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Subject: Re: [WIP 0/3] Memory model and atomic API in Rust
From: comex <comexk@gmail.com>
In-Reply-To: <ZgSNvzTkR4CY7kQC@boqun-archlinux>
Date: Wed, 27 Mar 2024 14:56:13 -0700
Cc: Kent Overstreet <kent.overstreet@linux.dev>,
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To: Boqun Feng <boqun.feng@gmail.com>


> On Mar 27, 2024, at 2:21=E2=80=AFPM, Boqun Feng <boqun.feng@gmail.com> =
wrote:
>=20
> I don't know whether I'm 100% correct on this, but Rust has =
references,
> so things like "you have a unique reference to a part of memory, no =
one
> would touch it in the meanwhile" are represented by `&mut`, to get a
> `&mut` from a raw pointer, you need unsafe, where programmers can
> provide the reasoning of the safety of the accesses. More like =
"pointers
> can alias anyone but references cannot" to me.

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`. For mutable references it=E2=80=99s =
justified because those references should be unique.  For immutable =
references it's justified because the memory behind the reference =
shouldn=E2=80=99t be mutated at all.  (There=E2=80=99s an exception for =
types with =E2=80=98interior mutability=E2=80=99, where =E2=80=98immutable=
' references actually can be used for mutation.)

The hope has always been that this gives Rust better overall =
optimizability than C or C++ and makes up for the losses from the lack =
of type-based aliasing rules.  If there=E2=80=99s any empirical data to =
justify or refute this, I=E2=80=99m not aware of it.  But that=E2=80=99s =
the hope, and by this point Rust is committed to the approach.

(Why only function parameters?  Mainly because of limitations of what =
LLVM IR can express.  =46rom the perspective of the work-in-progress =
Rust memory model spec, function parameters are special in *some* ways, =
but many of the optimizations could apply to all uses of references.  =
That's just not currently implemented.)