1. CSE 490/590, Spring 2011 CSE 490/590 Computer Architecture Pipelining III Steve Ko Computer Sciences and Engineering University at Buffalo

2. CSE 490/590, Spring 2011 2 Last Time… MIPS pipelining –5 stages Pipelining hazards –Structural hazards –Data hazards –Control hazards Structural hazards –Resource conflict –MIPS doesn’t have it Data hazards –Stall –Bypass

3. CSE 490/590, Spring 2011 3 Resolving Data Hazards (2) Strategy 2: Route data as soon as possible after it is calculated to the earlier pipeline stage  bypass

4. CSE 490/590, Spring 2011 4 Bypassing Each stall or kill introduces a bubble in the pipeline CPI > 1 time t0t1t2t3t4t5t6t7.... (I 1 ) r1 r0 + 10IF 1 ID 1 EX 1 MA 1 WB 1 (I 2 ) r4 r1 + 17IF 2 ID 2 ID 2 ID 2 ID 2 EX 2 MA 2 WB 2 (I 3 )IF 3 IF 3 IF 3 IF 3 ID 3 EX 3 MA 3 (I 4 ) stalled stagesIF 4 ID 4 EX 4 (I 5 ) IF 5 ID 5 timet0t1t2t3t4t5t6t7.... (I 1 ) r1 r0 + 10IF 1 ID 1 EX 1 MA 1 WB 1 (I 2 ) r4  r1 + 17IF 2 ID 2 EX 2 MA 2 WB 2 (I 3 )IF 3 ID 3 EX 3 MA 3 WB 3 (I 4 ) IF 4 ID 4 EX 4 MA 4 WB 4 (I 5 ) IF 5 ID 5 EX 5 MA 5 WB 5 A new datapath, i.e., a bypass, can get the data from the output of the ALU to its input

5. CSE 490/590, Spring 2011 5 Adding a Bypass ASrc... (I 1 )r1 r0 + 10 (I 2 )r4 r1 + 17 r4 r1r1  IR PC A B Y R MD1 MD2 addr inst Inst Memory 0x4 Add IR Imm Ext ALU rd1 GPRs rs1 rs2 ws wd rd2 we wdata addr wdata rdata Data Memory we 31 nop stall D EMW When does this bypass help? r1 M[r0 + 10] r4 r1 + 17 JAL 500 r4 r31 + 17 yesno

6. CSE 490/590, Spring 2011 6 Fully Bypassed Datapath ASrc IR PC A B Y R MD1 MD2 addr inst Inst Memory 0x4 Add IR ALU Imm Ext rd1 GPRs rs1 rs2 ws wd rd2 we wdata addr wdata rdata Data Memory we 31 nop stall D EMW PC for JAL,... BSrc

7. CSE 490/590, Spring 2011 7 Resolving Data Hazards (3) Strategy 3: Speculate on the dependence. Two cases: Guessed correctly  do nothing Guessed incorrectly  kill and restart …. We’ll later see examples of this approach in more complex processors.

8. CSE 490/590, Spring 2011 8 Control Hazards What do we need to calculate next PC? –For Jumps » Opcode, offset and PC –For Jump Register »Opcode and Register value –For Conditional Branches »Opcode, PC, Register (for condition), and offset –For all other instructions »Opcode and PC have to know it’s not one of above

9. CSE 490/590, Spring 2011 9 time t0t1t2t3t4t5t6t7.... (I 1 ) r1 (r0) + 10IF 1 ID 1 EX 1 MA 1 WB 1 (I 2 ) r3 (r2) + 17 IF 2 IF 2 ID 2 EX 2 MA 2 WB 2 (I 3 )IF 3 IF 3 ID 3 EX 3 MA 3 WB 3 (I 4 ) IF 4 IF 4 ID 4 EX 4 MA 4 WB 4 time t0t1t2t3t4t5t6t7.... IFI 1 nop I 2 nop I 3 nop I 4 IDI 1 nop I 2 nop I 3 nop I 4 EX I 1 nopI 2 nopI 3 nopI 4 MA I 1 nopI 2 nopI 3 nopI 4 WB I 1 nopI 2 nopI 3 nopI 4 PC Calculation Bubbles (assuming no branch delay slots for now) Resource Usage nop  pipeline bubble

10. CSE 490/590, Spring 2011 10 Harvard-Style Datapath for MIPS 0x4 RegWrite Add clk WBSrcMemWrite addr wdata rdata Data Memory we RegDst BSrc ExtSelOpCode z OpSel clk zero? clk addr inst Inst. Memory PC rd1 GPRs rs1 rs2 ws wd rd2 we Imm Ext ALU Control 31 PCSrc br rind jabs pc+4

11. CSE 490/590, Spring 2011 11 Speculate next address is PC+4 I 1 096ADD I 2 100J 304 I 3 104ADD I 4 304ADD kill A jump instruction kills (not stalls) the following instruction stall How? I2I2 I1I1 104 IR PC addr inst Inst Memory 0x4 Add nop IR E M Add Jump? PCSrc (pc+4 / jabs / rind/ br)

