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

Kent Overstreet
2024-03-27T19:41:44Z

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
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.