GC.verify_compaction_references

static VALUE
gc_verify_compaction_references(int argc, VALUE* argv, VALUE self)
{
    static ID keywords[3] = {0};
    if (!keywords[0]) {
        keywords[0] = rb_intern("toward");
        keywords[1] = rb_intern("double_heap");
        keywords[2] = rb_intern("expand_heap");
    }

    VALUE options;
    rb_scan_args_kw(rb_keyword_given_p(), argc, argv, ":", &options);

    VALUE arguments[3] = { Qnil, Qfalse, Qfalse };
    int kwarg_count = rb_get_kwargs(options, keywords, 0, 3, arguments);
    bool toward_empty = kwarg_count > 0 && SYMBOL_P(arguments[0]) && SYM2ID(arguments[0]) == rb_intern("empty");
    bool expand_heap = (kwarg_count > 1 && RTEST(arguments[1])) || (kwarg_count > 2 && RTEST(arguments[2]));

    rb_objspace_t *objspace = rb_gc_get_objspace();

    /* This verification machinery (heap expansion, toward_empty page ordering, the
     * moved-reference walk) is built for a single objspace, so with several demote it
     * to a plain full GC.  Plain GC.compact does compact them via the global GC. */
    if (!rb_gc_single_objspace_p()) {
        rb_gc_impl_start(objspace, true, true, true, false, false);
        return gc_compact_stats(self);
    }

    /* Clear the heap. */
    rb_gc_impl_start(objspace, true, true, true, false, false);

    unsigned int lev = RB_GC_VM_LOCK();
    {
        gc_rest(objspace);

        /* if both double_heap and expand_heap are set, expand_heap takes precedence */
        if (expand_heap) {
            struct desired_compaction_pages_i_data desired_compaction = {
                .objspace = objspace,
                .required_slots = {0},
            };
            /* Work out how many objects want to be in each size pool, taking account of moves */
            objspace_each_pages(objspace, desired_compaction_pages_i, &desired_compaction, TRUE);

            /* Find out which pool has the most pages */
            size_t max_existing_pages = 0;
            for (int i = 0; i < HEAP_COUNT; i++) {
                rb_heap_t *heap = &heaps[i];
                max_existing_pages = MAX(max_existing_pages, heap->total_pages);
            }

            /* Add pages to each size pool so that compaction is guaranteed to move every object */
            for (int i = 0; i < HEAP_COUNT; i++) {
                rb_heap_t *heap = &heaps[i];

                size_t pages_to_add = 0;
                /*
                 * Step 1: Make sure every pool has the same number of pages, by adding empty pages
                 * to smaller pools. This is required to make sure the compact cursor can advance
                 * through all of the pools in `gc_sweep_compact` without hitting the "sweep &
                 * compact cursors met" condition on some pools before fully compacting others
                 */
                pages_to_add += max_existing_pages - heap->total_pages;
                /*
                 * Step 2: Now add additional free pages to each size pool sufficient to hold all objects
                 * that want to be in that size pool, whether moved into it or moved within it
                 */
                objspace->heap_pages.allocatable_bytes = desired_compaction.required_slots[i] * heap->slot_size;
                while (objspace->heap_pages.allocatable_bytes > 0) {
                    heap_page_allocate_and_initialize(objspace, heap);
                }
                /*
                 * Step 3: Add two more pages so that the compact & sweep cursors will meet _after_ all objects
                 * have been moved, and not on the last iteration of the `gc_sweep_compact` loop
                 */
                pages_to_add += 2;

                for (; pages_to_add > 0; pages_to_add--) {
                    heap_page_allocate_and_initialize_force(objspace, heap);
                }
            }
        }

        if (toward_empty) {
            objspace->rcompactor.compare_func = compare_free_slots;
        }
    }
    RB_GC_VM_UNLOCK(lev);

    rb_gc_impl_start(rb_gc_get_objspace(), true, true, true, true, false);

    rb_objspace_reachable_objects_from_root(root_obj_check_moved_i, objspace);
    objspace_each_objects(objspace, heap_check_moved_i, objspace, TRUE);

    objspace->rcompactor.compare_func = NULL;

    return gc_compact_stats(self);
}