VHDL Symbolic simulator in OCaml

1. VHDL Symbolic simulator in OCaml Florent Ouchet florent.ouchet@imag.fr TIMA Labs – GINP – UJF – CNRS – VDS group OCaml Meeting 2009 Grenoble – France 04/02/2009

2. Outline VSYML – Vhdl Symbolic Simulator in ocaML: • Symbolic simulation goals • Why OCaml? • Application structure and limitations of OCaml in this context • Perspectives • Conclusion Florent Ouchet – OCaml meeting 2009 – Grenoble – France

3. Symbolic simulation goals Model is needed for: • Model checking: equivalence of IC implementation and specification; • Other formal methods based on the model: (ANR project FME3) analysis of error consequences using formal methods. Florent Ouchet – OCaml meeting 2009 – Grenoble – France

4. Symbolic simulation goals Integrated circuit synthesis flow: Modeling level Explicit time,behavioral Manual refinements Register Transfert Level FSM, complex types Automatic synthesis Gate Level Boolean type Placing and routing Drawing of transistors and wires Layout Manufacturing Chips Florent Ouchet – OCaml meeting 2009 – Grenoble – France

5. Symbolic simulation goals Existing symbolic simulators: Modeling level Explicit time,behavioral Register Transfert Level FSM, complex types VOSS/Forte (Intel CAD Labs) Gate Level Boolean type Drawing of transistors and wires Layout Chips Florent Ouchet – OCaml meeting 2009 – Grenoble – France

6. Symbolic simulation goals Existing symbolic simulators: Modeling level Explicit time,behavioral Register Transfert Level VSYML FSM, complex types Gate Level Boolean type Drawing of transistors and wires Layout Chips Florent Ouchet – OCaml meeting 2009 – Grenoble – France

7. Symbolic simulation goals Xt=0 ns Xt=1 ns … Integrated Circuit Description Design Under Test (DUT) Xt=0 + Yt=1 ? Yt=0 ns Yt=1 ns … Description written in VHDL (IEEE Std 1076.1TM – 2007) Symbolic values depend on the time. Florent Ouchet – OCaml meeting 2009 – Grenoble – France

8. Why OCaml? Expression patterns: • X@1ns • 1 • unary_operator X (not – abs) • X assoc_operator Y (+ * and or xor xnor) • X binary_operator Y (nand nor ^ = /= < > <= >=) • X other_operator Y (mod rem / -) • … (total of 28 patterns) Florent Ouchet – OCaml meeting 2009 – Grenoble – France

9. Why OCaml? type formula = | FormulaSymbol of symbol | FormulaImmediateInt of big_int | FormulaUnaryOperator of (unary_operator*formula) | FormulaAssociativeOperator of (assoc_operator*formula list) | FormulaBinaryOperator of (binary_operator*formula*formula) | FormulaOtherOperator of (other_operator*formula*formula list) | … Florent Ouchet – OCaml meeting 2009 – Grenoble – France

10. Why OCaml? • Known expression patterns are rewritten (numeric evaluation); • As in the standard, the simulation algorithm is intrinsically free of side effects. Florent Ouchet – OCaml meeting 2009 – Grenoble – France

11. Why OCaml? DUT Process A Process C Process B The next value of each signal is computed from current values. Florent Ouchet – OCaml meeting 2009 – Grenoble – France

12. Why OCaml? DUT Process A Process C Process B The processes are executed until the design is stabilized. Florent Ouchet – OCaml meeting 2009 – Grenoble – France

13. Why OCaml? DUT Process A Process C Process B The processes are executed until the design is stabilized. Florent Ouchet – OCaml meeting 2009 – Grenoble – France

14. Why OCaml? DUT Process A Process C Process B The processes are executed until the design is stabilized. Florent Ouchet – OCaml meeting 2009 – Grenoble – France

15. Why OCaml? DUT Process A Process C Process B The processes are executed until the design is stabilized. Florent Ouchet – OCaml meeting 2009 – Grenoble – France

16. Why OCaml? DUT Process A Process C Process B The processes are executed until the design is stabilized. Florent Ouchet – OCaml meeting 2009 – Grenoble – France

