alt Hacker NewsYou 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
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> You can roll stackful coroutines in C++ (or C) with 50-ish lines of Assembly
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)
Hmm. I'm fairly certain that most of that assembly code for saving/restoring registers can be replaced with setjmp/longjmp, and only control transfer itself would require actual assembly. But maybe not.
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.
C++ destructors and exception safety will likely wreak havoc with any "simple" assembly/longjmp-based solution, unless severely constraining what types you can use within the coroutines.
A much nicer code base to study is: https://swtch.com/libtask/
The stack save/restore happens in: https://swtch.com/libtask/asm.S