stackful 协程是指之前 boost 里面实现的 boost.context, boost.fiber 等等协程方案。基本原理是保存寄存器、jmp指令、恢复寄存器。

而 stackless 协程是指 c++20 实现的 co_await, co_yield 这个语法。它把协程的代码变换成为另外一段 c++ 的类型，类似于 lambda 那样继承一个专门的协程类型，然后调用它的方法。

这里写出了两种协程的对比。

stackless coroutines

    stackless coroutines (C++20) do code transformation (state machine)
    stackless in this case means, that the application stack is not used to store local variables (for instance variables in your algorithm)
    otherwise the local variables of the stackless coroutine would be overwritten by invocations of ordinary functions after suspending the stackless coroutine
    stackless coroutines do need memory to store local variables too, especially if the coroutine gets suspended the local variables need to be preserved
    for this purpose stackless coroutines allocate and use a so-called activation record (equivalent to a stack frame)
    suspending from a deep call stack is only possible if all functions in between are stackless coroutines too (viral; otherwise you would get a corrupted stack)
    some clang developers are sceptical that the Heap Allocation eLision Optimization (HALO) can always be applied

stackful coroutines

    in its essence a stackful coroutine simply switches stack and instruction pointer
    allocate a side-stack that works like a ordinary stack (storing local variables, advancing the stack pointer for called functions)
    the side-stack needs to be allocated only once (can also be pooled) and all subsequent function calls are fast (because only advancing the stack pointer)
    each stackless coroutines requires its own activation record -> called in a deep call chain a lot activation records have to be created/allocated
    stackful coroutines allow to suspend from a deep call chain while the functions in between can be ordinary functions (not viral)
    a stackful coroutine can outlive its caller/creator
    one version of the skynet benchmarks spawns 1 million stackful coroutines and shows that stackful coroutines are very efficient (outperforming version using threads)
    a version of the skynet benchmark using stackless coroutiens was not implemented yet
    boost.context represents the thread's primary stack as a stackful coroutine/fiber - even on ARM
    boost.context supports on demand growing stacks (GCC split stacks)

https://stackoverflow.com/questions/57163510/are-stackless-c20-coroutines-a-problem
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