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Guest Lecturer: Justin Hsia

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1 Guest Lecturer: Justin Hsia
CS 61C: Great Ideas in Computer Architecture Pipelining Hazards Guest Lecturer: Justin Hsia 4/12/2013 Spring Lecture #31

2 Great Idea #4: Parallelism
Software Hardware Parallel Requests Assigned to computer e.g. search “Garcia” Parallel Threads Assigned to core e.g. lookup, ads Parallel Instructions > 1 one time e.g. 5 pipelined instructions Parallel Data > 1 data one time e.g. add of 4 pairs of words Hardware descriptions All gates functioning in parallel at same time Warehouse Scale Computer Smart Phone Leverage Parallelism & Achieve High Performance Core … Memory Input/Output Computer Cache Memory Core Instruction Unit(s) Functional Unit(s) A0+B0 A1+B1 A2+B2 A3+B3 Logic Gates 4/12/2013 Spring Lecture #31

3 Review of Last Lecture Implementing controller for your datapath
Take decoded signals from instruction and generate control signals Use “AND” and “OR” Logic scheme Pipelining improves performance by exploiting Instruction Level Parallelism 5-stage pipeline for MIPS: IF, ID, EX, MEM, WB Executes multiple instructions in parallel What can go wrong??? 4/12/2013 Spring Lecture #31

4 Agenda Pipelining Performance Structural Hazards Administrivia
Data Hazards Forwarding Load Delay Slot Control Hazards 4/12/2013 Spring Lecture #31

5 Review: Pipelined Datapath
Morgan Kaufmann Publishers Review: Pipelined Datapath 4/12/2013 Spring Lecture #31 Chapter 4 — The Processor

6 Pipelined Execution Representation
Time IF ID EX MEM WB Each instruction has identical latency! Every instruction must take same number of steps, so some stages will idle e.g. MEM stage for any arithmetic instruction 4/12/2013 Spring Lecture #31

7 Graphical Pipeline Diagrams
1. Instruction Fetch 2. Decode/ Register Read 3. Execute 4. Memory 5. Write Back PC instruction memory +4 Register File rt rs rd ALU Data imm MUX Use datapath figure below to represent pipeline: IF ID EX Mem WB ALU I$ Reg D$ 4/12/2013 Spring Lecture #31

8 Graphical Pipeline Representation
RegFile: left half is write, right half is read Time (clock cycles) I n s t r O d e I$ ALU Reg Reg I$ D$ ALU I$ Reg I$ ALU Reg D$ I$ Load Add Store Sub Or D$ Reg ALU 4/12/2013 Spring Lecture #31

9 Pipelining Performance (1/3)
Morgan Kaufmann Publishers Pipelining Performance (1/3) Use Tc (“time between completion of instructions”) to measure speedup Equality only achieved if stages are balanced (i.e. take the same amount of time) If not balanced, speedup is reduced Speedup due to increased throughput Latency for each instruction does not decrease 7/24/2012 Summer Lecture #21 Chapter 4 — The Processor

10 Pipelining Performance (2/3)
Morgan Kaufmann Publishers Pipelining Performance (2/3) Assume time for stages is 100ps for register read or write 200ps for other stages What is pipelined clock rate? Compare pipelined datapath with single-cycle datapath Instr Instr fetch Register read ALU op Memory access Register write Total time lw 200ps 100 ps 800ps sw 700ps R-format 600ps beq 500ps 7/24/2012 Summer Lecture #21 Chapter 4 — The Processor

11 Pipelining Performance (3/3)
Morgan Kaufmann Publishers Pipelining Performance (3/3) Single-cycle Tc = 800 ps Here using Tc as “time between completion of instructions.” Pipelined Tc = 200 ps 7/24/2012 Summer Lecture #21 Chapter 4 — The Processor

12 Morgan Kaufmann Publishers
Pipelining Hazards A hazard is a situation that prevents starting the next instruction in the next clock cycle Structural hazard A required resource is busy (e.g. needed in multiple stages) Data hazard Data dependency between instructions Need to wait for previous instruction to complete its data read/write Control hazard Flow of execution depends on previous instruction 4/12/2013 Spring Lecture #31 Chapter 4 — The Processor

