The Analysis of Cyclic Circuits with Boolean Satisfiability John Backes, Brian Fett, and Marc Riedel Electrical Engineering, University of Minnesota

inputsoutputs The current outputs depend only on the current inputs. Combinational Circuits combinational logic

x a b c d AND OR AND OR x x 0 )))((( 1 fcdxab 1 f  0 Circuits with Cycles

x 1 x 1 x x a b c d AND OR AND OR )))((( 1 fcdab 1 f  Circuits with Cycles

1 1 x x x a b c d AND OR AND OR 1 ))((cdab 1 f  )( 2 abxcdf  Circuit is cyclic yet combinational; computes functions f 1 and f 2 with 6 gates. An acyclic circuit computing these functions requires 8 gates. Circuits with Cycles

Circuit Model Perform analysis in the “floating-mode”. At the outset: all wires are assumed to have unknown/undefined values ( ). the primary inputs assume definite values in {0, 1}. a “controlling” input full set of “non-controlling” inputs unknown/undefined output

Circuit Model During the analysis, we propagate controlling values. 1    ORAND   Perform analysis in the “floating-mode”. At the outset: all wires are assigned to have unknown/undefined values ( ). the primary inputs assigned definite values in {0, 1}.

Exhaustive Analysis Assign values to every wire Step through all primary inputs values Propagate all known values a b c d AND OR AND OR x x

Exhaustive Analysis Assign values to every wire Step through all primary inputs values Propagate all known values a b c d AND OR AND OR x x

Exhaustive Analysis Assign values to every wire Step through all primary inputs values Propagate all known values 1 1

Previous Work S. Malik, Analysis of Cyclic Combinational Circuits M. Riedel, J. Bruck, The Synthesis of Cyclic Combinational Circuits, DAC03: Design Automation Conference  Best Paper Award at DAC03 M. Riedel, J. Bruck, Timing Analysis of Cyclic Combinational Circuits.

Analysis Combinational

Analysis Not Combinational Analysis

Why use Boolean Satisfiability (SAT)? BDD-based analysis is slow for large problem sizes SAT-based methods are known to be a good solution for large problem sizes in practice

SAT-Based Analysis SAT-Based Analysis UNSAT (Combinational)

SAT-Based Analysis SAT (Not Combinational) SAT-Based Analysis

SAT Based Analysis of Cyclic Circuits Find feedback arc set Introduce dummy variables Encode the circuit computation for ternary- valued logic (0, 1, ) SAT Question: Is there any input assignment that produces values somewhere in the circuit? ┴ ┴

Feedback and Dummy Variables

Ternary Logic Conversion Ternary ANDEncoding SchemeBinary AND f 0 = a 0 b 0 + a 1 b 0 b 1 f 1 = a 1 b 1 + a 0 b 1 b 0

The SAT Question “For any input assignment (where all dummy variables are assigned their correct values) does a value persist?” ┴

1 1 Previous Example

The Final SAT Instance

Runtimes (seconds) CircuitAreaBDD BasedSAT BasedRatio 5xp bbara < clip cse dk duke ex keyb misex planet planet pma s s s sand average

Further Work Synthesis  Implement new synthesis algorithm using Craig interpolation  Builds off of algorithm proposed in: C.-C. Lee, J.-H. R. Jiang, C.-Y. Huang, and A. Mishchenko, “Scalable exploration of functional dependency by interpolation and incremental SAT solving”, ICCAD07: International Conference on Computer Aided Design

Further Work f 0 f 1 x 0 x x n f 2 f 3 f 0 f 1 f 2 f 3 x 0 x x n

Acknowledgements Alan Mishchenko ABC: A System for Sequential Synthesis and Verification was used to along with MiniSat to implement the SAT Based algorithm Research funding was provided by FENA