Introduction to Formal Equivalence Verification (FEV)

1. Introduction to Formal Equivalence Verification (FEV) Erik Seligman CS 510, Lecture 4, January 2009

2. Goals • Introduce basic concepts of FEV • Enable you to try FEV using Cadence Conformal • Examine some corner cases

3. FEV: The Basic Concepts

4. What Is FEV? • Best-established form of FV • Other names: Equivalence Checking • Answers: Are two models equivalent?

5. Main Uses of FEV • RTL-Netlist equivalence • Essential part of design flows • Also leveraged for late hand edits (ECOs) • Verifying quick changes to a model • Fast & easy if model almost the same

6. Types of FEV • Combinatorial / Synchronous • Models must be (mostly) state-matching • Very efficient due to no time calculations • Works very well for synthesized netlists • Most synthesis tools expect this • Cadence Conformal is leader • Others: Synopsys Formality, Magma Quartz • Sequential • Allows more abstract RTL, or HLM-RTL FEV • More flexibility for late netlist timing edits • Much more risk/expense • Few commercial tools (Calypto, NEC)

7. State-Matching FEV

8. Are these equivalent? a f1 f2 b out ck a f3 f4 b out ck

9. Step 1: Map key points a f1 f2 b out f4 f3 ck a f3 f4 b out ck Inputs?- Match. Outputs? Match. States? f1->f3, f2->f4

10. Step 2: Build Equations a f1 f2 b out f4 f3 ck a f3 f4 b out ck f3 = b, f4 = f3, out = !(a&f4) f3=b, f4 = !(!f3), out = !a | !f4

11. Step 3: Compare Equations a f1 f2 b out f4 f3 ck a f3 f4 b out ck f3 = b bEQUAL f4 = f3 !(!f3) EQUAL out =!(a&f4) !a | !f4 EQUAL

12. What if there was an error? a f1 f2 b out f4 f3 ck a f3 f4 b out ck f3 = b bEQUAL f4 = f3 !f3DIFFER out =!(a&f4) !a | !f4 EQUAL

13. Debugging: Where To Look • Fanin cones (“support set”) • Different fanin  major issue • Set of counterexample values • If only specific values cause cex, provides hint of root cause • “Intelligent” hints from tools • Is an overall inversion suspected? • Identify similar areas of logic within cone? • Isolate error

14. Debug Schematic View 1 1 f1 f2 f4 f3 ck 1 0 1 f3 f4 ck • Combinational  other logic irrelevant • Good tools provide annotated cex value

15. Introduction To Conformal

16. Conformal Terminology • Gold = golden model (often RTL) • Rev = revised model (often netlist) • Many commands have –gold/-rev option • Key Point = points to map • Basic ones: primary inputs/outputs, states • Others: blackboxes, dangling (Z) nodes, … • Can refer to by name or integer ID • Support Set = fanin cone

17. Conformal Modes • Setup Mode: initial state • Can load models, assign renaming rules • Can set various global options • Return to this mode: “set sys mode setup” • LEC Mode: checking state • Transition with “set sys mode lec” • Automatically tries to map key points • Models have been loaded, can compare

18. Conformal Usage Model • Based on command console • Startup with “LEC –nogui” • Capable of taking general tcl scripts • “help” available for any command • Example: “help read design” • Full manuals in /pkgs/cadence6/CONFRML71/doc • “set log file <filename>” to start logging • Always do this for homework! • “set gui on” / “set gui off” can be done any time • “dofile <filename>.do” to execute script • Script = any set of console commands

19. Mapping Key Points • LEC has good automapper • Can guess many mappings • But sometimes fails • View mapping as “renaming” • Temporarily rename RTL sig to match netlist • “add renaming rule” to specify mappings • Or “add mapped point” in LEC mode

20. Skeleton LEC Dofile set log file lec.log –replace read design –systemverilog –gold –f myrtl.filelist read design –systemverilog –rev –f mynetlist.filelist add renaming rule r1 foo bar –gold set sys mode lec report unmapped points add compare points –all compare report compare data

