assert(!(this_sp && this_sp->slavesMask.none()));
assert(Threads.size() > 2);
// Prefer to join to SP with few parents to reduce the probability
// that a cut-off occurs above us, and hence we waste our work.
assert(!(this_sp && this_sp->slavesMask.none()));
assert(Threads.size() > 2);
// Prefer to join to SP with few parents to reduce the probability
// that a cut-off occurs above us, and hence we waste our work.