Clang vs. Clang

When I saw the link was to DJB’s site, I figured the post would contain a vitriolic and hyperbolic rant. It’s pretty on-brand for him (although, to be fair, he’s usually right.)

But blaming the compiler devs for this is just misguided.

For one thing, CPU caches are wonders of technology, but a C compiler that only uses registers for computations but stores all results in memory and issues a load for every read will be unbearingly slow.

So, you need a register allocator and if you have that, you either need (A) an algorithm to spill data to memory if you run out of registers, or (B) have to refuse to compile such code.

If you make choice A, any change to the code for spilling back to memory can affect timings and that can introduce a timing bug in constant-time code that isn’t branch-free.

Also, there still is no guarantee that code that is constant-time on CPU X also will be on CPU Y. For example, one CPU has single-cycle 64-bit multiplication, but another doesn’t.

If you make choice B, you don’t have a portable language anymore. Different CPUs have different amounts of registers, and they can have different features, so code that runs fine in on one CPU may not do so on another one (even if it has the exact same amount of registers of the same size).

Phrased differently: C isn’t a language that supports writing constant-time functions. If you want that, you either have to try hard to beat a C compiler into submission, and you will fail in doing that, or choose a different language, and that likely will be one that is a lot like the assembly language of the target CPU. You could make it _look_ similar between CPUs, but there would be subtle or not so subtle differences in semantics or in what programs the language accepts for different CPUs.

Having said that: a seriously dumbed down C compiler (with a simple register allocator that programmers can mostly understand, no constant folding, no optimizations replacing multiplications by bit shifts or divisions by multiplications, 100% honors ‘inline’ phrases, etc.) probably could get close to what people want. It might even have a feature where code that requires register spilling triggers a compiler error. I am not aware of any compiler with that feature, though.

I wouldn’t call that C, though, as programs written in it would be a lot less portable.

Hence why games for 8 and 16 bit home computers were mostly written in Assembly, and there were books like the Zen of Assembly Programming.

It was the way that optimizating compilers started to exploit UB in C, that finally made it fast enough for modern times.

Modern hardware has nothing to do with C abstract machine.