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Chapter 4

CprE 381 Computer Organization and Assembly Level Programming, Fall 2013. Chapter 4. The Processor. Zhao Zhang Iowa State University Revised from original slides provided by MKP. Week 10 Overview. Expected project progress: Complete Mini-Project B, part 1 ALU data hazard and forwarding

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Chapter 4

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  1. CprE 381 Computer Organization and Assembly Level Programming, Fall 2013 Chapter 4 The Processor Zhao Zhang Iowa State University Revised from original slides provided by MKP

  2. Week 10 Overview • Expected project progress: Complete Mini-Project B, part 1 • ALU data hazard and forwarding • MEM data hazard, forwarding, and pipeline stall • Control hazard and branch execution Chapter 1 — Computer Abstractions and Technology — 2

  3. Data Hazards from ALU Instructions • An instruction depends on completion of data access by a previous instruction • add $s0, $t0, $t1sub $t2, $s0, $t3 • Consider this sequence: sub $2, $1,$3and $12,$2,$5or $13,$6,$2add $14,$2,$2sw $15,100($2) Chapter 4 — The Processor — 3

  4. Data Hazards from ALU Instructions • A naïve approach is to insert nops to wait out the dependence • add $s0, $t0, $t1sub $t2, $s0, $t3 Change to • add $s0, $t0, $t1noopnoopsub $t2, $s0, $t3 Chapter 4 — The Processor — 4

  5. Data Hazards in ALU Instructions • Another naïve approach is to stall the 2nd instruction in the dependence • add $s0, $t0, $t1sub $t2, $s0, $t3 Chapter 4 — The Processor — 5

  6. Data Hazards in ALU Instructions • Observations on this scenario • The first, ALU instruction produces a register value • The following instruction(s) consumes the register value sub $2, $1,$3and $12,$2,$5or $13,$6,$2 add $14,$2,$2sw $15,100($2) Chapter 1 — Computer Abstractions and Technology — 6

  7. Data Hazards in ALU Instructions • What is exactly the problem? • A register value is written to the register file in the WBstage, two cycles after the EX stage • The following instructions read the register value in the beginning of the IDstage IF ID EX MEM WB and sub … … … sub reads $1, $3 or and sub … … AND reads old $2 add or and sub … OR reads old $2 sub writes to $2 add reads new $2 sw add or and sub Chapter 1 — Computer Abstractions and Technology — 7

  8. Data Hazards in ALU Instructions Chapter 1 — Computer Abstractions and Technology — 8

  9. Forwarding (aka Bypassing) • Use result when it is computed • The result is already in the pipeline • Don’t wait for it to be stored in a register • Requires extra connections in the datapath Chapter 4 — The Processor — 9

  10. Dependencies & Forwarding Chapter 4 — The Processor — 10

  11. Data Forwarding • To what place: The two ALU inputs in the EX stage datapath • Forwarded register value may replace the values from ID • From where: The destination register value in pipeline registers • Source 1: EX/MEM register • Source 2: MEM/WB register Chapter 1 — Computer Abstractions and Technology — 11

  12. Data Forwarding • When to forward: Data dependence detected between • Instructions at the EX and MEM stage • Instructions at the EX and WB stage • How to detect: Compare source and destination register numbers Chapter 1 — Computer Abstractions and Technology — 12

  13. Data Forwarding • Example sub $2, $1,$3 # MEM=>EX forwarding and $12,$2,$5 # WB =>EX forwarding or $13,$6,$2 add $14,$2,$2 sw$15,100($2) • IF IDEX MEM WB or and sub … … AND gets forwarded new $2 value add or and sub … sw add or and sub SUB gets forwardednew $2 value Chapter 1 — Computer Abstractions and Technology — 13

  14. Data Forwarding Logic Design sub $2, $1,$3 # and $12,$2,$5 # comp $2 with $2, $5 or $13,$6,$2 # comp $2 with $6, $2 • Detection: Comparers and rt at EX, with rd at MEM and rd at WB • Those register numbers are in the IE/EX, EX/MEM, and MEM/WB registers • rs was not in IE/EX register, we can add it Chapter 1 — Computer Abstractions and Technology — 14

  15. Data Forwarding Logic Design • Register numbers in pipeline • Source registers of the instruction at the EX stage ID/EX.RegisterRs, ID/EX.RegisterRt • Destination register of the instruction at the MEM stage EX/MEM.RegisterRd • Destination register of the instruction at WB stage MEM/WB.RegisterRd • Potential data hazards when 1a. EX/MEM.RegisterRd = ID/EX.RegisterRs 1b. EX/MEM.RegisterRd = ID/EX.RegisterRt 2a. MEM/WB.RegisterRd = ID/EX.RegisterRs 2b. MEM/WB.RegisterRd = ID/EX.RegisterRt Fwd fromEX/MEMpipeline reg Fwd fromMEM/WBpipeline reg Chapter 4 — The Processor — 15

