On 2018-05-13 06:57, Serhiy Storchaka wrote:

https://github.com/python/cpython/commit/1e2ec8a996daec65d8d5a3d43b66a9909c6... commit: 1e2ec8a996daec65d8d5a3d43b66a9909c6d0653 branch: master author: Rolf Eike Beer committer: Serhiy Storchaka date: 2018-05-13T13:57:31+03:00 summary:

bpo-28055: Fix unaligned accesses in siphash24(). (GH-6123)

The hash implementation casts the input pointer to uint64_t* and directly reads from this, which may cause unaligned accesses. Use memcpy() instead so this code will not crash with SIGBUS on sparc.

https://bugs.gentoo.org/show_bug.cgi?id=636400

files: A Misc/NEWS.d/next/Core and Builtins/2018-04-25-20-44-42.bpo-28055.f49kfC.rst M Python/pyhash.c

Hi Serhiy, I was against the approach a good reason. The PR adds additional CPU instructions and changes memory access pattern in a critical path of CPython. There is no reason to add additional overhead for the majority of users or X86 and X86_64 architectures. The memcpy() call should only be used on architectures that do not support unaligned memory access. See comment https://bugs.python.org/issue28055#msg276257 At least for latest GCC, the change seems to be fine. GCC emits the same assembly code for X86_64 before and after your change. Did you check the output on other CPU architectures as well as clang and MSVC, too? Christian