Re: HAVE_FAST_UNALIGNED_ACCESS

[Vibhav Pant <vibhavp@gmail.com>]

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On Thu, 2023-03-30 at 11:34 +0200, Robert Pluim wrote:
> Instead, HAVE_FAST_UNALIGNED_ACCESS and UNALIGNED_LOAD_SIZE should be
> removed and memcpy used instead:
> 
>   word_t a, c;
> 
>   memcpy (&a, w1 + b / ws, sizeof a);
>   memcpy (&c, w2 + b / ws, sizeof c);
> 

I had recently made a few modifications to this on master. There,
Fstring_lessp in uses the macro UNALIGNED_LOAD_SIZE instead, which is
defined to __sanitizer_unaligned_loadXX in lisp.h if:

* We're building with AddressSaniziter,
* <sanitizer/common_interface_defs.h> is available, and
* USE_SANITIZER_UNALIGNED_LOAD is defined.

> I would like to install such a change on emacs-29.  Emacs currently
> crashes when built with various compilers performing pointer
> alignment
> checks.

Instead of removing the code entirely, you could try modifying the
macro definition by getting rid of the AddressSanitizer ifdef, and
building with USE_SANITIZER_UNALIGNED_LOAD.  In theory, this should
make the load not crash with other sanitizers as well. If that works, I
imagine that would be a slightly more acceptable change to install into
emacs-29.

Best,
Vibhav
-- 
Vibhav Pant
vibhavp@gmail.com
GPG: 7ED1 D48C 513C A024 BE3A  785F E3FB 28CB 6AB5 9598

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