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FWIW, I found a faster solution than calling `w_object()` twice.
Currently the logic for w_ref() (used for each "complex" object) looks like this:
* if ob_ref == 1
 * do not apply FLAG_REF
 * marshal normally
* else if seen for the first time
 * apply FLAG_REF
 * marshal normally
* otherwise
 * emit TYPE_REF
 * emit the ref index of the first instance
The faster solution looks like this:
* if seen for the first time
 * do not apply FLAG_REF
 * marshal normally
 * record the index of the type byte in the output stream
* else if seen for a second time
 * apply FLAG_REF to the byte at the earlier-recorded position
 * emit TYPE_REF
 * emit the ref index of the first instance
* otherwise
 * emit TYPE_REF
 * emit the ref index of the first instance
While this is faster, there are two downsides: extra memory usage and it isn't practical when writing to a file. However, I don't think either is a significant problem. For the former, it can be mostly mitigated by using the negative values in WFILE.hashtable to store the type byte position. For the latter, "marshal.dump()" is already a light wrapper around "marshal.dump()" and for PyMarshal_WriteObjectToFile() we simply stick with the current unstable approach (or change it to do what "marshal.dump()" does).
FYI, I mostly have that implemented in a branch, but am not sure when I'll get back to it.