13 Agenda Pipelining Performance Structural Hazards Administrivia
Data Hazards Forwarding Load Delay Slot Control Hazards 4/12/2013 Spring Lecture #31

14 Morgan Kaufmann Publishers
1. Structural Hazards Conflict for use of a resource MIPS pipeline with a single memory? Load/Store requires memory access for data Instruction fetch would have to stall for that cycle Causes a pipeline “bubble” Hence, pipelined datapaths require separate instruction/data memories Separate L1 I$ and L1 D$ take care of this 4/12/2013 Spring Lecture #31 Chapter 4 — The Processor

15 Structural Hazard #1: Single Memory
Load Instr 1 Instr 2 Instr 3 Instr 4 ALU Reg D$ I n s t r O d e Time (clock cycles) Trying to read same memory twice in same clock cycle 4/12/2013 Spring Lecture #31

16 Structural Hazard #2: Registers (1/2)
Load Instr 1 Instr 2 Instr 3 Instr 4 ALU Reg D$ I n s t r O d e Time (clock cycles) Can we read and write to registers simultaneously? 4/12/2013 Spring Lecture #31

17 Structural Hazard #2: Registers (2/2)
Two different solutions have been used: Split RegFile access in two: Write during 1st half and Read during 2nd half of each clock cycle Possible because RegFile access is VERY fast (takes less than half the time of ALU stage) Build RegFile with independent read and write ports Conclusion: Read and Write to registers during same clock cycle is okay 4/12/2013 Spring Lecture #31

18 Agenda Pipelining Performance Structural Hazards Administrivia
Data Hazards Forwarding Load Delay Slot Control Hazards 4/12/2013 Spring Lecture #31

19 Administrivia Project 2: Performance Optimization
Part 1 due Sunday (4/14) Part 2 released by Sunday night, due 4/21 Built-in performance competition for Part 2! Never too early to start looking at past exams! Dan has moved his OH because of traveling See Piazza 4/12/2013 Spring Lecture #31

20 Agenda Pipelining Performance Structural Hazards Administrivia
Data Hazards Forwarding Load Delay Slot Control Hazards 4/12/2013 Spring Lecture #31

21 2. Data Hazards (1/2) Consider the following sequence of instructions:
add $t0, $t1, $t2 sub $t4, $t0, $t3 and $t5, $t0, $t6 or $t7, $t0, $t8 xor $t9, $t0, $t10 4/12/2013 Spring Lecture #31

22 2. Data Hazards (2/2) Data-flow backwards in time are hazards
sub $t4,$t0,$t3 ALU I$ Reg D$ and $t5,$t0,$t6 or $t7,$t0,$t8 xor $t9,$t0,$t10 add $t0,$t1,$t2 IF ID/RF EX MEM WB I n s t r O d e Time (clock cycles) 4/12/2013 Spring Lecture #31

23 Data Hazard Solution: Forwarding
Forward result as soon as it is available OK that it’s not stored in RegFile yet sub $t4,$t0,$t3 ALU I$ Reg D$ and $t5,$t0,$t6 or $t7,$t0,$t8 xor $t9,$t0,$t10 add $t0,$t1,$t2 IF ID/RF EX MEM WB 4/12/2013 Spring Lecture #31

24 Datapath for Forwarding (1/2)
Morgan Kaufmann Publishers Datapath for Forwarding (1/2) What changes need to be made here? 4/12/2013 Spring Lecture #31 Chapter 4 — The Processor

25 Datapath for Forwarding (2/2)
Handled by forwarding unit Scan figure 4.54 on p. 368. 4/12/2013 Spring Lecture #31

26 Data Hazard: Loads (1/4) Recall: Dataflow backwards in time are hazards Can’t solve all cases with forwarding Must stall instruction dependent on load, then forward (more hardware) sub $t3,$t0,$t2 ALU I$ Reg D$ lw $t0,0($t1) IF ID/RF EX MEM WB 4/12/2013 Spring Lecture #31

