If you wanted two separate threads to share data on the stack of one thread, why not just pass the second thread a pointer to the first thread's actual stack data, instead of relying on stack-relative addressing?
For your ASM example, instead of relying on RSP, keep the first thread's stack address in RBP during execution of the second thread. Then the second thread can have its own entirely separate stack via its own RSP value.
> why not just pass the second thread a pointer to the first thread's actual stack data, instead of relying on stack-relative addressing?
That would probably be the only way to do it in a high-level language, but otherwise there really isn't a need to, just like I didn't see a need to setup another stack. I also was not thinking in terms of threads - it was like "swap between these blocks of code", and there were more than 2 of them. Being able to think about and do things like this is why I'm using Asm in the first place - I could restrict myself to writing code using the more limiting conventions of HLLs, almost like a compiler would generate, but I feel that rather defeats the point of using Asm (why not just use a compiler?) Not to say that I don't appreciate using a thread abstraction when it's appropriate, but at this level there are no threads, no functions, no strict relationships between calls and returns, nothing other than a series of instructions.
The "fast-stack-switch" using registers is also something I remember doing before, though it wasn't on x86.
For your ASM example, instead of relying on RSP, keep the first thread's stack address in RBP during execution of the second thread. Then the second thread can have its own entirely separate stack via its own RSP value.