void add_reference(OopOrNarrowOopStar from, int tid) {

  {- -------------------------------------------
  (1) Remembered Set に参照元情報を記録する. 2通りのパスがある.

      * もし GC 中であり (= PerRegionTable がセットされており)
        かつこのスレッドが 0 番スレッドでなければ(= tid が 0 でなければ),
        tid に該当する PerRegionTable オブジェクトに対して
        PerRegionTable::seq_add_reference() をよぶことで記録する.

      * それ以外の場合は,
        PerRegionTable::add_reference() を呼んで
        base table (= この PosParPRT オブジェクト自身) に記録する.
      ---------------------------------------- -}

        // Expand if necessary.
        PerRegionTable** pt = par_tables();
        if (pt != NULL) {
          // We always have to assume that mods to table 0 are in parallel,
          // because of the claiming scheme in parallel expansion.  A thread
          // with tid != 0 that finds the table to be NULL, but doesn't succeed
          // in claiming the right of expanding it, will end up in the else
          // clause of the above if test.  That thread could be delayed, and a
          // thread 0 add reference could see the table expanded, and come
          // here.  Both threads would be adding in parallel.  But we get to
          // not use atomics for tids > 0.
          if (tid == 0) {
            PerRegionTable::add_reference(from);
          } else {
            pt[tid-1]->seq_add_reference(from);
          }
        } else {
          // Not expanded -- add to the base table.
          PerRegionTable::add_reference(from);
        }
      }