Yingmin Li Ting Yan Qi Zhao 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

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

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

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.

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.

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

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

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

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

Control prediction

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

Data speculation

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

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

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

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

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

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