12. CSE 490/590, Spring 2011 12 Pipelining Jumps I 1 096ADD I 2 100J 304 I 3 104ADD I 4 304ADD kill I2I2 I1I1 104 stall IR PC addr inst Inst Memory 0x4 Add nop IR E M Add Jump? PCSrc (pc+4 / jabs / rind/ br) IRSrc D = Case opcode D J, JAL nop... IM To kill a fetched instruction -- Insert a mux before IR nop IRSrc D I2I2 I1I1 304 nop

13. CSE 490/590, Spring 2011 13 time t0t1t2t3t4t5t6t7.... IFI 1 I 2 I 3 I 4 I 5 IDI 1 I 2 nop I 4 I 5 EX I 1 I 2 nop I 4 I 5 MA I 1 I 2 nop I 4 I 5 WB I 1 I 2 nop I 4 I 5 Jump Pipeline Diagrams time t0t1t2t3t4t5t6t7.... (I 1 ) 096: ADDIF 1 ID 1 EX 1 MA 1 WB 1 (I 2 ) 100: J 304IF 2 ID 2 EX 2 MA 2 WB 2 (I 3 ) 104: ADDIF 3 nop nop nop nop (I 4 ) 304: ADD IF 4 ID 4 EX 4 MA 4 WB 4 Resource Usage nop  pipeline bubble

14. CSE 490/590, Spring 2011 14 Harvard-Style Datapath for MIPS 0x4 RegWrite Add clk WBSrcMemWrite addr wdata rdata Data Memory we RegDst BSrc ExtSelOpCode z OpSel clk zero? clk addr inst Inst. Memory PC rd1 GPRs rs1 rs2 ws wd rd2 we Imm Ext ALU Control 31 PCSrc br rind jabs pc+4

15. CSE 490/590, Spring 2011 15 Pipelining Conditional Branches I 1 096ADD I 2 100BEQZ r1 +200 I 3 104ADD I 4 304ADD BEQZ? I2I2 I1I1 104 stall IR PC addr inst Inst Memory 0x4 Add nop IR E M Add PCSrc (pc+4 / jabs / rind / br) nop IRSrc D Branch condition is not known until the execute stage what action should be taken in the decode stage ? A Y ALU zero?

16. CSE 490/590, Spring 2011 16 Pipelining Conditional Branches I 1 096ADD I 2 100BEQZ r1 +200 I 3 104ADD I 4 304ADD stall IR PC addr inst Inst Memory 0x4 Add nop IR E M Add PCSrc (pc+4 / jabs / rind / br) nop IRSrc D A Y ALU zero? If the branch is taken - kill the two following instructions - the instruction at the decode stage is not valid  stall signal is not valid I2I2 I1I1 108 I3I3 BEQZ? ?

17. CSE 490/590, Spring 2011 17 Pipelining Conditional Branches I 1 096ADD I 2 100BEQZ r1 +200 I 3 104ADD I 4 304ADD stall IR PC addr inst Inst Memory 0x4 Add nop IR E M PCSrc (pc+4/jabs/rind/br) nop A Y ALU zero? I2I2 I1I1 108 I3I3 BEQZ? Jump? IRSrc D IRSrc E If the branch is taken - kill the two following instructions - the instruction at the decode stage is not valid  stall signal is not valid Add PC

18. CSE 490/590, Spring 2011 18 time t0t1t2t3t4t5t6t7.... IFI 1 I 2 I 3 I 4 I 5 IDI 1 I 2 I 3 nop I 5 EX I 1 I 2 nop nop I 5 MA I 1 I 2 nop nop I 5 WB I 1 I 2 nop nop I 5 Branch Pipeline Diagrams (resolved in execute stage) time t0t1t2t3t4t5t6t7.... (I 1 ) 096: ADDIF 1 ID 1 EX 1 MA 1 WB 1 (I 2 ) 100: BEQZ +200IF 2 ID 2 EX 2 MA 2 WB 2 (I 3 ) 104: ADDIF 3 ID 3 nop nop nop (I 4 ) 108: IF 4 nop nop nop nop (I 5 ) 304: ADD IF 5 ID 5 EX 5 MA 5 WB 5 Resource Usage nop  pipeline bubble

19. CSE 490/590, Spring 2011 19 CSE 490/590 Administrivia Recitations notes will be up online too. Very important to attend –Recitations next week & the week after Quiz 1 –Fri, 2/4 –Closed book, in-class –Includes whatever we cover until today Next class (Wed): review

20. CSE 490/590, Spring 2011 20 Acknowledgements These slides heavily contain material developed and copyright by –Krste Asanovic (MIT/UCB) –David Patterson (UCB) And also by: –Arvind (MIT) –Joel Emer (Intel/MIT) –James Hoe (CMU) –John Kubiatowicz (UCB) MIT material derived from course 6.823 UCB material derived from course CS252