[issue28055] pyhash's siphash24 assumes alignment of the data pointer

[Christian Heimes]

Christian Heimes added the comment:

I'm a bit worried that the patch might slow down the general case of SipHash24. When I was working on SipHash24 I made sure that the general case in PyBytes_Object and PyUnicode_Object are fast and always aligned. Do all compilers optimize that case? For MSVC we still have a specialized Py_MEMCPY() variant in pyports.h.

I can see three more ways to fix the issue:

1) Have two loops, one for the aligned case with memcpy() and one for the unaligned case w/o memcpy()
2) Add a special variant of _le64toh() for PY_LITTLE_ENDIAN on ARM and use the current variant on X86_64.
3) Make it illegal to call _Py_HashBytes() with non-aligned pointer and require the caller to provide an aligned buffer. It's easy for datetime but requires an extra buffer memoryview. Memoryview already uses a buffer for all but single-strided C contiguous views. We can easily add another case for non-aligned buffers.

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stage:  -> needs patch
type:  -> crash

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue28055>
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