An introduction to Esterel and its compilation Dumitru Potop, fresh PhD

Outline (Long) Introduction Esterel Esterel translation schemes Embedded systems, Reactive systems, Synchronous paradigm Esterel Syntax and intuitive semantics Esterel translation schemes Automata-based Circuit-based “Simulation”-based

Embedded systems Airplanes, cars, phones, washing machines, video cameras contain computers! Complex, custom hardware/software architectures Specific problems in their development: Specification, due to interdisciplinarity Specific languages, methods Compilation Implementations must fit tight specifications Joint development of hardware and software Insuring correctness, fault tolerance, and robustness (expensive bugs)

Reactive systems (Harel, 1987) Emphasis on communication and control React to input events with appropriate output events, according to the state Examples: digital circuits, man-machine interfaces, communication protocols state … Inputs … Outputs

Synchronous paradigm Cycle-based execution model, global clock Perfect synchrony Causality Input event Output event computation memory

Synchronous paradigm Synchrony = abstraction of the real world: Natural in circuits Advantages: clear semantics, codesign, verification methods Problem: combining circuit semantics with a higher-level description style time Execution instants

Synchronous languages Esterel (Berry et al.), ECL Structural imperative (Ada-like) style Fit for control-dominated parts Lustre (Halbwachs et al.), Signal (LeGuernic et al) Dataflow networks Circuit-like, low-level Argos (Maranichi), SyncCharts (André) Visual Statecharts-like formalisms

The Esterel language Structural imperative style Basic constructs Intuitive, modular Basic constructs Classical control flow: p;q, p||q, loop p end Preemption:abort p when S, suspend p when S Exception handling: trap T in p end, exit T Division into instants: pause, await S Signals: signal S in p end, emit S, present S then p else q end

The Esterel language Example 1: loop [ await A || await B ]; emit O; halt every R

The Esterel language Example 2: signal S in await I ; emit S || await S ; emit O end signal

The Esterel language Constructive causality Correct causality cycles Instantaneous reaction to signal absence (analysis of not yet executed code) causality cycle signal S,T in emit S; present T then present S else emit T end end; end break the cycle

Old translation schemes Automata translation Fast, large code 1 2 3 loop [ await A || await B ]; emit O; halt every R 4 switch(state){ case 0: state=1; break; case 1: if(!R)if(A)if(B) {O=1;state=4;} else state=2; else if(B)state=3;break; case 2: if(R)state=1; else if(B){O=1;state=4;} break; case 3: if(R)state=1; else if(A){O=1;state=4;} break; case 4: if(R)state=1;break; }

Old translation schemes Circuit translation Slow, small code R O loop [ await A || await B ]; emit O; halt every R boot B RESET = R & !BOOT A_TRIGGER = A_OUT & !RESET A_THEN = A_TRIGGER & A A_ELSE = A_TRIGGER & !A A_TERM = A_THEN | ! A_TRIGGER A_IN = BOOT | RESET | A_ELSE B_TRIGGER = B_OUT & !RESET B_THEN = B_TRIGGER & B B_ELSE = B_TRIGGER & !B B_TERM = B_THEN | ! B_TRIGGER B_IN = BOOT | RESET | B_ELSE ACTIVE = A_TRIGGER | B_TRIGGER O = A_TERM & B_TERM & ACTIVE

Old translation schemes Direct semantic simulation (Edwards, Bertin) Fast, small code through static scheduling Restrict the class of accepted programs Resume: if(R) goto Start ; else if (A_active | B_active) { if (A_active & A) { A_active=0 ; } if (B_active & B) { B_active=0 ; } if (!(A_active | B_active)) { emit O ; } goto Out; Start: A_active=1 ; B_active=1 ; Out: loop [ await A || await B ]; emit O; halt every R

Old translation schemes Semantically complete Efficient code Intermediate model Explicit FSM Circuits ? Expensive, slow General Cheap, fast General* Cheap, fast Only “acyclic” programs Compiling method Bisimulation (fc2tools) RTL optimizations (SIS) Classical control-flow optimizations Optimization Generated code (without optim.) Very large, Very fast Small Slow Very small Very fast Semantics (acyclic=?) Less powerful optim. Problems Do not scale up well *=sccausal or slow simulation

What I wanted How Formalize the efficient approach Generate efficient code for “good” programs Generate code for all programs Understand cyclicity at a higher level Inexpensive optimizations based on static analysis Formalize the efficient approach New intermediate format/model (GRC) Hierarchical state representation Control-flow graph No specific encoding How

GRC format – a small example 1 boot: loop [ await A;emit B || await B ]; emit O; halt every R # 4 await A 3 || await B 2 5 # loop-every halt 6 enter 4 Empty boxes = program operations. Blue = state representation, encoding, and decoding Entered in order, exited in reverse order State representation = skeleton, flowgraph = muscles and nerves, influencing one another Flowgraph = unrolling of the Esterel source. Skeleton = static information repository. Boot = generated, technical detail exit 1 enter 2 enter 3 enter 5 [1] exit 2 Inactive[4] [2] R 4 exit 4 emit B K0[4] K0 exit 3 emit O enter 6 A K1[4] K1 Inactive[5] [3] 5 exit 5 K0[5] 2 B [6] K1[5]

GRC format – utility Supports the full Esterel semantics Analysis and optimization at GRC code level Supports optimized encoding and scheduling algorithms

Conclusion A very brief introduction to Esterel, its compilation, and my work During my PhD: New Esterel semantics Intermediate model for Esterel programs Static analysis and optimization at this level General code generation scheme The best compiler yet

Past Future PhD at Ecole des Mines de Paris, with Gérard Berry Complete the work started during the PhD Investigate aspects related to code distribution (Benveniste et al.) Future

Results Prototype compiler (acyclic case) Examples: Turbo channel bus Berry’s wristwatch Video generator Shock absorber Operating system model Avionics fuel controller Avionics cockpit Man-machine interface Test configuration: PIII/1GHz/128M/Linux gcc-2.96 –O, 1Mcycle random or given