Re: Coroutines

[Maurizio Vitale <maurizio_vitale at verizon dot net>]

I'm looking at the very same problem, hoping to get very lightweight  
user-level threads for use in discrete event simulation.

It would be very nice if it was possible to write an inlined piece of  
assembler that saved the program counter and the stack pointer and  
then be able to say to GCC that everything is clobbered by those few  
lines of assembler.
Ideally this clobber should include all registers, including callee- 
saved registers, otherwise you need to explicitly save at least  
those. With that "über"-clobber I think it would be possible to  
implement very cheap context switches, where only the registers that  
GCC knows are required when resuming the context are actually saved.
All attempts to say clobber-all that I've made result in problems,  
typically failure to reload.

Is there a safe way to say to GCC that everything is clobbered in a  
way that is safe across all possible optimization settings?
I'm particularly interested in the x86_64 target.

On a side note, when writing this type of context-switching code one  
would often need to insert (local) labels in the assembler to serve  
as target for thread resumption. GCC is very aggressive in removing  
labels which according to C/C++ semantics he can prove are never  
reached. It would be nice to have an attribute to say "please, GCC,  
for reasons I cannot explain to you, this label will actually be  
reached eventually".
This is particularly useful combined with the "über"-clobber of  
above, otherwise GCC could easily convince himself that not too much  
needs to be saved.

-- Maurizio

On Jun 17, 2006, at 5:28 AM, Gabriel Dos Reis wrote:

> Andrew Haley <aph@redhat.com> writes:
>
> | Dustin Laurence writes:
> |  > On Fri, Jun 16, 2006 at 02:05:13PM -0700, Mike Stump wrote:
> |  >
> |  > > If every language were going to have the feature, then,  
> moving it
> |  > > down into the mid-end or back-end might make sense, but I  
> don't think
> |  > > it does in this case.
> |  >
> |  > Personally, I'd like, and use, decent coroutines in C.  But  
> perhaps I am
> |  > the only one.
> |  >
> |  > > I wouldn't start with pthreads I don't think.
> |  >
> |  > That was my thought--I played with it some but I intended it  
> as a bit of
> |  > threads practice.  Using threads to emulate a synchronous  
> construct just
> |  > seems *wrong.*
> |
> | You need a way to switch from one stack to another, but why not  
> leave
> | open the possibility of implementing this in a number of different
> | ways?
>
> Yup.
>
> | You need detach() and resume() [in Simula notation] and these
> | can be provided either by low-level stack-switching or by invoking a
> | pthreads library.
>
> I wouldn't use a thread library because many uses of coroutine are to
> implement low-cost, efficient, alternatives to thread where, for  
> example,
> the full power of threads are not essential.
>
> -- Gaby