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From: "akihiko.odaki at daynix dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug sanitizer/114217] -fsanitize=alignment false positive with
 intended unaligned struct member access
Date: Sun, 03 Mar 2024 07:19:29 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114217

--- Comment #3 from Akihiko Odaki <akihiko.odaki at daynix dot com> ---
(In reply to Andrew Pinski from comment #1)
> >but also emits code to assert alignment.
>=20
>=20
> Yes because the code is broken still.
>=20
> The alignment is not about when the access happens but rather when the
> pointer is casted to.
>=20
> So in this case when passing in the argument to f, the argument entry sho=
uld
> be aligned to what the `struct dir_entry` is aligned to; otherwise it is
> undefined code.


I had a similar thought when I faced the same issue before and didn't repor=
t it
then, but this time I realized GCC still emits code to perform slow unalign=
ed
access for such a construct. Whichever is right, to assume an aligned or
unaligned access, it is not consistent.

Theoretically, it also makes sense to emit unaligned memory access for such=
 a
construct instead of ignoring it when -fsanitize=3Daddress, but I'm worried=
 that
such a change will break too many things.=