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

2. 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

3. 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)

4. 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

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

6. 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

7. 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

8. 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

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

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

11. 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

12. 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; }

13. 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

14. 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

15. 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

16. 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

17. GRC format – a small example1
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]

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

19. 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

20. 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

21. 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 –O, 1Mcycle random or given