27 Data Hazard: Loads (2/4) Hardware stalls pipeline lw $t0, 0($t1)
Schematically, this is what we want, but in reality stalls done “horizontally” Hardware stalls pipeline Called “hardware interlock” sub $t3,$t0,$t2 ALU I$ Reg D$ bubble and $t5,$t0,$t4 or $t7,$t0,$t6 lw $t0, 0($t1) IF ID/RF EX MEM WB How to stall just part of pipeline? 4/12/2013 Spring Lecture #31

28 Data Hazard: Loads (3/4) Stall is equivalent to nop lw $t0, 0($t1) nop
sub $t3,$t0,$t2 and $t5,$t0,$t4 or $t7,$t0,$t6 I$ ALU Reg D$ lw $t0, 0($t1) bubble nop 4/12/2013 Spring Lecture #31

29 Data Hazard: Loads (4/4) Slot after a load is called a load delay slot
If that instruction uses the result of the load, then the hardware interlock will stall it for one cycle Letting the hardware stall the instruction in the delay slot is equivalent to putting a nop in the slot (except the latter uses more code space) Idea: Let the compiler put an unrelated instruction in that slot  no stall! 4/12/2013 Spring Lecture #31

30 Code Scheduling to Avoid Stalls
Morgan Kaufmann Publishers Code Scheduling to Avoid Stalls Reorder code to avoid use of load result in the next instruction! MIPS code for D=A+B; E=A+C; # Method 1: 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) # Method 2: 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) 11 cycles Stall! Stall! 13 cycles 4/12/2013 Spring Lecture #31 Chapter 4 — The Processor

31 Agenda More Pipelining Structural Hazards Administrivia Data Hazards
Forwarding Load Delay Slot Control Hazards 4/12/2013 Spring Lecture #31

32 Morgan Kaufmann Publishers
3. Control Hazards Branch (beq, bne) determines flow of control Fetching next instruction depends on branch outcome Pipeline can’t always fetch correct instruction Still working on ID stage of branch Simple Solution: Stall on every branch until we have the new PC value How long must we stall? 4/12/2013 Spring Lecture #31 Chapter 4 — The Processor

33 Branch Stall When is comparison result available? Time (clock cycles)
beq Instr 1 Instr 2 Instr 3 Instr 4 ALU Reg D$ I n s t r O d e Time (clock cycles) TWO bubbles required per branch! 4/12/2013 Spring Lecture #31

34 Summary Hazards reduce effectiveness of pipelining Structural Hazards
Cause stalls/bubbles Structural Hazards Conflict in use of datapath component Data Hazards Need to wait for result of a previous instruction Control Hazards Address of next instruction uncertain/unknown More to come next lecture! 4/12/2013 Spring Lecture #31

35 No stalls with forwarding B)
Question: For each code sequences below, choose one of the statements below: 1: lw $t0,0($t0) add $t1,$t0,$t0 2: addi $t2,$t0,5 addi $t4,$t1,5 3: addi $t1,$t0,1 addi $t2,$t0,2 addi $t3,$t0,2 addi $t3,$t0,4 addi $t5,$t1,5 No stalls as is A) No stalls with forwarding B) Must stall C)

36 Code Sequence 1 Time (clock cycles) I n s Must stall lw t r O add d e
instr ALU Reg D$ I n s t r O d e Time (clock cycles) Must stall 7/25/2012 Summer Lecture #22

37 Code Sequence 2 Time (clock cycles) I n s add t
addi instr ALU Reg D$ I n s t r O d e Time (clock cycles) forwarding no forwarding No stalls with forwarding 7/25/2012 Summer Lecture #22

38 Code Sequence 3 Time (clock cycles) I n No stalls as is s addi t r O d
ALU Reg D$ I n s t r O d e Time (clock cycles) No stalls as is 7/25/2012 Summer Lecture #22

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