Python cryptography, Rust, and Gentoo [LWN.net]

Python cryptography, Rust, and Gentoo

Posted Feb 12, 2021 3:29 UTC (Fri) by zev (subscriber, #88455)
In reply to: Python cryptography, Rust, and Gentoo by Wol
Parent article: Python cryptography, Rust, and Gentoo

C is the perfect language for programming a PDP-11. It's just that modern computers behave completely differently to a PDP-11. [...] Someone needs to do a "C", and design a new low-level language for programming x64.

I see this kind of thing said a lot, and frankly it's never made the slightest bit of sense to me. What exactly about C itself is remotely PDP-specific? It doesn't strike me as terribly specialized for a PDP or any other particular ISA as it is for the Von Neumann model of computation, which was still pretty ubiquitous last time I checked. If we were all doing dataflow on FPGAs or whatnot, then sure, it'd be a poor fit, but we're still fetching and executing instructions (semantically) one at a time that load and store bytes in memory, pretty much just like Ken and Dennis did on their DECs.

"But modern machines have out-of-order execution and branch prediction and multi-level cache hierarchies!" I've seen some people argue...sure, but the whole point of that kind of microarchitectural sophistication is that it's microarchitectural -- it's not even directly visible at the assembly level, let alone in a high-level language. (Itanium exposed bits of its microarchitecture at the ISA level and look what a raging success that was.)

C's not without its shortcomings, but this notion that it's inappropriate for today's machines because it was initially run on a PDP-11 seems rather silly. Some of those shortcomings:

unsafety: if a 1970 PDP had been exposed to the variety of hostile inputs today's internet-connected machines are, this would have been just as much an issue.
pointer aliasing: C's challenges are much more entangled with modern compilers than they are with our hardware.
lack of abstractions/facilities for "programming in the large": pretty clearly unrelated to the underlying hardware.

None of these seem at all connected to its PDP origins. How would a "language for programming x64" differ?

Python cryptography, Rust, and Gentoo

Posted Feb 12, 2021 11:20 UTC (Fri) by Wol (subscriber, #4433) [Link] (2 responses)

> C's not without its shortcomings, but this notion that it's inappropriate for today's machines because it was initially run on a PDP-11 seems rather silly. Some of those shortcomings:

It's not that it's inappropriate. One of its major failings is that people *think* it's low level, but it doesn't map that well to what modern processors actually DO. In short, we treat it like the low-level language it *was*.

And it's that disconnect between what we think, and what actually happens, that causes all the problems.

Let's take your "unsafety" point, for example. On a PDP-11, I could have easily reasoned about what was ACTUALLY HAPPENING inside the CPU. That's not to say my programming is perfect, but my mental model of reality would have been reasonably close to reality. Nowadays, that's not true AT ALL.

And that's what bites kernel programmers all the time. Especially the noobs, their mental model of what's going on is wildly out of kilter with reality. The compiler takes the code they wrote and massively rewrites it behind their backs. And then the CPU effectively runs the object code in an interpreter I often get the impression ...

That's the point of a low-level language. Imho, if you have well-written code in a low-level language, the compiler SHOULD NOT be able to do that much optimisation. That's not a description of modern C !!!

And therein lies our problem.

Cheers,
Wol

Python cryptography, Rust, and Gentoo

Posted Feb 12, 2021 19:08 UTC (Fri) by khim (subscriber, #9252) [Link] (1 responses)

> That's the point of a low-level language. Imho, if you have well-written code in a low-level language, the compiler SHOULD NOT be able to do that much optimisation.

If we would use that definition then modern systems don't have any low-level languages. Not even machine code conforms: CPUs with speculative execution may do massive changes to what you wrote in your code!

Python cryptography, Rust, and Gentoo

Posted Feb 12, 2021 23:13 UTC (Fri) by Wol (subscriber, #4433) [Link]

Didn't I say that the CPU was an object code INTERPRETER? :-)

But if I'm unable to REASON LOGICALLY about what the CPU is going to do, how on earth am I going to get deterministic (ie it does what I want it to do) behaviour from my program?

It's turtles all the way down and logic (and the ability to debug!) has just gone down the plughole with the bathwater ...

Cheers,
Wol