FLOATING POINT with rtems

[Jiri Gaisler]

On 12/02/2013 09:32 AM, Sebastian Huber wrote:
> Hello,
> 
> yes, this approach works, but I wouldn't do it that way.  Maybe we should add floating-point enable BSP variants?  For example a "leon3_fpu" BSP?
> 

The SPARC FPU issue has been discussed several times on the list.
One of the problems in the past was that gcc could use %fp registers
for integer operations (!), unless -msoft-float was used. This
will obviously break the kernel. Newer gcc versions might have changed
behaviour, but I am not sure. My proposal was to always compile
the kernel with SPARC_HAS_FPU=1, *and* -msoft-float. If a system
has an FPU, the application will be compiled without -msoft-float
and the %fp registers will be properly saved since SPARC_HAS_FPU=1.
If a system does not have an FPU, the application should be compiled
with -msoft-float, *and* RTEMS_FLOATING_POINT should NOT be added
to the task attributes. Soft-float will use integer registers,
and the %fp saving code will never be invoked.

In this way the same bsp can be used for both FPU/non-FPU system,
and we also guarantee that no %fp registers are used by gcc in
kernel or non-FPU tasks.

Jiri.