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Approximation of the Worst-Case Execution Time Using Structural Analysis Matteo Corti and Thomas Gross Zürich

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EMSOFT 2004, © Matteo Corti2 Goal Worst-case execution time estimation of soft- real time Java applications. We focus on semantic analysis: –compute a tight bound on the max and min number of iterations for every block –consider different path frequencies inside loops –avoid path enumeration

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EMSOFT 2004, © Matteo Corti3 Outline Goal Loop bounds Block bounds Complexity and related work Testing environment Results Concluding remarks

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EMSOFT 2004, © Matteo Corti4 Semantic analysis Systems overview bytecode annotated asm partial abstract interpretation loop header bounds block bounds instruction duration analysis WCET

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EMSOFT 2004, © Matteo Corti5 Java Whole program analysis. Variable type based analysis to resolve polymorphism. We consider only local integer variables for the loop analysis. Our block iterations bounding technique is language independent.

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EMSOFT 2004, © Matteo Corti6 Semantic analysis Systems overview bytecode annotated asm partial abstract interpretation loop header bounds block bounds instruction duration analysis WCET

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EMSOFT 2004, © Matteo Corti7 Partial abstract interpretation We perform a limited abstract interpretation pass over linear code. We discover some false paths (not containing cycles). We gather information on possible variables values. void foo(int i) { if (i > 0) { for(;i<10;i++) { bar(); }

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EMSOFT 2004, © Matteo Corti8 Partial abstract interpretation BenchmarkInfeasible paths _201_compress2 _202_jess3 _205_raytrace7 _209_db2 _213_javac240 _222_mpegaudio19 _228_jack22

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EMSOFT 2004, © Matteo Corti9 Partial abstract interpretation BenchmarkInfeasible longest path JavaLayer2 linpack2 whetstone1

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EMSOFT 2004, © Matteo Corti10 Semantic analysis Systems overview bytecode annotated asm partial abstract interpretation loop header bounds block bounds instruction duration analysis WCET

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EMSOFT 2004, © Matteo Corti11 Loop bounds Bounds on the loop header computed similarly to C. Healy [RTAS98]. We introduce noncontiguous sets of integers to easily handle equality operators. Iteration branch: a block where the conditional jump could be responsible for a loop exit. For each edge e and iteration branch ib we compute the possible number of iterations.

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EMSOFT 2004, © Matteo Corti12 [101] [50] [100] Loop bounds The bounds on the iterations of the header are safe for the whole loop. But: some parts of the loop could be executed less frequently: for(int i=0; i<100; i++) { if (i < 50) { A; } else { B; } AB [101] [1] [100]

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EMSOFT 2004, © Matteo Corti13 Semantic analysis Systems overview bytecode annotated asm partial abstract interpretation loop header bounds block bounds instruction duration analysis WCET

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EMSOFT 2004, © Matteo Corti14 Basic block iterations The number of iterations of a block is not a local property (based on immediate predecessors). P0P0 P1P1 B [10] [0..10] void foo(boolean b) { for(int i=0; i<10; i++) { if (b) { P0; } else { P1; } B; }

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EMSOFT 2004, © Matteo Corti15 Basic block iterations The number of iterations of a block is not a local property (based on immediate predecessors). void foo(boolean b) { for(int i=0; i<20; i++) { if (i<10) { P0; } else { P1; } B; } P0P0 P1P1 B [20] [10]

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EMSOFT 2004, © Matteo Corti16 Basic block iterations The number of iterations of a block is not a local property (based on immediate predecessors). void foo(boolean b) { int i = 0; if (b) { do { i++; P0; } while (i<10); } else { do { i++; P1; } while (i<10); } B; } B P1P1 [1] [0] [10] P0P0

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EMSOFT 2004, © Matteo Corti17 Basic block iterations The number of iterations of a block is not a local property (based on immediate predecessors). [1] [0] [10] [20] [10] [0..10] [0] [10]

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EMSOFT 2004, © Matteo Corti18 Structural analysis Powerful interval analysis. Recognizes semantic constructs. Useful when the source code is not available. Iteratively matches the blocks with predefined patterns.

