1. TRACES Code Padding to Improve the WCET Calculability Christine Rochange and Pascal Sainrat Institut de Recherche en Informatique de Toulouse Toulouse

2. TRACES WCET evaluation  Static WCET analysis § IPET: Implicit Path Enumeration Technique flow analysislow-level analysis WCET computation

3. TRACES Implicit Path Enumeration Technique A B C E D x A = 1 + x DA = 1 + x AB x B = x AB = x BC + x BE x C = x BC = x CD x D = x CD + x ED = x DA x E = x BE = x ED x BC = x BE x DA ≤ N T =  x i.t i max +  x ij.  ij

4. TRACES Pipelined execution F FU1 FU2 C FETCH FU1 FU2 COMPL. 12345 FETCH FU1 FU2 COMPL. 12345 FETCH FU1 FU2 COMPL. 123456 5 5 -4  B1,B2

5. TRACES Long Timing Effects (1) F FU1 FU2 C FETCH FU1 FU2 COMPL. 12345 FETCH FU1 FU2 COMPL. 12345678 FETCH FU1 FU2 COMPL. 12345 FETCH FU1 FU2 COMPL. 12345 FETCH FU1 FU2 COMPL. 123456 FETCH FU1 FU2 COMPL. 123456 5 5 5 -4 T A-B-C = 7  = 8 +1

6. TRACES Long Timing Effects (2) t ABC t ABCD t 1…n =  t i +   j…k i=1 n 1 ≤ j ≤ k ≤n tAtA tBtB tCtC tDtD tEtE  AB  BC  CD  DE t AB  ABC  BCD  DEF  ABCD  BCDE  ABCDE J. Engblom

7. TRACES Motivation  Long timing effects are: § difficult to quantify  they might span over very long sequences § difficult to integrate into WCET computation  Long timing effects increase the variability of execution times  Our goal: eliminate long timing effects

8. TRACES Outline  Our approach: code padding  Implementation § software framework § analysis algorithms  to identify resource requirements  to compute safe padding lengths  Experimental results  Concluding remarks

9. TRACES Code padding FETCH FU1 FU2 COMPL. 12345678 FETCH FU1 FU2 COMPL. 12345678 filler instruction FETCH FU1 FU2 COMPL. 123456

10. TRACES Exemple (1) inst i 1 inst i 2 … inst i ni inst j 1 inst j 2 … inst j nj inst k 1 … inst k nk block i block j block k requires a 4-cycle delay requires a 3-cycle delay requires a 1-cycle delay

11. TRACES Exemple (2) inst i 1 inst i 2 … inst i ni inst j 1 inst j 2 … inst j nj inst k 1 … inst k nk block i block j block k nop 4-cycle delay 3-cycle delay 1-cycle delay

12. TRACES Exemple (3) inst i 1 inst i 2 … inst i ni inst j 1 inst j 2 … inst j nj inst k 1 … inst k nk block i block j block k bl delay4 bl delay3 nop 4-cycle delay 3-cycle delay 1-cycle delay delay4:nop nop delay3: nop nop delay2:blr filler block

13. TRACES Code padding framework C source code gcc compiler assembly code gas assembler object code CFG extractor cycle-level simulator interference analysis code padding safe padded assembly code list of basic blocks execution traces of block sequences padding lengths

14. TRACES Analyzing resource requirements (1)  Requirements of a basic block foreach block B do { ff[B]  first fetch cycle of B; lf[B]  last fetch cycle of B + 1; foreach resource R do { n[R]  cycle at which R is needed; r[R]  cycle at which R is released; // 0 if R not used by B n[R,B]  n[R] – ff[B]; r[R,B]  r[R] – lf[B]; // 0 if R not used by B } d[B]  0; } FETCH FU1 FU2 COMPL. 12345 ff[B 2 ] = 1 lf[B 2 ] = 2 n[FU1,B 2 ] = 0 r[FU1,B 2 ] = 0 n[FU2,B 2 ] = 1 r[FU2,B 2 ] = 3

15. TRACES Analyzing resource requirements (2)  Requirements of a sequence foreach sequence B 1 -…-B x (x < n) do { lf[B x ]  last fetch cycle of B x + 1; foreach resource R do { r[R]  cycle at which R is released; // 0 if R not used by any B i r[R,B 1 -…B x ]  r[R] – lf[B x ]; } } FETCH FU1 FU2 COMPL. 123456 lf[B 2 ] = 3 r[FU1,B 1 -B 2 ] = 2 r[FU2,B 1 -B 2 ] = 3 r[FU1,B 2 ] = 0

16. TRACES Computing padding lengths (1)  Depth-1 strategy § objective: r[R,A-B] == r[R,B] § algorithm: § example: foreach sequence A-B do foreach resource R do if r[R,A-B] ≠ r[R,B] then { d  StrictDelay(R,A-B); if d > d[B] then d[B]  d; } computes the padding length (iterative trials) r[FU1,B 2 ] = 0 r[FU1,B 1 -B 2 ] = 2 >

17. TRACES Computing padding lengths (2)  Depth-n strategy § analyze (n+1)-block sequences (B 0 -B 1 -…-B n ) § objectives:  for i < n : if r[R,B 0 -…-B i ] > n[R,B i+1 ] : r[R,B 0 -…-B i ] == r[R,B 1 -…-B i ]  r[R,B 0 -…-B n ] == r[R,B 1 -…-B n ]

18. TRACES Computing padding lengths (3)  Example: depth-4 algorithm foreach sequence A-B-C-D-E do foreach resource R do if (n[R,C] > 0)&& (r[R,A-B] > n[R,C]) && (r[R,A-B] > r[R,B]) then { d  MinimumDelay(R,A-B-C); if d > d[B] then d[B]  d; } elsif (n[R,D] > 0)&& (r[R,A-B-C] > n[R,D]) && (r[R,A-B-C] > r[R,B-C]) then { d  MinimumDelay(R,A-B-C);...

19. TRACES Experimental results (1)  Code size increase 2-way4-way matmul35.24%76.19% ludcmp16.51%28.20% jfdctint11.37%126.97% bsort31.25%76.25% heapsort25.00%51.47% insertsort23.81%59.52% MEAN23.86%69.77% depth-1

20. TRACES Experimental results (2)  WCET increase

21. TRACES Concluding remarks  Inter-block long timing effects make the WCET analysis complex and pessimistic  Code padding prevents long timing effects and limit the variability of partial execution times  The cost of padding can be acceptable § code size (  20% for a 2-way pipeline) § real WCET increase (  20%)  future work: cost on the estimated WCET?

22. TRACES Thank you! Traces stands for Research group on Architectures and Compilers for Embedded Systems www.irit.fr/TRACES