The line marked below isn't 64-bit clean:

  void GenMarkSweep::allocate_stacks() {
    GenCollectedHeap* gch = GenCollectedHeap::heap();
    // Scratch request on behalf of oldest generation; will do no
    // allocation.
    ScratchBlock* scratch = gch->gather_scratch(gch->_gens[gch->_n_gens-1], 0);

    // $$$ To cut a corner, we'll only use the first scratch block, and then
    // revert to malloc.
    if (scratch != NULL) {
      _preserved_count_max =
-----> scratch->num_words * sizeof(juint) / sizeof(PreservedMark);
    } else {
      _preserved_count_max = 0;
    }