- Simon Tatham, author of Putty, has quite a detailed blog post [0] on using the C++20's coroutine system. And yep, it's a lot to do on your own, C++26 really ought to give us some pre-built templates/patterns/scaffolds.
[0] https://web.archive.org/web/20260105235513/https://www.chiar...
by nananana9
8 subcomments
- You can roll stackful coroutines in C++ (or C) with 50-ish lines of Assembly. It's a matter of saving a few registers and switching the stack pointer, minicoro [1] is a pretty good C library that does it. I like this model a lot more than C++20 coroutines:
1. C++20 coros are stackless, in the general case every async "function call" heap allocates.
2. If you do your own stackful coroutines, every function can suspend/resume, you don't have to deal with colored functions.
3. (opinion) C++20 coros are very tasteless and "C++-design-commitee pilled". They're very hard to understand, implement, require the STL, they're very heavy in debug builds and you'll end up with template hell to do something as simple as Promise.all
[1] https://github.com/edubart/minicoro
by cherryteastain
6 subcomments
- Not an expert in game development, but I'd say the issue with C++ coroutines (and 'colored' async functions in general) is that the whole call stack must be written to support that. From a practical perspective, that must in turn be backed by a multithreaded event loop to be useful, which is very difficult to write performantly and correctly. Hence, most people end up using coroutines with something like boost::asio, but you can do that only if your repo allows a 'kitchen sink' library like Boost in the first place.
by abcde666777
4 subcomments
- More broadly the dimension of time is always a problem in gamedev, where you're partially inching everything forward each frame and having to keep it all coherent across them.
It can easily and often does lead to messy rube goldberg machines.
There was a game AI talk a while back, I forget the name unfortunately, but as I recall the guy was pointing out this friction and suggesting additions we could make at the programming language level to better support that kind of time spanning logic.
- As the author lays out, the thing that made coroutines click for me was the isomorphism with state machine-driven control flow.
That’s similar to most of what makes C++ tick: There’s no deep magic, it’s “just” type-checked syntactic sugar for code patterns you could already implement in C.
(Occurs to me that the exceptions to this … like exceptions, overloads, and context-dependent lookup … are where C++ has struggled to manage its own complexity.)
- > turns it into some sort of ugly state machine
Why are people afraid of state machines? There's been sooo much effort spent on hiding them from the programmer...
- This is one reason why I built coroutines into my game programming language Easel (https://easel.games). I think they let you keep the flow of the code matching the flow of the your logic (top-to-bottom), rather than jumping around, and so I think they are a great tool for high-level programming. The main thing is stopping the coroutines when the entity dies, and in Easel that is done by implying ownership from the context they are created in. It is quite a cool way of coding I think, avoids the state machines like the OP stated, keeps everything straightforward step-by-step and so all the code feels more natural in my opinion. In Easel they are called behaviors if anyone is interested in more detail: https://easel.games/docs/learn/language/behaviors
- Looking at C++ made me understand the point of Rust.
- Always jarring to see how Unity is stuck on an ancient version of C#. The use of IEnumerable as a "generator" mechanic is quite a good hack though.
- Coroutines generally imply some sort of magic to me.
I would just go straight to tbb and concurrent_unordered_map!
The challenge of parallelism does not come from how to make things parallel, but how you share memory:
How you avoid cache misses, make sure threads don't trample each other and design the higher level abstraction so that all layers can benefit from the performance without suffering turnaround problems.
My challenge right now is how do I make the JVM fast on native memory:
1) Rewrite my own JVM.
2) Use the buffer and offset structure Oracle still has but has deprecated and is encouraging people to not use.
We need Java/C# (already has it but is terrible to write native/VM code for?) with bottlenecks at native performance and one way or the other somebody is going to have to write it?
by appstorelottery
0 subcomment
- I've been doing a lot of work with ECS/Dots recently and once I wrapped my head around it - amazing.
I recall working on a few VR projects - where it's imperative that you keep that framerate solid or risk making the user physically sick - this is where really began using coroutines for instantiating large volumes of objects and so on (and avoiding framerate stutter).
ECS/Dots & the burst compiler makes all of this unnecessary and the performance is nothing short of incredible.
by tliltocatl
0 subcomment
- Stackless coroutines in C when? As an embedded dev, I miss them deeply. Certainly not enough RAM to give a separate stack for everything and rewriting every async call as a callback sequence sucks.
- As I mentioned on the Reddit thread,
This is quite understandable when you know the history behind how C++ coroutines came to be.
They were initially proposed by Microsoft, based on a C++/CX extension, that was inspired by .NET async/await implementation, as the WinRT runtime was designed to only support asynchronous code.
Thus if one knows how the .NET compiler and runtime magic works, including custom awaitable types, there will be some common bridges to how C++ co-routines ended up looking like.
- Coroutines is just a way to write continuations in an imperative style and with more overhead.
I never understood the value. Just use lambdas/callbacks.
- In Haskell this technique has been called ‘reinversion of control’: http://blog.sigfpe.com/2011/10/quick-and-dirty-reinversion-o...
- >> To misquote Kennedy, “we chose to focus coroutines on generator in C++23, not because it is hard, but because it is easy”.
Appreciate this humor -- absurd, tasteful.
by nice_byte
1 subcomments
- I don't know, I'm not convinced with this argument.
The "ugly" version with the switch seems much preferable to me.
It's simple, works, has way less moving parts and does not require complex machinery to be built into the language. I'm open to being convinced otherwise but as it stands I'm not seeing any horrible problems with it.
- Most game engines seem to have some coroutine kludge.
- The 'primitive' SCUMM language used for writing Adventure Games like Maniac Mansion had coroutines - an ill fated attempt to convert to using Python was hampered by Python (at the time) having no support for yield.
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- I do not find so called "green threads" useful at all. In my opinion except some very esoteric cases they serve no purpose in "native" languages that have full access to all OS threading and IO facilities. Useful only in "deficient" environments like inherently single threaded request handlers like NodeJS.
by momocowcow
4 subcomments
- No serious devs even uses Unity coroutines. Terrible control flow and perf. Fine for small projects on PC.
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