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

2. If-conversion 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

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

4. Phi-Predication Instructions

5. Compiler Transformation 4 Classes

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

7. Main Idea 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

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

9. Predicate Misprediction Recovery 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

10. Pipeline comparison Flush vs. Rename-replay

11. Selective Replay Examples

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

13. Comparison Speedups

14. Wrap-up

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