Ch. 13 Pipelining

1. Ch. 13 Pipelining Comp Sci 251 -- pipelining

2. Pipelining Comp Sci 251 -- pipelining

3. Performance of Pipeline • One instruction completes every clock cycle • n stages in pipeline  up to n times faster • speedup < n because some instructions do not need every stage • Note: individual instructions are not faster Comp Sci 251 -- pipelining

4. MIPS Pipeline 5 stages • IF: instruction fetch • ID: instruction decode (read registers) • EX: instruction execution, address calc • MEM: memory access • WB: write back (to register) Comp Sci 251 -- pipelining

5. Implementing a single-cycle pipeline IF ID EX MEM WB IF ID EX MEM WB IF ID EX MEM WB • Every stage takes the same time, whether there is work or not • Each stage must be stretched to accommodate the slowest instruction Comp Sci 251 -- pipelining

6. Total time for each instruction Comp Sci 251 -- pipelining

7. Single-cycle non-pipelined execution lw $7, 100($5) IR R ALU MEM R IR R ALU MEM R lw $8, 200($5) 800 ps IR lw $9, 300($5) 800 ps 3 independent lw instructions will take 3 x 800 ps = 2400 ps Comp Sci 251 -- pipelining

8. Pipelined execution 200 400 600 800 1000 1200 1400 lw $7, 100($5) IF R ALU MEM R IF R ALU MEM R lw $8, 200($5) 200ps IF R ALU MEM R lw $9, 300($5) 200ps 200ps 3 independent lw instructions takes 3 x 200 ps = 600 ps 4 times faster than the single-cycle non-pipelined execution Comp Sci 251 -- pipelining

9. Pipeline Hazards Control Hazards • caused by conditional branch instruction • cannot decide which instruction is next until stage 3 (or stage 2 with beefed up processor) • pipeline wants to start next instruction during stage 2 Comp Sci 251 -- pipelining

10. Solutions to Control Hazards • Stall: • start next instruction during stage 3 (assume branch is resolved in stage 2) • equivalent to placing a “nop” after every branch • Predict: • if incorrect, flush the bad instruction • Some prediction strategies • assume all branches not taken • static: assume some always taken, others never taken • dynamic: use past history, keep stats on branches Comp Sci 251 -- pipelining

11. Solutions to Control Hazards • Delayed decision: (used in MIPS and SPARC) • instruction following branch always executes • branch takes place after this instruction • compiler or assembler fills “delay slots” with useful instructions (or nop’s) • Change order of neighboring instructions, if logically acceptable • Or nop Comp Sci 251 -- pipelining

12. Data Hazards • Assume register write happens in WB stage add $s0, $t0, $t1 sub $t2, $s0, $t3 • Example requires three pipeline stalls • too costly to allow • too frequent for compiler to resolve Comp Sci 251 -- pipelining

13. Solution to Data Hazards Forwarding: getting the missing item early from internal resources • sub gets $s0 value from ALU, not reg. file • sometimes forwarding avoids stalls Comp Sci 251 -- pipelining

14. Solution to Data Hazards • sometimes forwarding only reduces stalls Comp Sci 251 -- pipelining

15. Advanced Pipelining Techniques • Superpipelining: large number of stages • Superscalar • multiple copies of each stage • several instructions started/finished per cycle Comp Sci 251 -- pipelining

16. Advanced Pipelining Techniques • Dynamic Pipeline Scheduling Comp Sci 251 -- pipelining

17. Pentium Pro / Power PC 604 Comp Sci 251 -- pipelining