The GC does marking using the capability pointer, not the pointer's integer value. So, there's no need for a general "hey GC, what object does this point into and where does it start" query.

The capability is the start of the allocation most of the time. It might have a bit in it saying that it's an aligned allocation (indicating alignment greater than 16 bytes), in which case there's an alignment hole between the start of what the GC thinks is the allocation and where the capability starts. Also capabilities might be global, meaning that there is no need to mark them. So, to mark a capability (which the runtime calls an "object"), the algorithm is something like:

    if (object->is_global)
        return;
    void* thing_to_mark;
    if (UNLIKELY(object->is_aligned))
        thing_to_mark = object & -object->alignment;
    else
        thing_to_mark = object;
    mark(thing_to_mark);