 2a. MEM/WB.RegisterRd = ID/EX.RegisterRs Fwd from 2b. MEM/WB.RegisterRd = ID/EX.RegisterRt MEM/WB pipeline reg CSE-2021 July-19-2012 11

  12. Detecting the Need to Forward • But only if forwarding instruction will write to a register! – EX/MEM.RegWrite, MEM/WB.RegWrite • And only if Rd for that instruction is not $zero – EX/MEM.RegisterRd ≠ 0, MEM/WB.RegisterRd ≠ 0 CSE-2021 July-19-2012 12

  13. Forwarding Paths CSE-2021 July-19-2012 13

  14. Forwarding Conditions • EX hazard – if (EX/MEM.RegWrite and (EX/MEM.RegisterRd ≠ 0) and (EX/MEM.RegisterRd = ID/EX.RegisterRs)) ForwardA = 10 – if (EX/MEM.RegWrite and (EX/MEM.RegisterRd ≠ 0) and (EX/MEM.RegisterRd = ID/EX.RegisterRt)) ForwardB = 10 CSE-2021 July-19-2012 14

  15. Forwarding Conditions • MEM hazard – if (MEM/WB.RegWrite and (MEM/WB.RegisterRd ≠ 0) and (MEM/WB.RegisterRd = ID/EX.RegisterRs)) ForwardA = 01 – if (MEM/WB.RegWrite and (MEM/WB.RegisterRd ≠ 0) and (MEM/WB.RegisterRd = ID/EX.RegisterRt)) ForwardB = 01 CSE-2021 July-19-2012 15

  16. Double Data Hazard • Consider the sequence: add $1,$1,$2 add $1,$1,$3 add $1,$1,$4 • Both hazards occur – want to use the most recent • Revise MEM hazard condition – only fwd if EX hazard condition isn’t true CSE-2021 July-19-2012 16

  17. Revised Forwarding Condition • MEM hazard – if (MEM/WB.RegWrite and (MEM/WB.RegisterRd ≠ 0) and not (EX/MEM.RegWrite and (EX/MEM.RegisterRd ≠ 0) and (EX/MEM.RegisterRd = ID/EX.RegisterRs)) and (MEM/WB.RegisterRd = ID/EX.RegisterRs)) ForwardA = 01 – if (MEM/WB.RegWrite and (MEM/WB.RegisterRd ≠ 0) and not (EX/MEM.RegWrite and (EX/MEM.RegisterRd ≠ 0) and (EX/MEM.RegisterRd = ID/EX.RegisterRt)) and (MEM/WB.RegisterRd = ID/EX.RegisterRt)) ForwardB = 01 CSE-2021 July-19-2012 17

  18. Datapath with Forwarding CSE-2021 July-19-2012 18

  19. Load-Use Data Hazard Need to stall for one cycle CSE-2021 July-19-2012 19

  20. Load-Use Hazard Detection • Check when using instruction is decoded in ID stage • ALU operand register numbers in ID stage are given by – IF/ID.RegisterRs, IF/ID.RegisterRt • Load-use hazard when – ID/EX.MemRead and ((ID/EX.RegisterRt = IF/ID.RegisterRs) or (ID/EX.RegisterRt = IF/ID.RegisterRt)) • If detected, stall and insert bubble CSE-2021 July-19-2012 20

  21. How to Stall the Pipeline • Force control values in ID/EX register to 0 – EX, MEM and WB do nop (no-operation) • Prevent update of PC and IF/ID register – using instruction is decoded again – following instruction is fetched again – 1-cycle stall allows MEM to read data for lw • can subsequently forward to EX stage CSE-2021 July-19-2012 21

  22. Stall/Bubble in the Pipeline Stall inserted here CSE-2021 July-19-2012 22

  23. Stall/Bubble in the Pipeline Or, more accurately… CSE-2021 July-19-2012 23

  24. Datapath with Hazard Detection CSE-2021 July-19-2012 24

  25. Stalls and Performance • Stalls reduce performance – but are required to get correct results • Compiler can arrange code to avoid hazards and stalls – requires knowledge of the pipeline structure The he B BIG IG P Pictur icture CSE-2021 July-19-2012 25

  26. Branch Hazards • If branch outcome determined in MEM Flush these instructions (Set control values to 0) PC CSE-2021 July-19-2012 26

  27. Reducing Branch Delay • Move hardware to determine outcome to ID stage – move target address adder (easy) – add register comparator (hard) • need additional forwarding h/w as operands might depend on previous instruction CSE-2021 July-19-2012 27

  28. Example: Branch Taken 36: sub $10, $4, $8 40: beq $1, $3, 7 44: and $12, $2, $5 48: or $13, $2, $6 52: add $14, $4, $2 56: slt $15, $6, $7 ... 72: lw $4, 50($7) CSE-2021 July-19-2012 28

  29. Example: Branch Taken CSE-2021 July-19-2012 29

  30. Example: Branch Taken CSE-2021 July-19-2012 30

  31. Data Hazards for Branches • If a comparison register is a destination of 2 nd or 3 rd preceding ALU instruction add $1, $2, $3 IF ID EX MEM WB add $4, $5, $6 IF ID EX MEM WB … IF ID EX MEM WB beq $1, $4, target IF ID EX MEM WB  Can resolve using forwarding CSE-2021 July-19-2012 31

  32. Data Hazards for Branches • If a comparison register is a destination of preceding ALU instruction or 2 nd preceding load instruction – need 1 stall cycle lw $1, addr IF ID EX MEM WB add $4, $5, $6 IF ID EX MEM WB beq stalled IF ID beq $1, $4, target ID EX MEM WB CSE-2021 July-19-2012 32

  33. Data Hazards for Branches • If a comparison register is a destination of immediately preceding load instruction – need 2 stall cycles lw $1, addr IF ID EX MEM WB beq stalled IF ID beq stalled ID beq $1, $0, target ID EX MEM WB CSE-2021 July-19-2012 33

  34. Dynamic Branch Prediction • In deeper and superscalar pipelines, branch penalty is more significant • Use dynamic prediction – branch prediction buffer (aka branch history table) – indexed by recent branch instruction addresses – stores outcome (taken/not taken) – to execute a branch • check table, expect the same outcome • start fetching from fall-through or target • if wrong, flush pipeline and flip prediction CSE-2021 July-19-2012 34

  35. 1-Bit Predictor: Shortcoming • Inner loop branches mispredicted twice! outer: … … inner: … … beq …, …, inner … beq …, …, outer  Mispredict as taken on last iteration of inner loop  Then mispredict as not taken on first iteration of inner loop next time around CSE-2021 July-19-2012 35

  36. 2-Bit Predictor • Only change prediction on two successive mispredictions CSE-2021 July-19-2012 36

  37. Calculating the Branch Target • Even with predictor, still need to calculate the target address – 1-cycle penalty for a taken branch • Branch target buffer – cache of target addresses – indexed by PC when instruction fetched • if hit and instruction is branch predicted taken, can fetch target immediately CSE-2021 July-19-2012 37

  38. Concluding Remarks • ISA influences design of datapath and control • Datapath and control influence design of ISA • Pipelining improves instruction throughput using parallelism – more instructions completed per second – latency for each instruction not reduced • Hazards: structural, data, control • Multiple issue and dynamic scheduling (ILP) – dependencies limit achievable parallelism – complexity leads to the power wall CSE-2021 July-19-2012 38

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