  16. Data Forwarding Logic Design • But only if forwarding instruction will write to a register! • EX/MEM.RegWrite=1, MEM/WB.RegWrite=1 • It’s possible an instruction has a matching rd but doesn’t write to register • And only if Rd for that instruction is not $zero • EX/MEM.RegisterRd ≠ 0,MEM/WB.RegisterRd ≠ 0 • It’s allowed for an instruction to write to $0 Chapter 4 — The Processor — 16

  17. Forwarding Paths The forwarding unit accesses three pipeline registers Note rs is added to IE/EX pipeline register Chapter 4 — The Processor — 17

  18. Forwarding Conditions • EX hazard: Data forwarding from EX/MEM register • 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 • MEM hazard: Data forwarding from MEM/WB register • 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 This is not the final version (see slides 23) Chapter 4 — The Processor — 18

  19. Datapath with Forwarding Chapter 4 — The Processor — 19

  20. Caveats • Data forwarding happens in the beginning of the cycle • The forwarding unit is in the EX stage, with its inputs from three pipeline stages • A small overhead added to the critical path latency of the EX stage • For EX hazard, data forwarding is from MEM to EX • Precisely, the register value of the instruction being executed at the MEM stage is forwarded to the instruction being executed at the EX stage Chapter 1 — Computer Abstractions and Technology — 20

  21. Caveats • For MEM hazard, the forwarding is from the WB to EX • From the instruction at WB to the instruction at EX • Data forwarding is to EX not to ID • An instruction may read obsolete register values at ID, with the values latched at ID/EX register • The correct values may be at EX (EX Hazard) or MEM (MEM Hazard) • Any obsolete values get replaced at EX • There is no WB hazard • Register write at WB and register read at ID, for the same register, may complete within one cycle Chapter 1 — Computer Abstractions and Technology — 21

  22. Double Data Hazard • Consider the sequence: add $1,$1,$2add $1,$1,$3add $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 Chapter 4 — The Processor — 22

  23. Revised Forwarding Condition • MEM hazard (revision from slide 18) • 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 Chapter 4 — The Processor — 23

  24. Load-Use Data Hazard • Load-use data hazard: A load instruction is followed immediately by an instruction using the value of load • How is a load instruction different from an ALU instruction? • ALU inst: destination register value available at the end of the EX stage • Load inst: destination register value available at the end of the MEM stage • Note the next instruction may need the value in the beginning of its EX stage Chapter 1 — Computer Abstractions and Technology — 24

  25. Load-Use Data Hazard • Can’t always avoid stalls by forwarding • If value not computed when needed • Can’t forward backward in time! Chapter 4 — The Processor — 25

  26. Load-Use Data Hazard Need to stall for one cycle Chapter 4 — The Processor — 26

  27. Load-Use Hazard How to insert a pipeline bubble (lost cycle)? lw $2, 20($1) sub $4, $2, $5 or $8, $2, $6 When the load instruction is at the EX stage • Hold the instruction at the IF stage • Do not update the PC • Hold the instruction at the ID stage • Do not change the IF/ID register • Insert a nopat the EX stage • Make all control signals in ID/EX register to zero • Particularly, RegWrite = 0 and MemWrite = 0 • Move forward MEM and WB Chapter 1 — Computer Abstractions and Technology — 27

  28. Load-Use Hazard Detection To detect, check if • Aload instruction is at the EX stage • ID/EX.MemRead= 1 • The instruction at the ID stage reads the register value of load • ID/EX.RegisterRt = IF/ID.RegisterRs, orID/EX.RegisterRt = IF/ID.RegisterRt (for R-type) If detected, stall IF and ID, insert bubble at EX, move forward MEM and MB Chapter 4 — The Processor — 28

  29. Pipeline Stall • The nop has all control signals set to zero • It does nothing at EX, MEM and WB • Prevent update of PC and IF/ID register • Using instruction is decoded again (OK) • Following instruction is fetched again (OK) • 1-cycle stall allows MEM to read data for lw • Can subsequently forward from WB to EX • Need to add new control lines • PCWrite for holding or updating PC • IF/IDWrite for holding or update IF/ID register Chapter 4 — The Processor — 29

  30. Stall/Bubble in the Pipeline Stall inserted here Chapter 4 — The Processor — 30

  31. Stall/Bubble in the Pipeline Or, more accurately… Chapter 4 — The Processor — 31

  32. Datapath with Hazard Detection Chapter 4 — The Processor — 32

  33. 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 BIG Picture Chapter 4 — The Processor — 33

  34. Code Scheduling to Avoid Stalls • Reorder code to avoid use of load result in the next instruction • C code for A = B + E; C = B + F; lw $t1, 0($t0) lw $t2, 4($t0) add $t3, $t1, $t2 sw $t3, 12($t0) lw $t4, 8($t0) add $t5, $t1, $t4 sw $t5, 16($t0) lw $t1, 0($t0) lw $t2, 4($t0) lw $t4, 8($t0) add $t3, $t1, $t2 sw $t3, 12($t0) add $t5, $t1, $t4 sw $t5, 16($t0) stall stall 13 cycles 11 cycles Chapter 4 — The Processor — 34

