Original and learnt clauses in Conflict-Driven Clause Learning (CDCL) SAT solvers often contain redundant literals. This may have a negative impact on performance because redundant literals may deteriorate both the effectiveness of Boolean constraint propagation and the quality of subsequent learnt clauses. To overcome this drawback, we propose a clause vivification approach that eliminates redundant literals by applying unit propagation. The proposed clause vivification is activated before the SAT solver triggers some selected restarts, and only affects a subset of original and learnt clauses, which are considered to be more relevant according to metrics like the literal block distance (LBD). Moreover, we conducted an empirical investigation with instances coming from the hard combinatorial and application categories of recent SAT competitions. The results show that a remarkable number of additional instances are solved when the proposed approach is incorporated into five of the best performing CDCL SAT solvers (Glucose, TC_Glucose, COMiniSatPS, MapleCOMSPS and MapleCOMSPS_LRB). More importantly, the empirical investigation includes an in-depth analysis of the effectiveness of clause vivification. It is worth mentioning that one of the SAT solvers described here was ranked first in the main track of SAT Competition 2017 thanks to the incorporation of the proposed clause vivification. That solver was further improved in this paper and won the bronze medal in the main track of SAT Competition 2018.

Chavira and Darwiche 2008; Apsel and Brafman 2012)) and forms of planning (see e.g., (Palacios et al. 2005; It and more importantly the variable elimination rule (replacing proves useful when the problem under consideration (e.g., in the input CNF formula all the clauses containing a the satisfiability issue) can be solved more efficiently when given variable x by the set of all their resolvents over x) or the input formula has been first preprocessed (of course, the blocked clause elimination rule (removing every clause the preprocessing time is taken into account in the global containing a literal such that every resolvent obtained by resolving solving time). Some preprocessing techniques are nowadays on it is a valid clause).