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

Fred L. Drake, Jr. fdrake at acm.org
Thu Dec 4 11:30:19 EST 2003

Andrew Koenig writes:

An int is a 31-bit integer, INCLUDING sign. One extra bit indicates whether the integer is really a number or, alternatively, a pointer to the rest of the representation.

Now, you may object that implementing this strategy in C will require lots of shifting and masking. I would have thought so, too. However, every C implementation of which I am aware puts all but the most trivial data structures on boundaries of two or more bytes. This fact frees the LOW-order bit of the integer to indicate whether it is a pointer.

I recently mentioned this approach to coaxing an extra bit from an existing structure to Tim, and he pointed out (rightly!) that this only works for machines with byte-addressable memories. On word-addressable memories, there's no chance to coax the extra bit from a pointer at all, and Python still runs on at least one such platform (some Cray thing, IIRC).

Otherwise I like the approach. ;-)

-Fred

-- Fred L. Drake, Jr. PythonLabs at Zope Corporation

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