[Python-Dev] bpo-28055: Fix unaligned accesses in siphash24(). (GH-6123) (original) (raw)
Christian Heimes christian at python.org
Sun May 13 13:42:48 EDT 2018
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On 2018-05-13 06:57, Serhiy Storchaka wrote:
https://github.com/python/cpython/commit/1e2ec8a996daec65d8d5a3d43b66a9909c6d0653 commit: 1e2ec8a996daec65d8d5a3d43b66a9909c6d0653 branch: master author: Rolf Eike Beer <eike at sf-mail.de> committer: Serhiy Storchaka <storchaka at gmail.com> 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 uint64t* 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/showbug.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
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