PPC float context only 4 Bytes per register?

[Till Straumann]

To quote from a lesson out of my first English textbook:

"What a shock for Mrs. Dent..."

I had a look at the code in question and I must agree. This simple
test program confirms our suspicions:


void fpctxt_tst(void *parm)
{
volatile double val;
register double tst;
rtems_interrupt_level l;
int     i,j;
char    mval[50];

        rtems_interrupt_disable(l);
        tst = val = sqrt(2.);
        rtems_interrupt_enable(l);

        j=0;
        do {
                if (j) rtems_task_wake_after(2);
                j++;
                rtems_interrupt_disable(l);
                i = (tst == val);
                rtems_interrupt_enable(l);
        } while( i );

        rtems_interrupt_disable(l);
        sprintf(mval,"%.20g",val);
        rtems_interrupt_enable(l);

        printf("FP register mismatch in %ith loop, should be %s but is
%.20g\n",j,mval,tst);
}

I must admit that I'm quite a bit shocked -
thanks to Thomas for pointing this out. How come that this
has been known for a while (see Sergej's message) but not fixed?
IMO, this deserved at least a 'known-issues' entry or similar.

Luckily, it should be easy - is somebody working on this already?

-- Till

Thomas Doerfler wrote:

> Hi,
>
> I have just had some looks into the
> libcpu/powerpc/new_exception_processing/cpu_asm.S code to
> save/restore a floating-point context. In there it seems that
> for every context switch all the float registers are saved
> into 4 byte locations (in "single" precision format).
>
> From my point of view this results in rounding errors for
> "double precision" (64 bit) float arithmetics. So if I have
> written some code using "double precision" variables I will
> get roundoff errors, if there has been a context switch during
> calculations. (I did not try that out, but it seems logical to
> me...)
>
> Did I miss something there?
>
> Any comments welcome...
>
>         Thomas Doerfler.
> --------------------------------------------
> IMD Ingenieurbuero fuer Microcomputertechnik
> Thomas Doerfler           Herbststrasse 8
> D-82178 Puchheim          Germany
> email:    Thomas.Doerfler at imd-systems.de
> PGP public key available at: http://www.imd-
> systems.de/pgp_key.htm