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Re: HAVE_FAST_UNALIGNED_ACCESS
From: |
Arsen Arsenović |
Subject: |
Re: HAVE_FAST_UNALIGNED_ACCESS |
Date: |
Sat, 01 Apr 2023 17:22:48 +0200 |
Eli Zaretskii <eliz@gnu.org> writes:
> You are again trying to push for a change without showing any actual
> bug with the existing code. Please humor me, and please show me an
> actual bug due to the existing code before suggesting a solution. See
> above for the description of the details I'd like to know about such
> actual bug.
For a somewhat contrived example, UBsan flags this code (and, when
running the testsuite, it'd seem that it flags only this code).
~/gnu/emacs-29/_build 2 $ gcc --version
gcc (Gentoo Hardened 13.0.1_pre20230326-r1 p9) 13.0.1 20230326 (experimental)
Copyright (C) 2023 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
~/gnu/emacs-29/_build$ git rev-parse HEAD
b39c3cd1125590bf4b77880b41ac08b29cdfcff6
~/gnu/emacs-29/_build$ gcc -dumpmachine
x86_64-pc-linux-gnu
I can't speak to Po's example, but IIUC, he ran into a problem somewhere
(maybe the same example?).
>> .. or something similar to it, assuming I made an error, which is likely
>> given the circumstances. This does pass the testsuite, anyway. It
>> should just expand deferences into explicit memcpys.
>>
>> No actual memcpy calls are produced, and this is at least functional on
>> a superset of compilers, and I suspect replacing the whole thing with a
>> naive-looking while (*(w1++) != *(w2++)); loop would be even better (but
>> I can settle for that being too experimental).
>
> Sorry, I don't want to risk any errors, and I would like to avoid any
> experiments with the release branch. Which is why I'm asking for hard
> evidence. It isn't that I don't understand what you and others are
> saying, or don't believe you. It's just that we need to see the
> problems before we can judge the solutions that must be safe on this
> branch.
I was writing the patch for demonstration purposes more than for actual
application.
I understand why you're arguing this, and I have to thank you for it -
such efforts keep Emacs as stable as it is - but this feels like an
example that is too trivial to apply such judgment to, hence my
position. I am certain that you have an understanding of the issue at
hand, I'm only trying to provide a solution that's safe enough for 29.
Have a lovely day.
--
Arsen Arsenović
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- Re: HAVE_FAST_UNALIGNED_ACCESS, Eli Zaretskii, 2023/04/01
- Re: HAVE_FAST_UNALIGNED_ACCESS, Po Lu, 2023/04/01
- Re: HAVE_FAST_UNALIGNED_ACCESS, Eli Zaretskii, 2023/04/01
- Re: HAVE_FAST_UNALIGNED_ACCESS, Mattias Engdegård, 2023/04/01
- Re: HAVE_FAST_UNALIGNED_ACCESS, Eli Zaretskii, 2023/04/01
- Re: HAVE_FAST_UNALIGNED_ACCESS, Po Lu, 2023/04/01
- Re: HAVE_FAST_UNALIGNED_ACCESS, Eli Zaretskii, 2023/04/01
- Re: HAVE_FAST_UNALIGNED_ACCESS, Arsen Arsenović, 2023/04/01
- Re: HAVE_FAST_UNALIGNED_ACCESS, Eli Zaretskii, 2023/04/01
- Re: HAVE_FAST_UNALIGNED_ACCESS,
Arsen Arsenović <=
- Re: HAVE_FAST_UNALIGNED_ACCESS, Eli Zaretskii, 2023/04/01
- Re: HAVE_FAST_UNALIGNED_ACCESS, Po Lu, 2023/04/01
- Re: HAVE_FAST_UNALIGNED_ACCESS, Po Lu, 2023/04/01