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OOO vs. EPIC

OOO vs. EPIC. Yingmin Li Ting Yan Qi Zhao. Outline. “Advantages” of EPIC Critique Conclusion. EPIC: Main Idea. “Smart compiler, dumb machine” Finding parallelism Processor  compiler Software/hardware synergy Processor design Avoid complexity and difficulty ILP, SMT & CMP.

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OOO vs. EPIC

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  1. OOO vs. EPIC Yingmin Li Ting Yan Qi Zhao

  2. Outline • “Advantages” of EPIC • Critique • Conclusion

  3. EPIC: Main Idea • “Smart compiler, dumb machine” • Finding parallelism • Processor  compiler • Software/hardware synergy • Processor design • Avoid complexity and difficulty • ILP, SMT & CMP

  4. EPIC: Predication • In OOO: dynamic branch prediction. • Larger basic blocks. • Control dep.  Data dep. • Eliminate misprediction & penalties.

  5. EPIC: Speculation • OOO: dynamic hardware • Data speculation & control speculation • Bigger window • Reduce impact of memory latencies

  6. EPIC: Large Register Set • OOO: register renaming. • Easier to design than reg. Renaming. • “Real” registers benefits some apps. • Encryption alg., Numerical alg. • Avoid loss of invisible registers. • Interruptions in OOO.

  7. EPIC: Unique Features • Register Stack Engine (RSE). • To deal with call/ return costs. • Seems an unlimited stack of phys. Reg. • Rotating register file. • Software pipelining. • Multiple loops at the same time.

  8. Function Call • Register saving/restoring • Processor? • Compiler? • Register file • Expensive • Always idle

  9. Predication • Computation of the branch condition is on the critical path • Increase ICache footprint • Half of the functional units effectively used if both “then” and “else” are scheduled • Hard to implement out-of-order with full predication

  10. Predication To compute if (a) x = t+1:

  11. Control Speculation • Why not just use prefetch which will not cause unexpected exception? • Technique to exploit control speculation such as superblock increase code length

  12. Control prediction

  13. Data Speculation • Moving a load above a possibly conflicting store • An advanced load and a checking load (IA64) • A run-time predictor

  14. Data speculation

  15. Software Pipelining • For high performance technical computing • High trip-count loops • For commercial applications • Low trip-count loops

  16. EPIC: at least not a breakthrough • Design Object of EPIC: • Moving hardware complexity to compiler

  17. EPIC: at least not a breakthrough • The failure of EPIC: • The compiling technique used for EPIC almost also apply well to OOO • Hardware simplicity is not so obvious to offset EPIC’s overhead • Without dynamic information, compiler essentially can’t do sth well enough

  18. The tragedy of cycle time • Why no obvious improvement in cycle time • mechanisms like RSA increase die complexity • Compare and dependent branch in one cycle • Predicted execution dependent on the existence of many function units

  19. Dynamic path length: hey, IA64, you wasted too much here • Speculation • Half of the predicted instructions discarded • Restricted bundling • One base register • No sign-extended loads • No integer multiply or divide in general register

  20. CPI • No dynamic prediction • Longer source code (more GR, Predicate register, template bit, restricted bundling, recovery code) is burdensome for instruction fetching • Recovery code may induce ICache pollution or just a page-fault

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