Predicate Execution2008/01/10

Presented by Jinho

If-conversion

Performance degradationmov 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;

ProblemMultiple definition

Performance degradation due to re-naming

Phi-Predication for Light-Weight If-ConversionWeihaw Chuang, Brad Calder, Jeanne Ferrante - CGO’03

Phi-Predication

Instructions

Compiler Transformation

4 Classes

Predicate Predication for Efficient Out-of-order ExecutionWeihaw Chuang, Brad Calder - ICS’03

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

Predicate Early Evaluation

REN1◦Predicate prediction is completed

REN2◦The predicted predicates and the true predicate

values are early-evaluated

Predicate Misprediction Recovery

Flush Predicate Misprediction◦Naïve approach

Rename-Replay for Predicate Mispredic-tion◦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 Mispredic-tions◦All instructions are put into the issue queue◦Replay selectively

Pipeline comparison

Flush vs. Rename-replay

Selective Replay

Examples

Evaluation

Methodology◦Trace

David Mosberger’s “utrace.c”◦Simulator

Modified SimpleScalar 3.0 to handle IA64◦Benchmarks

Spec2000 Integer and Floating-Point

Comparison

Speedups

Wrap-up

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 renameOnly for some operations

Predicate predictionGenerally better performanceProblem in hard-to-predict branch