Looking at Unity made me understand the point of C++ coroutines

I'm not normally keen to "well actually" people with the C standard, but .. if you're writing in assembly, you're not writing in C. And the obvious consequence is that it stops being portable. Minicoro only supports three architectures. Granted, those are the three most popular ones, but other architectures exist.

(just double checked and it doesn't do Windows/ARM, for example. Not that I'm expecting Microsoft to ship full conformance for C++23 any time soon, but they have at least some of it)

That's the problem with register machines, I guess. Interestingly enough, BCPL, its main implementation being a p-code interpreter of sorts, has pretty trivially supported coroutines in its "standard" library since the late seventies — as you say, all you need to save is the current stack pointer and the code pointer.

> require the STL

That it has to heap-allocate if non-inlined is a misconception. This is only the default behavior.

One can define:

void *operator new(size_t sz, Foo &foo)

in the coro's promise type, and this:

- removes the implicitly-defined operator new

- forces the coro's signature to be CoroType f(Foo &foo), and forwards arguments to the "operator new" one defined

Therefore, it's pretty trivial to support coroutines even when heap cannot be used, especially in the non-recursive case.

Yes, green threads ("stackful coroutines") are more straightforward to use, however:

- they can't be arbitrarily destroyed when suspended (this would require stack unwinding support and/or active support from the green thread runtime)

- they are very ABI dependent. Among the "few registers" one has to save FPU registers. Which, in the case of older Arm architectures, and codegen options similar to -mgeneral-regs-only (for code that runs "below" userspace). Said FPU registers also take a lot of space in the stack frame, too

Really, stackless coros are just FSM generators (which is obvious if one looks at disasm)

That was over 20 years ago. No idea what the current hotness is.

The stack save/restore happens in: https://swtch.com/libtask/asm.S

Why? You can just as well execute all your coroutines on a single thread. Many networking applications are doing fine with just use a single ASIO thread.

Another example: you could write game behavior in C++ coroutines and schedule them on the thread that handles the game logic. If you want to wait for N seconds inside the coroutine, just yield it as a number. When the scheduler resumes a coroutine, it receives the delta time and then reschedules the coroutine accordingly. This is also a common technique in music programming languages to implement musical sequencing (e.g. SuperCollider)

In a Unity context, the engine provides the main loop and the developer is writing behaviors for game entities.

You can call a function that makes use of coroutines without worrying about it. That's the core intent of the design.

That is, if you currently use some blocking socket library, we could replace the implementation of that with coroutine based sockets, and everything should still work without other code changes.

Multithreaded? Nope. You can do C++ coroutines just fine in a single-threaded context.

Event loop? Only if you're wanting to do IO in your coroutines and not block other coroutines while waiting for that IO to finish.

> most people end up using coroutines with something like boost::asio

Sure. But you don't have to. Asio is available without the kitchen sink: https://think-async.com/Asio/

Coroutines are actually really approachable. You don't need boost::asio, but it certainly makes it a lot easier.

I recommend watching Daniela Engert's 2022 presentation, Contemporary C++ in Action: https://www.youtube.com/watch?v=yUIFdL3D0Vk

This what Rich Hickey (Clojure author) has termed “place oriented programming”, when the focus is mutating memory addresses and having to synchronize everything, but failing to model time as a first class concept.

I’m not aware of any general purpose programming language that successfully models time explicitly, Verilog might be the closest to that.

Sounds interesting. If it's not too much of an effort, could you dig up a reference?

For example, generators. Also known as semicoroutines.

https://langdev.stackexchange.com/a/834

This:

  generator fib() {
      a, b = 1, 2
      while (a<100) {
          b, a = a, a+b
          yield a
      }
      yield a-1
  }

  struct fibState {
      a,
      b,
      position
  }

  int fib(fibState state) {
      switch (fibState.postion) {
          case 0:
              fibState.a, fibState.b = 1,2
              while (a<100) {
                  fibState.b, fibState.a = fibState.a, fibState.a+fibState.b
                  // switching the context
                  fibState.position = 1;
                  return fibState.a;
          case 1:
              }

              fibState.position = 2;
              return fibState.a-1
          case 2:
              fibState.position = -1;
      }
  }

The programming language I made is a concrete example of what programming these things manually is like. I had to write every primitive as a state machine just like the one above.

https://www.matheusmoreira.com/articles/delimited-continuati...