https://github.com/openjdk/jdk/blob/090cdfc7a2e280c620a0926512fb67f0ce7f3c21/src/hotspot/share/cds/archiveHeapLoader.cpp#L90-L104

The current implementation reads the narrow oop, decodes it using the "dump time" encoding, encodes it using "runtime" encoding, and then stores it back into the heap:

// Patch all the embedded oop pointers inside an archived heap region,
// to be consistent with the runtime oop encoding.
class PatchCompressedEmbeddedPointers: public BitMapClosure {
  narrowOop* _start;

 public:
  PatchCompressedEmbeddedPointers(narrowOop* start) : _start(start) {}

  bool do_bit(size_t offset) {
    narrowOop* p = _start + offset;
    narrowOop v = *p;
    assert(!CompressedOops::is_null(v), "null oops should have been filtered out at dump time");
    oop o = ArchiveHeapLoader::decode_from_archive(v);
    RawAccess<IS_NOT_NULL>::oop_store(p, o);
    return true;
  }
};

In some cases, the decoding/encoding is not necessary. For example, if the the same number of shifts are used in both dump time and runtime, it can be simplified as:

const uintx delta = (runtime_heap-base - dumptime_heap_base)
                              >> num_shifts;

  bool do_bit(size_t offset) {
    narrowOop* p = _start + offset;
    narrowOop v = *p;
    v += delta;
    *p = v;
    return true;
  }