  35. Control Hazards • Branch determines flow of control • Two branch outcomes: Taken or Not-Taken • Fetching next instruction depends on branch outcome • Pipeline can’t always fetch correct instruction • Still working on ID stage of branch • In MIPS pipeline • Need to compare registers and compute target early in the pipeline • Add hardware to do it in ID stage Chapter 4 — The Processor — 35

  36. Control Hazards Several caveats • The CPU doesn’t recognize a branch until it reaches the end of the ID stage • Every cycle, the CPU has to fetch one instruction • Cannot afford to wait and see • Must predict the next PC every cycle • The CPU may predict “always not-taken” (MIPS 5-stage pipeline) • Alternatively, the CPU may predict branch outcome dynamically (advanced CPU design) Chapter 1 — Computer Abstractions and Technology — 36

  37. Control Hazards • This MIPS pipeline always predicts Not-Taken • Easy prediction: The next PC is current PC plus 4 • No need to design complex branch prediction unit • More Taken than Not-Taken in most programs • What happens if the branch is wrong? • Will have mis-fetched instructions • Flush those instructions before they take effect i.e. Before they write to memory or register • A Taken branch incurs a performance penalty Chapter 1 — Computer Abstractions and Technology — 37

  38. Performance Impact §4.8 Control Hazards • If branch outcome determined in MEM Flush theseinstructions (Set controlvalues to 0) PC Three cycles wasted on a taken branch Chapter 4 — The Processor — 38

  39. Performance Impact • The performance loss is 3 cycles per taken branch • Ifbranch outcome determined in MEM • Move execution of branch to the ID stage! • Only beq and bne are supported in original MIPS • Testing equality and inequality is very fast, do it at the end of ID • Branch target can be calculate in ID • Branch target = PC + extended offset • PC and offset are known in the beginning of ID • At the of ID, the CPU knows the branch outcome and branch target Chapter 1 — Computer Abstractions and Technology — 39

  40. Reducing Branch Delay • Move hardware to determine outcome to ID stage • Target address adder • Register comparator • Example code with branch taken 36: sub $10, $4, $840: beq $1, $3, 744: and $12, $2, $548: or $13, $2, $652: add $14, $4, $256: slt $15, $6, $7 ...72: lw $4, 50($7) Chapter 4 — The Processor — 40

  41. Example: Branch Taken Chapter 4 — The Processor — 41

  42. Early Branch Outcome • Pipeline changes for early branch outcome • 2ndPC adderand the shifter moved to ID • Comparator added to ID • No zero any more from ALU • CPU flushes one instruction for every taken branch • CPU detects taken branch at ID • The instruction at the IF will be flushed • 1 lost cycles instead of 3 lost cycles per taken branch Chapter 1 — Computer Abstractions and Technology — 42

  43. Pipeline Flushing When CPU detects a taken branch at ID • Update PC with branch target (already have) • Flush the instruction IF stage • Add flush signal to IF/ID pipeline • When flushing, convert the instruction in IF/ID register to 32-bit zeros • 0x00000000 is “and $0, $0, $0”, effectively a nop Chapter 1 — Computer Abstractions and Technology — 43

  44. Example: Branch Taken Note: Branch does nothing in EX, MEM and WB Chapter 4 — The Processor — 44

  45. Pipeline Bubble on Branch • Taken branch incurs a pipeline bubble because of instruction flushing Chapter 4 — The Processor — 45

  46. Data Hazards for Branches Moving branch execution to ID is not so easy • May need another forwarding unit • The forwarding unit has to be in the ID stage • The current forwarding unit, in the EX stage, obviously doesn't work • Need extensions to the hazard detection unit, and more pipeline stalls • Branch uses register values at ID, ALU and load produce register values at EX and MEM Chapter 1 — Computer Abstractions and Technology — 46

  47. IF IF IF IF ID ID ID ID EX EX EX EX MEM MEM MEM MEM WB WB WB WB Data Hazards for Branches • If a comparison register is a destination of 2nd or 3rd preceding ALU instruction add $1, $2, $3 add $4, $5, $6 … beq$1, $4, target • Can resolve using forwarding • From MEM to ID, and from WB to ID Chapter 4 — The Processor — 47

  48. IF IF ID ID EX EX MEM MEM WB WB Data Hazards for Branches • If a comparison register is a destination of preceding ALU instruction or 2nd preceding load instruction • May need 1 stall cycle • However, beq needs the value at the end of ID lw$1, addr add $4, $5, $6 IF ID beqstalled ID EX MEM WB beq$1, $4, target Chapter 4 — The Processor — 48

  49. IF ID EX MEM WB Data Hazards for Branches • If a comparison register is a destination of immediately preceding load instruction • May need 2 stall cycles • Again, beq needs the value at the end of ID, so it’s possible to reduce stall to one cycle lw$1, addr IF ID beqstalled ID beqstalled ID EX MEM WB beq$1, $0, target Chapter 4 — The Processor — 49

  50. Mini-Project C • In Mini-Project C, implement • The simple MIPS pipeline • Data forwarding and hazard detection • Not-taken branch prediction with pipeline flushing Chapter 1 — Computer Abstractions and Technology — 50

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