17. Why OCaml? DUT Process A Process C Process B The processes are executed until the design is stabilized. Florent Ouchet – OCaml meeting 2009 – Grenoble – France

18. Why OCaml? • Strong type checking (compared to C, Pascal…): confidence in computations; • Lexer/parser generator; • « Free » software, its ancestor was written in Mathematica; • Lightweight and easy to redist (Java, C#, F#). Florent Ouchet – OCaml meeting 2009 – Grenoble – France

19. Application structure VHDL source files Design model VSYML parameters Standalone and automated application. Florent Ouchet – OCaml meeting 2009 – Grenoble – France

20. Application structure • Some facts: • Over 26500 lines of code • 24 dedicated modules (types, basics, lexer, parser, resources, evaluators, converters, volume extrusion, top-level…) Florent Ouchet – OCaml meeting 2009 – Grenoble – France

21. Application structure • Straightforward conversion from BNF rules (language spec) • No syntax for optional tokens • Debugging conflicts is a nightmare VHDL source files (thousands lines) Lexer (59 states, 1058 transitions) Parser (695 grammar rules,1318 states) Abstract syntax tree Florent Ouchet – OCaml meeting 2009 – Grenoble – France

22. Some basic functions are missing in OCaml standard libraries Issue 4662 closed as « won’t fix » Application structure Abstract syntax tree Design elaboration Simulation structure tree Florent Ouchet – OCaml meeting 2009 – Grenoble – France

23. Application structure • A function to map a list of functions to the same parameter: (‘a -> ‘b) list -> ‘a -> ‘b list • A function to remove the n first list elements: ‘a list -> int -> ‘a list Florent Ouchet – OCaml meeting 2009 – Grenoble – France

24. Unbounded integers in VHDL (32 bits at least -"Time" requires 64 bits) Simulation of design on n bits  Big_int Application structure Abstract syntax tree Design elaboration Simulation structure tree Florent Ouchet – OCaml meeting 2009 – Grenoble – France

25. Big_int is an abstract type (=) (<) (>) on Big_int raise runtime exceptions Associative list functions based on (=) Application structure Abstract syntax tree Design elaboration Simulation structure tree Florent Ouchet – OCaml meeting 2009 – Grenoble – France

26. Many executions of the functional structure with different evaluation contexts Application structure Simulation structure tree Simulation algorithm Simulation results Florent Ouchet – OCaml meeting 2009 – Grenoble – France

27. Application structure • Signature of process simulation function: process -> structure_context -> evaluation_context -> changed_object list Florent Ouchet – OCaml meeting 2009 – Grenoble – France

28. Application structure • Prototype of process simulation function: process -> structure_context -> evaluation_context -> changed_object list • Functors for efficiency (+50%) Florent Ouchet – OCaml meeting 2009 – Grenoble – France

29. Application structure • Prototype of process simulation function: process -> structure_context -> evaluation_context -> changed_signal list • evaluation_context = { simulation_time: Big_int; currentvalues: (signal*formula) list; … } • Test with array, hashtbl and references Florent Ouchet – OCaml meeting 2009 – Grenoble – France

30. Application structure • Some timings: FIR filter simulation, 1000 clock cycles. Florent Ouchet – OCaml meeting 2009 – Grenoble – France

31. Application structure DUT Process A Process C Process B Load averaging for a multi-thread simulation. Communication efficiency for a distributed simulation. Florent Ouchet – OCaml meeting 2009 – Grenoble – France

32. Perspectives • Spread the simulation over cores and computers: an integrated circuit is a fixed finite set of concurrent process •  concurrent simulation but: • lots of rendez-vous and communications • Irregular load • Mathematics: loop invariants… Florent Ouchet – OCaml meeting 2009 – Grenoble – France

33. Conclusion • Development for project FME3 is now finished. • Research prototype for parallel computations? • Release to the public? CeCill Licence? Florent Ouchet – OCaml meeting 2009 – Grenoble – France

34. Questions? Thanks for your attention Florent Ouchet – OCaml meeting 2009 – Grenoble – France