[Python-Dev] Re: Re: Int FutureWarnings and other 2.4 TODOs (original) (raw)

Tim Peters tim.one at comcast.net
Thu Dec 4 11:44:09 EST 2003

[Andrew Koenig]

So here's the strategy: If the low-order bit of an integer is off, it's really a pointer to the rest of the implementation. If the low-order bit is on, then it represents an integral value that can be obtained by doing a one-bit arithmetic right shift.

Yes, it's sleazy. But I imagine it would be much faster than using inheritance.

[Phillip J. Eby]

I imagine it wouldn't, because it'd add an extra test to not only every PyINCREF and PyDECREF, but every PyObjectsomething call.

[Michael Hudson]

It wouldn't have to be that bad if you put the pointer/int thingy in the obival slot.

Not all HW Python runs on is byte-addressed, so the base idea that "the last bit" of a pointer-to-non-trivial-structure is always 0 doesn't get off the ground. C doesn't have an arithmetic right-shift operator either, but that one is easier to worm around (pyport.h already has a Py_ARITHMETIC_RIGHT_SHIFT macro to "do the right thing").

OTOH, the only current Python platform I know of that's word-addressed is the Cray T3E, which also happens to be the only one I know of where C's signed ">>" zero-fills (and is also the only one I know of that has no 16-bit integral type -- the T3E porters had an interesting time).

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