undefined | Better HN

0 pointsbayindirh2y ago0 comments

C is only portable between systems which emulate PDP-11 at hardware level and if and only if you don't use any compiler-specific extensions.

If you use sys calls, work between different breeds of operating systems (UNIX, POSIX and Windows are not compatible with each other), you need to rewrite or wrap relevant parts, or write the relevant part beforehand inside ifdefs to be able "port" it between systems.

The gist of the piece is, hardware is evolving to please C's programming model, hiding all the complexities C is not aware of, and behave like a PDP-11 on steroids. This is why we have truckload of side-channel attacks in X86 to begin with. To "emulate" PDP-11s faster and faster.

0 comments

5 comments · 2 top-level

AnimalMuppet2y ago· 3 in thread

I think we'd have the side-channel attacks on x86 even if we wrote in assembler - unless we wrote the assembler specifically with an eye to preventing (the known kinds of) side-channel attacks.

Put differently, I don't think the side-channel attacks would disappear if we wrote in Rust or Haskell or Agda.

bayindirhOP2y ago

The side channel attacks are not a result of programming in C, but the design of the hardware which doesn't upset the view of the system w.r.t. C compilers.

All programming languages, regardless of their type (imperative, functional) or interfacing method with the system (JIT, interpreted, compiled) are not immune from these attacks, because it's the hardware which is designed to emulate PDP-11.

In other words, all programming languages target a modern PDP-11 at the end of the day. If hardware has shown all of its tricks (esp. cache management, invalidation, explicit prefetching, etc.), and lacked speculative, out of order execution, these problems will go away, but getting the highest performance would become much harder and complicated, and even impossible in some cases.

Intel tried this with IA64, with a "No tricks, compiler shall optimize" approach, and it tanked to put it mildly (esp. after AMD64 came out).

AnimalMuppet2y ago

Let's say we have two chips. Chip A requires the programmer to handle all the "magic" stuff. Chip B is like current chips; it hides that stuff. Chip B is subject to side channel attacks. Chip A likely is also unless the programmer is very careful.

Which chip would have sold more? I assert that chip B would have, by a massive volume, because it didn't require the programmer to mess with all that stuff.

So I don't think that it's fair to say that the chip is trying to look like a PDP-11 because of C. I think it's trying to look like a simpler chip, so that mere mortals can program it and still get most of the maximum performance.

1 more reply

Yoric2y ago

I believe that the point is the processor was designed to please C (by emulating PDP-11). And this design complicates things immensely, which is how we end up with side-channel attacks on our processors.

Joker_vD2y ago

It's not even that faithful to PDP-11, either. PDP-11 has unified integer division/modulo instruction (and it operates in double-width: it takes 32-bit dividend and 16-bit divisor and produces 16-bit quotient and remainder, just like x86), it has double-width integer multiplication (again, just like x86), it has instruction for addition/subtraction with carry — nothing of that is available from (standard) C, and it's quite a pity. And also, while PDP-12 it has built-in support for post-increment and pre-decrement for pointers, it doesn't has built-in pre-increment or post-decrement.

j / k navigate · click thread line to collapse