Simplified Circuit-Based SAT Solver by Alan Mishchenko

1. Simple Circuit-Based SAT Solver Alan Mishchenko

2. Outline • Motivation • Implementation • Experiment

3. Motivation for a New SAT Solver • Runtime of several applications is dominated by SAT • SAT sweeping • Sequential SAT sweeping (register/signal correspondence) • Accumulation of structural choices • Computing don’t-cares in a window • The problems solved by SAT solver in these applications have the following in common • Incremental (each problem has +/- 10 AIG nodes, compared to the previous problem solved) • Relatively easy (less than 100 conflicts) • Numerous (10K-100K problems)

4. Motivation for a Circuit-Based Solver • CNF is an intermediate step • MiniSat uses more memory to represent CNF, which is not needed for easy problems • Circuit is a helpful data-structure • It is the primary problem representation • It is more compact than CNF • It has useful information for the solver • Some of these observations are well known in the ATPG community

5. Implementation • High-level view of the solver • Solver data structures • Recursive procedure • Additional implementation details

6. High-Level View of the Solver Input: Multi-output combinational AIG Output: SAT solving status for each output SAT: satisfiable (provide a counter-example) UNSAT: unsatisfiable UNDECIDED: the solver’s resource limit is reached status_array SatSolver( Sat_Solver * p, Aig_Man * pMan ) { for each primary output Out of pMan { if ( Out is driven by a constant ) status = DeriveStatusSimple( Out ); else { PQueueAssume( p, DrivingNode( Out ) ); status = SatSolve_rec( p ); } AddStatusToArray( status_array, status ); } return status_array; }

7. Data Structures • Constraint data-base (AIG) • Each node is labeled with two binary flags • Assign: Set to 1 iff the node is assigned a value • Value: Set to 1 iff the node’s current value is 1 • Propagation queue (PQueue) • The sequence of assignments made • Justification queue (JQueue) • The set of nodes to be justified • The output is assigned 0 while inputs are unassigned • Node vector containing a SAT assignment • Used to PI nodes participating in a counter-example after a satisfiable SAT run

8. Recursive SAT Procedure status SatSolve_rec( Sat_Solver * p ) { if ( PQueuePropagate( p ) == UNSAT ) return UNSAT; if ( JQueueIsEmpty( p ) ) return SAT; Aig_Node * Var = JQueueSelectVariable( p ); int mark = PQueueMarkCurrentPosition( p ); PQueueAssume( p, !Var ); if ( SatSolve_rec( p ) == SAT ) return SAT; PQueueCancelUntil( mark ); PQueueAssume( p, Var ); if ( SatSolve_rec( p ) == SAT ) return SAT; return UNSAT; }

9. Implementation Details • JQueue is not implemented as an array (as opposed to the traditional implementation as a linked list) • Before each decision JQueue is duplicated and stored away • Non-chronological backtracking can be added • Need two additional data-structures • For each assigned node, save its level • For each assigned node, save its reason (NULL, if decision) • Recording learned clauses can be added • A learned clause is derived after each conflict • Constraint propagation on learned clauses can be added • Watched literal scheme can be used • It is important to keep only learned clauses in the data-base • this allows the solver to derive incomplete assignments

10. Experimental Results (SAT)

11. Experimental Results (CEC) CEC results for 8 hard industrial instances. Runtime in minutes on Intel Q9450 @ 2.66 Ghz. Time1 is “cec” in ABC809xx. Time2 is “&cec” in abc90329. Timeout is 1 hour. Less than 100 Mb of RAM was used in these experiments.

12. Why MiniSAT Is Slower? • Requires multiple intermediate steps • Window  AIG  CNF  Solving • Instead of Window  Solving • Uses too much memory • Solver + CNF = 140 bytes / AIG node • Instead of 8-16 bytes / AIG node • Decision heuristics • Are not aware of the circuit structure • Instead of Using circuit information