Remove global variable RUNTIME.

[MashPlant]

More detailed modifications are recorded in CHANGELOG.md. My English may be poor, so if you are confused with my explanation, just ask me directly.

[cfsamson]

First of all. This looks like an impressive improvement. Some cool techniques here I haven't seen before. Using global_asm! was a smart move.

Have you tried running the program under Valgrind? Regarding the wasted space on the stack I think I added some space at the bottom because I got an issue reporting some errors in Valgrind using offsets of -16 and -8 but I might be mistaken.

I'll get back to you on this one once I've had some time to go through everything thoroughly.

[liufuyang]

I tried to compile your code on my mac and it seems giving this error? 🤔 Have you tested your code on mac os or?

error: linking with `cc` failed: exit code: 1
  |
  = note: Undefined symbols for architecture x86_64:
            "_switch", referenced from:
                green_threads::Runtime::do_yield::h6963f530cc3ab8ee in green_threads-77913a0e134e42f4.nexreiqecebn77q.rcgu.o
          ld: symbol(s) not found for architecture x86_64
          clang: error: linker command failed with exit code 1 (use -v to see invocation)

[cfsamson]

What I'll most likely do here is that I'll merge this into a separate branch, and point readers to it in the start of the book.

It requires a substantial rewrite of the Windows branch as well in addition to rewrite (and proofread) quite a bit of the text in the book if I merge this to master (and I also think the added complexity might make the example less accessible to readers which also argues for keeping it in a separate branch).

I'll still have to run the code on macos and through Valgrind though. I can't see any obvious reason this should work on Linux and not macos as reported by @liufuyang.

[cfsamson]

Ok, so I have had some time to have a look at this now. It works on Linux, but there is a problem on macos, with the following error:

= note: Undefined symbols for architecture x86_64:
            "_switch", referenced from:
                green_threads::Runtime::do_yield::h2c1879a601ea63e4 in green_threads-3e6b868f1b554120.3xumwfqgwyz6otwr.rcgu.o
          ld: symbol(s) not found for architecture x86_64
          clang: error: linker command failed with exit code 1 (use -v to see invocation)

Turns out that there are some subtle differences in the ABI, one is that symbols are prefixed with a leading underscore for function names on macos. Changing the name of the function defined in the assembly to _switch solved the linker error but resulted in a segfault at runtime.

I'd love for this to work on both macos and linux (it's ok to skip windows for this), but I don't have time to debug this rignt now.

I'll consider referencing this in the book anyways stating the limitations just because I think it used some interesting techniques that.