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

Boqun Feng
2024-03-27T21:22:02Z

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.

Regards,
Boqun

> problem. A lot of C programmers have stockholm syndrome when it comes to
> this, we end up writing a lot of code in weirdly baroque and artificial
> styles to partially work around this when we care about performance -
> saving things into locals because at least the _stack_ generally can't
> alias to avoid forced reloads, or passing and returning things by
> reference instead of by value when that's _not the semantics we want_
> because otherwise the compiler is going to do an unnecessary copy -
> again, that's fundamentally because of aliasing.