> No, the original question really was the question that I meant to ask. :)
Ok. Then the reference to issue 7406 is really confusing, as this is about undefined behavior - why does the answer to your question affect the resolution of this issue? > I absolutely agree that CPython shouldn't invoke undefined behaviour, > precisely because of the risk of gcc (or some other compiler) > optimizing based on the assumption that undefined behaviour never > happens. This is why I opened issue 7406. > > So my question, and the listed assumptions, are about implementation- > defined behaviour, not undefined behaviour. For the 3 assumptions listed, > gcc obeys all those assumptions (see section 4.5 of the GCC manual). > > http://gcc.gnu.org/onlinedocs/gcc-4.4.2/gcc/Integers-implementation.html I think gcc makes promises here beyond resolving implementation-defined behavior. For bitshift operators, C99 says (6.5.7) [#4] The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are filled with zeros. If E1 has an unsigned type, the value of the result is E1×2E2, reduced modulo one more than the maximum value representable in the result type. If E1 has a signed type and nonnegative value, and E1×2E2 is representable in the result type, then that is the resulting value; otherwise, the behavior is undefined. [#5] The result of E1 >> E2 is E1 right-shifted E2 bit positions. If E1 has an unsigned type or if E1 has a signed type and a nonnegative value, the value of the result is the integral part of the quotient of E1 divided by the quantity, 2 raised to the power E2. If E1 has a signed type and a negative value, the resulting value is implementation- defined. Notice that only right-shift is implementation-defined. The left-shift of a negative value invokes undefined behavior, even though gcc guarantees that the sign bit will shift out the way it would under two's complement. So I'm still opposed to codifying your assumptions if that would mean that CPython could now start relying on left-shift to behave in a certain way. For right-shift, your assumptions won't help for speculation about the result: I think it's realistic that some implementations sign-extend, yet others perform the shift unsigned (i.e. zero-extend). I'd rather prefer to explicitly list what CPython assumes about the outcome of specific operations. If this is just about &, |, ^, and ~, then its fine with me. Regards, Martin _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
