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Predicate Execution

Predicate Execution. 2008/01/10 Presented by Jinho. Performance degradation. mov r1 = 0; if( r2 == r3 ) mov r1 = 1; else mov r1 = 2; mov r4 = r1;. mov r1 = 0; cmp.eq p1,p2=r2,r3; ( p1) mov r1 = 1; ( p2) mov r1 = 2; mov r4 = r1;. Problem Multiple definition

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Predicate Execution

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  1. Predicate Execution 2008/01/10 Presented by Jinho

  2. Performance degradation mov r1 = 0; if( r2 == r3 ) mov r1 = 1; else mov r1 = 2; mov r4 = r1; mov r1 = 0; cmp.eq p1,p2=r2,r3; (p1) mov r1 = 1; (p2) mov r1 = 2; mov r4 = r1; • Problem • Multiple definition • Performance degradation due to renaming If-conversion

  3. Phi-Predication for Light-Weight If-Conversion Weihaw Chuang, Brad Calder, Jeanne Ferrante - CGO’03

  4. Instructions Phi-Predication

  5. 4 Classes Compiler Transformation

  6. Predicate Predication for Efficient Out-of-order Execution Weihaw Chuang, Brad Calder - ICS’03

  7. Predicate Predictor • Predicates were branches before if-conversion • Value prediction instead of branch prediction • Implementation • Separate from branch predictor • Only for branch history table • No needs for the return-address stack or the branch target buffer Main Idea

  8. REN1 • Predicate prediction is completed • REN2 • The predicted predicates and the true predicate values are early-evaluated Predicate Early Evaluation

  9. Flush Predicate Misprediction • Naïve approach • Rename-Replay for Predicate Misprediction • Instructions on false predicates are not put into the issue queue • Replay from predicate early evaluation • Instructions are stored in recovery queue(RecQ) • Selective-Replay for Predicate Mispredictions • All instructions are put into the issue queue • Replay selectively Predicate Misprediction Recovery

  10. Flush vs. Rename-replay Pipeline comparison

  11. Examples Selective Replay

  12. Methodology • Trace • David Mosberger’s “utrace.c” • Simulator • Modified SimpleScalar 3.0 to handle IA64 • Benchmarks • Spec2000 Integer and Floating-Point Evaluation

  13. Speedups Comparison

  14. Wrap-up

  15. Solution? mov r1 = 0; if( r2 == r3 ) mov r1 = 1; else mov r1 = 2; mov r4 = r1; mov r1 = 0; cmp.eq p1,p2=r2,r3; (p1) mov r1 = 1; (p2) mov r1 = 2; mov r4 = r1; Predicate prediction Generally better performance Problem in hard-to-predict branch Phi-prediction mov r1 = 0; mov r5 = 2; cmp.eq p1,p2=r2,r3; phi r4 = (p1)1,r5; Doesn’t need to rename Only for some operations Multiple definition

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