struct handle {
  void *v;
  struct handle *next;
};

#define HANDLE_TO(T) union { T* v; struct handle handle; }
#define HANDLE_REF(h) h.v
#define HANDLE_SET(h,val) do { h.v = val; } while (0)
#define PUSH_HANDLE(cx, h) push_handle(&(cx)->roots.roots, &h.handle)
#define POP_HANDLE(cx) pop_handle(&(cx)->roots.roots)

static inline void push_handle(struct handle **roots, struct handle *handle) {
  handle->next = *roots;
  *roots = handle;
}

static inline void pop_handle(struct handle **roots) {
  *roots = (*roots)->next;
}

static inline void visit_roots(struct handle *roots,
                               void (*trace_edge)(struct gc_edge edge,
                                                  void *trace_data),
                               void *trace_data) {
  for (struct handle *h = roots; h; h = h->next)
    trace_edge(gc_edge(&h->v), trace_data);
}