Efficient Green Thread Context-Switching

I propose that the LLVM devs consider adding “swapstack” and “newstack” into the LLVM project officially.

Thanks for the reference. I was going to suggest the ‘preserve_none’ calling convention, applied to asm which does the stack pointer swapping, would be equivalent to swapstack. However that calling convention doesn’t appear to exist - it would be one where the caller is required to save all live values to the stack before jumping.

‘preserve_none’ may be a useful calling convention independent of green threads. Functions which will always use all the available registers benefit from not having to save/restore any of them, if the caller knew to preserve any live values. A green thread context switch is roughly a call that preserved nothing.

Newstack I’m not as sure about. Green thread libraries are likely to put a lot of effort into how stacks are sized, allocated, reused. That seems better as a runtime call.


I could live with that. You’re right that “newstack" is probably more use-case-dependant. A “preserve_none” calling convention would be more general than swapstack, which is perhaps more preferred for LLVM. I don’t think that would be a big deal for swapstack-seekers, because I think the stack pointer swapping could be achieved using @llvm.read_register() and @llvm.write_register().

I’m glad to know other people are in support of this functionality.