SPARC Floating Point Context

[Cláudio Silva]

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

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?

Regards,


On Wed, Mar 5, 2014 at 2:26 PM, Sebastian Huber
<sebastian.huber at embedded-brains.de> wrote:
> Hello,
>
> I just noticed that the SPARC floating point unit (FPU) support is a bit
> odd:
>
> 1. the context switch saves and restores all 32 floating point registers
> (%f0 up to %f31) and the FSR
>
> http://git.rtems.org/rtems/tree/cpukit/score/cpu/sparc/rtems/score/cpu.h#n551
>
> 2. the interrupt entry/exit doesn't save/restore the floating point
> registers.
>
> Since we use the System V ABI, this makes no sense.  Here all floating point
> registers are volatile.
>
> The SPARC V8 trap entry sequence doesn't disable the FPU via the PSR_EF bit.
> So it seems that interrupt handler using the FPU can destroy the FPU context
> of the interrupted thread?
>
> For a System V ABI conforming implementation it should be:
>
> 1. the interrupt entry/exit must save/restore the floating point registers
> to/from the stack of the interrupted thread
>
> 2. nothing more
>
> What was the reason for the existing implementation?
>
> --
> Sebastian Huber, embedded brains GmbH
>
> Address : Dornierstr. 4, D-82178 Puchheim, Germany
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