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Subject: Re: [WIP 0/3] Memory model and atomic API in Rust
From: comex <comexk@gmail.com>
In-Reply-To: <5246D3E2-E503-40BA-9A72-1876BCF1186B@gmail.com>
Date: Wed, 27 Mar 2024 15:02:41 -0700
Cc: Kent Overstreet <kent.overstreet@linux.dev>,
 Linus Torvalds <torvalds@linux-foundation.org>,
 "Dr. David Alan Gilbert" <dave@treblig.org>,
 Philipp Stanner <pstanner@redhat.com>,
 rust-for-linux <rust-for-linux@vger.kernel.org>,
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 linux-arch@vger.kernel.org,
 llvm@lists.linux.dev,
 Miguel Ojeda <ojeda@kernel.org>,
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 =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
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References: <c51227c9a4103ad1de43fc3cda5396b1196c31d7.camel@redhat.com>
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 <CAHk-=wgLGWBXvNODAkzkVHEj7zrrnTq_hzMft62nKNkaL89ZGQ@mail.gmail.com>
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 <ZgSNvzTkR4CY7kQC@boqun-archlinux>
 <5246D3E2-E503-40BA-9A72-1876BCF1186B@gmail.com>
To: Boqun Feng <boqun.feng@gmail.com>


> On Mar 27, 2024, at 2:56=E2=80=AFPM, comex <comexk@gmail.com> wrote:
>=20
> Right.  When I said =E2=80=9Cstrict aliasing=E2=80=9D I meant =
type-based aliasing rules, which is what GCC calls =E2=80=9Cstrict =
aliasing".  But Rust does have stricter aliasing rules than C in a =
different way.  Both mutable and immutable references are annotated with =
LLVM `noalias` by default, equivalent to C `restrict`.

=E2=80=A6oops, this should say =E2=80=9Creference-typed function =
parameters=E2=80=9D.

> On Mar 27, 2024, at 2:49=E2=80=AFPM, Kent Overstreet =
<kent.overstreet@linux.dev> wrote:
>=20
> That's not really a workable rule because in practice every data
> structure has unsafe Rust underneath. Strict aliasing would mean that
> unsafe Rust very much has to follow the aliasing rules too.


There have indeed been a lot of issues where some innocent-seeming piece =
of unsafe Rust turns out to violate the reference aliasing rules.  Miri =
helps (it=E2=80=99s a tool that can detect violations at runtime), and =
there have been attempts to loosen the rules where possible.  But it is =
definitely a case where Rust=E2=80=99s UB rules are more subtle than one =
would like.  At least it only applies to unsafe code.=