This is part for of a five part series on coroutines in C++.
You've probably seen libraries that provide a "cooperative threading" API, e.g. threads of execution that yield to each other at specific times, rather than when the Kernel decides to. Windows Fibers, Boost Fibers, ucontext on Linux. I've been programming for a very long time, so I have written code for MacOS's cooperative "Thread Manager", back before cooperative multi-tasking was cool - at the time we were really annoyed that there was no pre-emption and that one bad worker thread would hang your whole machine. We were truly savages back then.
The gag for all of these "fiber" systems is the same - you have these 'threads' that only yield the CPU to another one via a yield API call - the yield call may name its successor or just pick any runnable non-running thread. Each thread has its own stack, thus our function (and its callstack and all state) is suspended mid-execution at the yield call.
Sounds a bit like coroutines, right?
There are two flavors of coroutines - stackful, and stackless. Fibers are stackful coroutines - the whole stack is saved to suspend the routine, allowing us to suspend at any point in execution anywhere.
C++20 coroutines are stackless, which means you can only suspend inside a coroutine itself - suspension depends on the compiler transforms of the coroutine (into a state machine object) to achieve the suspend.
When I first heard this, I thought: "well, performance is good but I wonder if I'll miss having full stack saves."
As it turns out, we don't need stack saves - here's why.
First, a coroutine is a function that has at least one potential suspend point. If there are zero potential suspend points, it's just a function, like from the 70s with the weed an the K&R syntax, and to state the obvious, we don't have to worry about a plain old function trying to suspend on us.
So if a coroutine calls a plain old function, the function can't suspend, so not being able to save the whole stack isn't important. We only need ask: "what happens if a coroutine calls a coroutine, and the child coroutine suspends?"
Well, we don't really "call" a child coroutine - we co_await it! And co_awaiting Just Works™.
- When our parent coroutine co_awaits the child routine, the child coroutine (via the awaitable mediating this transfer of execution) gets a handle to the parent coroutine to stash for later. We can think of this as the "return address" from a stack function call, and the child coroutine can stash it somewhere in its promise storage.
- When the child coroutine co-awaits (e.g. on I/O) this works as expected - we don't return to the parent coroutine because it's already suspended.
- When the child coroutine finishes for real, its final suspend awaitable can return the parent coroutine handle (that we stashed) to transfer control back to the parent - this is the equivalent of using a RET opcode to pop the stack and jump to the return address.