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They would run under GIL (I can't see CPython C API being thread-friendly unless gilectomy succeeds)

Ah, ok. So this approach doesn't completely remove the GIL, but removes it as a barrier for pure python code running in PyPy?

Or does it break the current support for porting cpython extensions?

It removes it for pure python code. The C extensions run under the lock (which is unfair to call interpreter any more)