⚛ (0xe2.0x9a.0x9b.delete@this.gmail.com) on September 23, 2021 9:37 am wrote:
>
> In other words, if I read your posts about segments correctly, you seem to be claiming
> that there is zero correlation between the overly simplistic type system of the C programming
> language (plus other languages from that era) and the downfall of segments.

I don't think that's at all what I said.

I said that segments are an extra complication, and that they don't add any value, so people dropped their use.

I said nothing about the C language. In fact, it's an entirely language-agnostic statement.

People love to blame C for various things, and claim that the "C model" made computer architecture much less interesting. You seem to be in that camp, and seem to try to claim that C had something to do with segments going away.

Not so.

C not wanting segments is not the cause, it's the correlation. Nobody wants segments. It's not that "C made computer architecture boring", it's that "learning what works and what does not" made for crazy useless features going away.

Those crazy useless features may have been "exciting", but that doesn't make them good.

If anything, the C language people bent over backwards to make excuses for horrible architecture features, and weakened the language more than they should have in order to allow for bad architecture decisions. Pretty much every single "undefined behavior" is about the language allowing for silly things.

So what I am claiming is that segments - and by extension, "capability pointer architectures" - are a bad idea. They add unwanted complexity that doesn't actually buy users anything at all into one of the most important part of the CPU pipeline - the memory units.

Don't do it. It's been a research subject for many decades, it's still a research subject (I see pointers to the ARM capability pointers in this thread), and it has never ever been successful. At what point do people just admit defeat?

The fact is, the whole notion of passing magic pointers and trying to associate random state with them between two random untrusted but somehow cooperative entities is pure BS. And I claim that no amount of segmentation - whatever you call it - will make it anything else.

Side note: that doesn't mean that you can't do other things. By all means add features like pointer coloring that is for debugging. The ARM memory tagging is actually very successful. It's not about some absolute pointer security, it's not about arbitrary segments, it's a reasonably low-overhead debugging aid that can catch bad uses early. And it still lives in the context of a regular flat address space, just one where you can add some region markers.

So you can add exciting value add in the memory unit.

It's just that segments or capabilities isn't it. Too much pain for no real gain.

Linus