Re: [ECOS] Re: Ecos Port to PPC509/PPC555

[Jonathan Larmour <jlarmour at cygnus dot co dot uk>]

Bart Veer wrote:
> 
> >>>>> "Sergei" == Sergei Organov <osv@Javad.RU> writes:
>     Sergei> I don't want to have FP overhead for every task
>     Sergei> in my system. Actually only few tasks should have floating
>     Sergei> point context, and the rest of tasks never use floating
>     Sergei> point. Therefore it is required to somehow specify if
>     Sergei> given task has FP context or not. This requires change(s)
>     Sergei> to the kernel API.
> 
> Mostly correct, although one possible implementation requires no
> changes to existing interfaces: add a boolean field to the context
> structure to indicate whether or not the current thread has performed
> any floating point; trap FP exceptions and set this boolean for the
> current thread; [snip]

But you can't rely on FP exceptions being generated - they don't for normal
code, so I don't believe this is a viable option unless I've misunderstood
you.

>     <snip>
>     Sergei> You was right so far, but I can't agree with the last
>     Sergei> statement. I'd prefer to don't save/restore FP context on
>     Sergei> interrupts and all context switches. First, it is possible
>     Sergei> to compile code that should never use FP with -msoft-float
>     Sergei> to prevent compiler from generating FP instructions.
> 
> This is the first complication. Even if you compile most of eCos with
> -msoft-float, there are some bits such as libm which you will probably
> want to compile with -mhard-float. Furthermore these flags are only
> applicable to the PowerPC architecture, other architectures may need
> something different. In the current eCos build system there is no easy
> way to support this, although an ugly kludge would be possible.

Actually yes we do - the existing Configuration Tool allows the overriding
of flags on a per-package basis (although no finer granularity than that).
Its under Build->Options...

Certainly one problem with doing this for (e.g.) libm is that sometimes
functions in there get called from all around the place, e.g.
*printf/*scanf. So there's a lot of scope for programmer error if they think
a thread doesn't ever need an FP context. Something to bear in mind anyway.

Jifl
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