Operation Reuse on Handheld Devices

1. Operation Reuse on Handheld Devices Yonghua Ding and Zhiyuan Li For LCPC 2003

2. Outline • Introduction • Computation reuse • Branch reuse by IF-merging • Conclusions

3. Introduction • Handheld devices have • Limited processing power • Limited energy resource • Operation reuse • Computation reuse • Branch reuse • Hardware solutions • Software solutions

4. Computation Reuse • Can be viewed as an extension of CSE • Redundancy among different instances of a code segment • Code segment with repetitive inputs • A hashing table records the input values and the computed output values • Replace the computation with a table look-up if the input is in the table

5. An Example code • Int quan(int val) { • int I; • for (i=0; i<15; i++) { • if (val < power2[i]) • break; • } • return (i); • }

6. Transformation Code • Int quan(int val) { • int I, key • if (check_hash(val,hash_tab,&key)==0) { • for (i=0; i<15; i++) { • if ( val<power2[i] ) • break; • } • hash_tab[key].output = I; • } • else • I = hash_tab[key].output; • return (i); • }

7. Framework of the Scheme Identify candidate code segments Data flow analysis to determine input/output Estimate hashing overhead Granularity analysis Choose code segments for value profiling Determine code segments to transform

8. Important factors • Computation granularity ( C ) • Hashing overhead ( O ) • Hashing function complexity • The size of input/output • Reuse rate ( R ) • R = 1 – Nds/N

9. Cost-Benefit Analysis • Cost of computation reuse • (C+O)(1-R)+O.R • The gain of computation reuse • C - (C+O)(1-R)+O.R Ξ R.C – O • Criteria to choose code segments • R.C – O > 0 or R > O/C

10. Experimentation Setup • Compaq iPAQ 3650 PDA • 206MHZ StrongARM SA1110 processor • 32MB RAM • 16KB I-cache and 8KB D-cache • Digital multi-meter HP 3458a • 6 MediaBench programs and a GNU GO game

11. Performance Improvement

12. Energy Saving

13. Performance Improvement for Different Input Files

14. Related Work • Richardson’s result cache • Sodani and Sohi’s instruction reuse • Huang and Lilja’s basic block level reuse • Connors and Hwu’s code region level reuse

15. Branch Reuse by IF-Merging • Motivation • Branch instructions degrade the efficiency of deep pipelining • Branches reduce the size of basic blocks • Branches introduce control dependences • Source-level code transformation

16. An Example Code • If ( sign ) { • diff = -diff; • } • …… • If ( sign ) • valpred -= vpdiff; • Else • valpred += vpdiff;

17. Transformation by IF-merging • If ( sign ) { • diff = -diff; • …… • valpred -= vpdiff; • } • Else { • …… • valpred += vpdiff; • }

18. Three Schemes of IF-Merging • A basic IF-merging scheme • Merge IF statements with identical condition • An IF-condition Factoring scheme • Factor and merge common sub-predicates • A path profiling scheme • IF-merging with path profiling information

19. A Basic IF-Merging Scheme • Symbolic analysis to identify IF statements with identical IF condition • Data dependence analysis to determine intermediate statements

20. A Factoring Scheme • Non-identical conditions have common sub-predicates (a&&b, a&&c) • Factor the common sub-predicates to construct a common IF statement • The new IF statement encloses the original IF statements with the remaining sub-predicates as conditions

21. A Path Profiling Scheme • Merge IF statements with high rate of alltaken • Exchange nested IF statements whose conditions are dependent

22. Experimental Results

23. Related Work • Kreahling et al’s profile-based condition merging • Branch prediction • Predicated execution • Muller and Whalley’s avoiding branches by code replication • Yang et al’s branch reordering

24. Conclusions • Operation reuse techniques are desirable for both program speed and energy saving on handheld devices • Computation reuse • Branch reuse by IF-merging