21. Skeleton LEC Dofile set log file lec.log –replace read design –systemverilog –gold –f myrtl.filelist read design –systemverilog –rev –f mynetlist.filelist add renaming rule r1 foo bar –gold set sys mode lec report unmapped points add compare points –all compare report compare data

22. Skeleton LEC Dofile set log file lec.log –replace read design –systemverilog –gold –f myrtl.filelist read design –systemverilog –rev –f mynetlist.filelist add renaming rule r1 foo bar –gold set sys mode lec report unmapped points add compare points –all compare report compare data

23. Skeleton LEC Dofile set log file lec.log –replace read design –systemverilog –gold –f myrtl.filelist read design –systemverilog –rev –f mynetlist.filelist add renaming rule r1 foo bar –gold set sys mode lec report unmapped points add compare points –all compare report compare data

24. Skeleton LEC Dofile set log file lec.log –replace read design –systemverilog –gold –f myrtl.filelist read design –systemverilog –rev –f mynetlist.filelist add renaming rule r1 foo bar –gold set sys mode lec report unmapped points add compare points –all compare report compare data

25. Skeleton LEC Dofile set log file lec.log –replace read design –systemverilog –gold –f myrtl.filelist read design –systemverilog –rev –f mynetlist.filelist add renaming rule r1 foo bar –gold set sys mode lec report unmapped points add compare points –all compare report compare data

26. Skeleton LEC Dofile set log file lec.log –replace read design –systemverilog –gold –f myrtl.filelist read design –systemverilog –rev –f mynetlist.filelist add renaming rule r1 foo bar –gold set sys mode lec report unmapped points add compare points –all compare report compare data

27. Debugging Mismatches • Debug commands available in console • “diagnose <point>”: Display basic info • But easier to debug in gui • Report->Compare Data to see all points • Red dots indicate mismatches • Right-click at mismatch point, and “Diagnose” • Gives support set, cex values, and LEC’s hints • From Diagnose window can launch sch view

28. Report -> Compare Data

29. Example: Fanin Cone

30. Example: Inversion

31. Example: Messy Error

32. Schematic View

33. Model “Flattening” Minor exceptions to state-matching Useful if flops/latches don’t map

34. Are These Equal? rst d rst d DLAT

35. Are These Equal? rst d rst d DLAT set flatten model –dff_to_dlat_zero

36. Are These Equal? rst ck rst DLAT ck

37. Are These Equal? rst ck rst DLAT ck set flatten model –dff_to_dlat_feedback

38. Are These Equal? ck

39. Are These Equal? ck set flatten model –seq_constant

40. Are These Equal? d ck d DLAT DLAT ck

41. Are These Equal? d ck d DLAT DLAT ck set flatten model –latch_fold

42. Are These Equal? DLAT

43. Are These Equal? DLAT set flatten model –latch_transparent

44. Model Flattening • Tool modified cases on previous slides • Internally changes view of logic • Only on request, not automatic • May cause mismatches rather than curing! • Often useful if key point imbalance • In Conformal: “set flatten model…” • Many options, not just ones on slides • Can also use “remodel …” on single point

45. FEV Constraints

46. Are these equivalent? a f1 f2 b out ck f3 f4 b out ck

47. Are these equivalent? a f1 f2 b out ck f3 f4 b out ck • No! BUT– What if ‘a’ is always 1?

48. FEV: Why Constraints? • RTL is often very general `ifdef CHIP_VERSION_1 `define A 1 `else `define A 2 `endif • Design reuse: irrelevant RTL remains assign A = 1’b1; … if (!A) …

49. Why Do Contraints Matter? • Good synthesis tools take advantage • Assume constants to reduce size/scope • Don’t synthesize masked-out RTL • Allow out-of-band constraint specs in control files • FEV must recognize constraints • Otherwise get mismatches • No effort *if* constraints visible at FEV level • But may be only in wrapper RTL • Or inside analog blackbox • Or could be due to software / outside specs • If not visible to tool, may need to specify • add pin constraint 0 /foo/bar

50. Some References • http://en.wikipedia.org/wiki/Formal_equivalence_checking • http://cad-for-vlsi.blogspot.com/2007/03/111-art-of-equivalence-checking.html • Full Conformal docs at /pkgs/cadence6/CONFRML71/doc