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EMSOFT 2004, © Matteo Corti19 Structural analysis Powerful interval analysis. Recognizes semantic constructs. Useful when the source code is not available. Iteratively matches the blocks with predefined patterns.

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EMSOFT 2004, © Matteo Corti20 Structural analysis if (A || (B && C)) { D; } else { E; } F; A D B E C F Static patterns: Dynamic patterns:

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EMSOFT 2004, © Matteo Corti21 Block iterations Block iterations are computed using the CFG root and the iteration branches. The header and the type of the biggest semantic region that includes all the predecessors of a node determine its number of iterations. Complete algorithm in the paper. P0P0 B H P1P1

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EMSOFT 2004, © Matteo Corti22 Example P0P0 P1P1 B H [20] [10] void foo(boolean b) { for(int i=0; i<20; i++) { if (i<10) { P0; } else { P1; } B; }

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EMSOFT 2004, © Matteo Corti23 Example B H P1P1 [1] [0..10] P0P0 void foo(boolean b) { int i = 0; if (b) { do { i++; P0; } while (i<10); } else { do { i++; P1; } while (i<10); } B; }

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EMSOFT 2004, © Matteo Corti24 Example B PH L H [1] [11] [10] void foo(boolean b) { PH; for(int i=0; i<10; i++) { L; } B; }

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EMSOFT 2004, © Matteo Corti25 Related work Automatically detected value-dependent constraints [Healy, RTAS99]: –per block bounds –requires path enumeration (in the loop body) We propagate the header bounds to the blocks in quadratic time: –Structural analysis: O(B 2 ) –Loop bounds: O(B) –Block bounds: O(B)

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EMSOFT 2004, © Matteo Corti26 Evaluation: hardware-level analysis The semantic analysis is platform independent. Evaluation: Pentium III on Linux. We approximate the effects of caches and pipelines: –we assume that the effects of an instruction fade over time. –caches and pipelines are analyzed locally. Possible sources of inaccuracies: –cache misses and pipeline stalls –but not the number of iterations of an instruction (conservative)

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EMSOFT 2004, © Matteo Corti27 Evaluation Base: the bounds on the iterations of the loop header are used for the whole loop. Enhanced: structural analysis is used to consider different path frequencies in loop bodies.

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EMSOFT 2004, © Matteo Corti28 Results: synthetic benchmarks for (i=0; i<10000; i++) { if (i<5000) { array[i] = -array[i]; } if (array[i] > max) { max = array[i]; } for(i=0; i<10; i++) { for (j=0; j<10; j++) { if(j<9) { m[i][j] *= m[i][j]; } else { for(k=0; k<9; k++) { m[i][j]+=m[i][k]; } } } } B1B2B3B4 Base10000 100 Enhanced5000100009010 B1 B2 B3 B4 Example 1 Example 2

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EMSOFT 2004, © Matteo Corti29 Results BenchmarkMax observed [cycles] Enhanced [cycles] Base [cycles] MatMult2.68·10 9 2.73·10 9 2%2.73·10 9 2% Jacobi0.88·10 10 1.08·10 10 22%1.08·10 10 22% JavaLayer2.67·10 9 1.30·10 10 487%1.49·10 9 558% SciMark2.47·10 10 1.42·10 11 579%2.12·10 11 858% _201_compress9.45·10 9 1.11·10 10 117%4.76·10 12 50370%

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EMSOFT 2004, © Matteo Corti30 _201_compress for (data = 0; data < N; data++) { // compress data if (data == 10000) { // update structures }

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EMSOFT 2004, © Matteo Corti31 Concluding remarks We tighten the bounds of basic blocks iteration considering different paths inside loop bodies. We do not perform path enumeration Tests with real applications validate the semantic analysis.

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