[Python-Dev] ANSI strict aliasing and Python (original) (raw)

Ben Laurie ben@algroup.co.uk
Sat, 19 Jul 2003 11:38:15 +0100

Martin v. L�wis wrote:

"Tim Peters" <tim.one@comcast.net> writes:

Notice that a compiler is allowed to infer that the test is always false.

I don't buy that. I'll buy that the result of the comparison is undefined by C, and that a hostile implementation of C could arbitrarily decide to call all such expressions false -- or arbitrarily decide to call all such expressions true. You are right: it is undefined, so an implementation that always gives false might still be conforming. The closest statement to defining behaviour is 6.3.2.3p7: # A pointer to an object or incomplete type may be converted to a # pointer to a different object or incomplete type. If the resulting # pointer is not correctly aligned57) for the pointed-to type, the # behavior is undefined. Otherwise, when converted back again, the # result shall compare equal to the original pointer. So if you convert two pointers to the same PyIntObject to PyObject*, and convert them back, then compare them, you are guaranteed to get true. If you compare them while they are PyObject*, no guarantees are given.

FWIW, K&R 2nd Ed. says in A6.6 that this is only true if the conversion is to a type whose alignment requirement is less or equally strict. It also says that chars have the least strict alignment requirement.

Cheers,

Ben.

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