SPARC Floating Point Context

[Sebastian Huber]

Hello,

On 2014-03-05 17:35, Cláudio Silva wrote:
> Hello Sebastian,
>
> Regarding this issue, there was also a old problem where "Newer
> versions of gcc can generate FPU instructions even if
> no floating point operations are made in the C code" (Paraphrasing Jiri):
>
> http://www.rtems.org/ml/rtems-users/2006/may/msg00040.htm

yes, this is why I ask.  I think the current implementation is extremely 
dangerous.  The GCC at least on ARM and PowerPC is pretty aggressive in its use 
of floating point registers for integer only tasks.  For the SPARC, I don't know.

>
> Discussed Again in:
> http://www.rtems.org/ml/rtems-users/2009/september/msg00008.html
>
> The threads includes other discussions about FP usage in SPARC/RTEMS.
>
> I don't know if this is still applicable, but it is information that
> has been circulating between SPARC/RTEMS users. Anyway the proposed
> solution was that RTEMS should always be compiled with soft-float or
> all tasks must be FP.
>
> My only comment regarding saving/restoring the FPU context in every
> interrupt is the performance penalty. How would you do it? Save it on
> Executing Thread FPU context and then skip the save part if a Context
> Switch is necessary? Further extend the ISF?

My proposal is this:

1. Disable the FPU on interrupt entry, before the high-level code is called.

2. Compile RTEMS with -msoft-float.

3. Remove the FPU context from the thread context.

4. In an interrupt initiated thread dispatch check the PSR_EF bit and in this 
case save/restore the floating point registers.

5. Add a BSP implemented method which tells the RTEMS kernel if the processor 
has an FPU.  If it has an FPU enable the PSR_EF bit in 
_CPU_Context_initialize() for floating point tasks.

Advantages:

* In case the GCC uses the FPU in interrupt handlers, you get a trap and know 
right now what is wrong and not later if FPU register corruption is noticed.

* Interrupt entry is fast since no FPU context needs to be saved.

* One library set for a BSP supports FP/non-FP applications.

* Works on SMP.

Disadvantages:

* No deferred floating point context switch.

-- 
Sebastian Huber, embedded brains GmbH

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