Re: [WIP 0/3] Memory model and atomic API in Rust

Kent Overstreet
2024-03-27T21:50:08Z

On Wed, Mar 27, 2024 at 02:21:03PM -0700, Boqun Feng wrote:
> On Wed, Mar 27, 2024 at 03:41:16PM -0400, Kent Overstreet wrote:
> > On Wed, Mar 27, 2024 at 12:07:26PM -0700, Linus Torvalds wrote:
> > > On Wed, 27 Mar 2024 at 11:51, Kent Overstreet <kent.overstreet@linux.dev> wrote:
> > > >
> > > > On Wed, Mar 27, 2024 at 09:16:09AM -0700, comex wrote:
> > > > > Meanwhile, Rust intentionally lacks strict aliasing.
> > > >
> > > > I wasn't aware of this. Given that unrestricted pointers are a real
> > > > impediment to compiler optimization, I thought that with Rust we were
> > > > finally starting to nail down a concrete enough memory model to tackle
> > > > this safely. But I guess not?
> > >
> > > Strict aliasing is a *horrible* mistake.
> > >
> > > It's not even *remotely* "tackle this safely". It's the exact
> > > opposite. It's completely broken.
> > >
> > > Anybody who thinks strict aliasing is a good idea either
> > >
> > > (a) doesn't understand what it means
> > >
> > > (b) has been brainwashed by incompetent compiler people.
> > >
> > > it's a horrendous crock that was introduced by people who thought it
> > > was too complicated to write out "restrict" keywords, and that thought
> > > that "let's break old working programs and make it harder to write new
> > > programs" was a good idea.
> >
> > Strict aliasing is crap in C and C++ because we started out with
> > unrestricetd pointers, and it just doesn't work in C and C++ with the
> > realities of the kind of code we have to write, and we never got any
> > kind of a model that would have made it workable. Never mind trying to
> > graft that onto existing codebases...
> >
> > (Restrict was crap too... no scoping, nothing but a single f*cking
> > keyword? Who ever thought _that_ was going to work?)
> >
> > _But_: the lack of any aliasing guarantees means that writing through
> > any pointer can invalidate practically anything, and this is a real
>
> 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.

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.