[issue1621] Do not assume signed integer overflow behavior

[Martin Panter]

Martin Panter added the comment:

I committed the fix for negation in audioop.

slice-step.patch includes a better fix for the remaining part of trapv.patch, with Element Tree slicing. I think this fix is much less intrusive, and I have copied it to other places that handle slicing, and added corresponding test cases.

The undefined behaviour sanitizer produces lots of errors about bit shifting signed integers in low-level modules like ctypes, struct, audioop. Typically this is for code converting signed integers to and from bytes, and big/little-endian conversions. This is technically undefined behaviour, but I think it may be less serious than the other overflows with traditional arithmetic like addition and multiplication. E.g. GCC explicitly documents <https://gcc.gnu.org/onlinedocs/gcc/Integers-implementation.html> that this is handled as expected with twos-complement, so with GCC there should be no nasty surprises with optimizing out undefined behaviour. My set-overflow.patch would also be in this boat.

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versions: +Python 3.6
Added file: http://bugs.python.org/file43785/slice-step.patch

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