A Modular and Retargetable Framework for Tree-based WCET analysis Antoine Colin Isabelle Puaut IRISA - Solidor Rennes, France

ECRTS 2001, Delft, The Netherlands 2 Hard real time  Real time tasks must meet their deadlines  Hard real-time: critical applications  Deadline miss  catastrophic consequences  Scheduling algorithm  Must ensure that all tasks will meet their deadlines  Schedulability analysis (off-line)  Require information on scheduled task  WCET : worst case execution time

ECRTS 2001, Delft, The Netherlands 3 Estimating the WCET  Test and Measurement  How to exhibit the worst case behaviour of the program ?  Exhaustive testing: practically impossible è Unsafe  Static analysis  Safe, but pessimist  Mainly automatic  Requires the source code of the analysed programs  The programming language must be adapted:  restrictions: no indirect calls,...  annotations: loop bounds,... Unsafe estimates Exact WCET Overestimated WCETs MeasurementsStatic analysis

ECRTS 2001, Delft, The Netherlands 4 Static WCET analysis  The static analysis result  Should be a safe and tight estimate of the worst execution time  Depends on a specific hardware  Is provided for isolated code  The two levels of WCET analysis  High level analysis: statically determine the longest execution path in the program, and estimate the WCET along this path  Low level analysis: determine the execution time of basic blocks, taking hardware effects into account

ECRTS 2001, Delft, The Netherlands 5 High level: tree-based analysis  Requires well-structured programs  WCET calculation using a timing schema Loop [4] If BB 1 BB 2 BB 0 BB 5 BB 4 BB 3 Sequence BB 6 BB 7 int x,p=0,i=0; for(x=0;x<5;x++) { if(i%2) { p++; } else { i++; }} Assembly code BB 0 BB 1 BB 7... Basic blocks WCET(SEQ) WCET(S1) + … + WCET(Sn) S1;…;Sn WCET(IF) WCET(test) + if(test) max( WCET(then), WCET(else) ) then else WCET(LOOP) maxiter*(WCET(tst)+WCET(body)+WCET(inc)) for(;tst;inc) + WCET(test)+WCET(exit) {body} Timing schema Equation system

ECRTS 2001, Delft, The Netherlands 6 Low level: hardware effects  Goal: reducing the pessimism of the low level analysis  Pipeline effect: WCET(Basic Block) < WCET(instruction)  Cache and branch prediction effect: WCET(instruction) is variant, depending on the internal hardware state  Last decade  Various architectural features have been considered (Caches, Pipeline, Branch prediction, …)  Several methods proposed, and often designed independently è Leads to an integration issue  instruction  BB

ECRTS 2001, Delft, The Netherlands 7 Integration issue: the modular approach  Definition of modules  Modules are in charge of analysing architectural features effects  Co-operation through well defined interfaces  New WCET representation and extended timing schema  Retargetability  Changing modules in the framework  Using a new architecture description file

ECRTS 2001, Delft, The Netherlands 8 Salto : Assembly manipulation tool Assembly description file Modular and Retargetable static analysis framework Heptane Syntactic tree Control flow graph Source file WCET Maple.maple Front-end BB I-CacheBranch Pred. Pipeline WCET of BBs Extended Timing schema Modular parts Data Framework

ECRTS 2001, Delft, The Netherlands 9 Loop  Ln-levels are associated with loop constructs  ex: [ ], [0], [0.0], [0.1], [0.1.0], etc.  Partial order on ln-levels  Useful for characterising analysis results  Analysis results (events/estimates) depend on the considered ln-level  Ex: I-Cache conflict  (BB a,[0]) (BB b,[0.1.0])  BB b prefetch   Cache miss when loop [0] is executed, hit otherwise  (BB b -miss,[0]) : ln-level < [0] = hit, ln-level [0] = miss Loop nesting level information Loop Seq [ ] [0] [0.0] [0.1.0] [0.1]  

ECRTS 2001, Delft, The Netherlands 10 Instruction cache analysis  I-Cache analysis module Adaptation layer I-Cache Analysis Results adaptation Basic block I-Cache WCET information  Basic block -> Instruction blocks  Portion of basic block that fit exactly into a cache line (instructions and instruction fragments)  Allows to take into account various instruction sets  Existing I-Cache analysis technique  Example: static cache simulation  Iblock misses are expressed using ln-levels  Pair: (Iblock,miss-level)  I-Cache analysis result: a set of pairs

ECRTS 2001, Delft, The Netherlands 11 Branch prediction analysis  Similar to I-Cache module Adaptation layer Branch Pred. Analysis Results adaptation Basic block B.Pred. WCET information  Basic block -> Control Transfer Instructions  At most two branching possibilities at the end of the BB  Existing branch prediction analysis technique  Example: static BTB simulation [JRTS00]  Two miss-prediction levels (ln-level): jmp/seq

ECRTS 2001, Delft, The Netherlands 12 Pipeline analysis (1/2) Adaptation layer Pipeline simulation I-Cache WCET information I-Cache WCET information Representation of the WCET of basic blocks Branch Pred. WCET information  Use I-Cache and BTB analysis results  WCET info require adaptation  Ex: I-Blocks -> Instructions  Existing pipeline simulation technique  Reservation table, simulator,...  Inter/intra basic block effect  Results expressed using ln-levels  2 WCETs (jmp/seq) per ln-level

ECRTS 2001, Delft, The Netherlands 13  Incremental representation of the WCET  (1) basic WCET associated with the lowest ln-level of the basic block  (2) difference between WCET of subsequent ln-levels  One WCET representation per outgoing edge WCET seq (BB) = Pipeline analysis (2/2) [0.1.0]  15 [0.1]  15 [0]  17 [ ]  20 Pipeline analysis results WCET jmp (BB) = [0.1.0]  15 [0.1]  19 [0]  21 [ ]  24 Seqjmp

ECRTS 2001, Delft, The Netherlands =  One basic block  Two WCETs  select WCET seq /WCET jmp  Use sets of pairs : + and  are redefined  Operator : union of two sets of pair  Operator : M (wcet,ln-lev) = (M  wcet,L) if L ln-lev = (wcet,ln-lev) otherwise Adapted timing schema  L + L  L  [0] [ ] [0] Seq Loop [10] WCET Repr.

ECRTS 2001, Delft, The Netherlands 15 Conclusion and future work  Co-operation of several HW analysis techniques  Prototype configuration: Intel Pentium  Reduction of the pessimism of estimates  Analysis results: presentation on Friday, session 10  Future work  New modules:  ex: pipeline analysis (super-scalar, out of order execution)  Extend the framework to handle data caches,...  Retarget the analyser  Make the analyser available for community use  Further information: