[M3devel] registers and garbage collector?

[Tony Hosking]

Yes, this is safe.  Some platforms require that we account for the red- 
zone, but generally the registers have been saved on the stack in the  
signal handler frame.  I do not want to go the global array route that  
you suggest -- no need and very slow!

On 12 Jan 2009, at 09:28, Jay wrote:

> Is this a safe assumption?
>
>
> PROCEDURE ProcessOther (act: Activation;  p: PROCEDURE (start, stop:  
> ADDRESS)) =
> ...
>     IF RTMachine.GetState # NIL THEN
>       (* process explicit state *)
>       sp := RTMachine.GetState(act.handle, state);
>     ELSE
>       (* assume registers are saved in suspended thread's stack *)
>       sp := act.sp;
>     END;
>
>
> As I understand..insert question marks a bunch here:
>
>
> The threads are all sitting waiting in their signal handler, with a  
> ucontext pointer...which could be in a register and not on the  
> stack..which contains a pointer to the registers...which could be to  
> a thread local..and not on the stack..and not in the Modula-3  
> heap..would be a simple matter to strengthen this..be sure to store  
> the ucontext in a thread local (no, actually)..or somehow be sure to  
> get it on the stack (might as well skip the ucontext and use the  
> gregs or whatever is in it)?
> Storing the ucontext in a thread local doesn't work, because you  
> can't access thread locals that aren't yours. Instead a global array  
> would likely be needed, that the signal handlers would all store  
> into. I'd suggest, like, storing the ucontext/gregs into a local, or  
> maybe a volatile local in C, maybe a struct/record, but I don't  
> think there's any guaranteeing these aren't registers, I think a  
> global array would be the way.
> I guess the assumption is fairly safe, but I'm not 100% sure.
> No, I'm not suspecting any problem here. I was just remembering that  
> I'd seen platform specific stuff for some platforms (Darwin) that  
> I'd managed to avoid, somehow, and went and looked closer, see if I  
> was legitimately avoiding it. Most platforms do avoid it.
>
> ??
>
>
>  - Jay

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