@pitrou:
<ranting mode>
Argh, reference counting hinders even that?
</ranting mode>
I just discovered another problem caused by refcounting.

Various techniques allow to create binary code from the interpreter
binary, by just pasting together the code for the common interpreters
cases and producing calls to the other. But, guess what, on most
platforms (except plain x86, but including x86_64 and maybe x86 for the
shared library case) this does not work if the copied code includes
function calls (on x86_64 that's due to RIP-relative addressing, and on
similar issues on other platforms).

So, Python could not even benefit from that! That's a real pity...
I'll have to try with subroutine threading, to see if that's faster than
indirect threading on current platforms or if it is still slower.