mark_context_stack(mrb_state *mrb, struct mrb_context *c)
{
  size_t i;
  size_t e;
  mrb_value nil;

  if (c->stack == NULL) return;
  e = c->stack - c->stbase;
  if (c->ci) {
    e += ci_nregs(c->ci);
  }
  if (c->stbase + e > c->stend) e = c->stend - c->stbase;
  for (i=0; i<e; i++) {
    mrb_value v = c->stbase[i];

    if (!mrb_immediate_p(v)) {
      mrb_gc_mark(mrb, mrb_basic_ptr(v));
    }
  }
  e = c->stend - c->stbase;
  nil = mrb_nil_value();
  for (; i<e; i++) {
    c->stbase[i] = nil;
  }
}