Performance bug fix in the SAT solver (clearing variable activity after rollback).

src/sat/bsat/satSolver2.c

@@ -1573,6 +1573,9 @@ void sat_solver2_rollback( sat_solver2* s )
     // reset watcher lists
     for ( i = 2*s->iVarPivot; i < 2*s->size; i++ )
         s->wlists[i].size = 0;
+    // clear variable activity
+    for ( i = s->iVarPivot; i < s->size; i++ )
+        s->activity[i] = (1<<10);
 
     // initialize other vars
     s->size = s->iVarPivot;