PyTypeObject'stp_flags field is defined as anunsigned long. In effect this means that we have only 32 bits we can use for flags. We have already used all 32 of them.

Bits 15 and 16 are reserved for Stackless Python. Since Stackless Python is pining for the fjords, we could recycle those two bits. However that's not going to last long with likely changes necessary for parallelism and performance.

Any change toPyTypeObject breaks both API and ABI, but that's OK because:

• Adding a field or changing the size of a field is a backwards compatible API change.
• The existence ofPyTypeObject is part of the stable ABI, but its layout is not.

The cleanest change, IMO, is to changeunsigned long tp_flags touint64_t tp_flags which gives us another 32 bits, which should last many more years.