SPARC Floating Point Context

[Sebastian Huber]

On 2014-03-06 08:54, Sebastian Huber wrote:
> 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.
>

There is one problem with the -msoft-float for RTEMS approach.  At link time a 
multi-lib must be selected to provide the libc, libm, libgcc, etc.  In case the 
floating-point enabled variant is selected, then RTEMS will use e.g. memcpy() 
compiled with floating-points enabled.  So it is possible that the integer only 
RTEMS library will use floating-point instructions indirectly via library calls.

-- 
Sebastian Huber, embedded brains GmbH

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