Q powerpc new exception processing.

[Charles-Antoine Gauthier]

Sergei Organov wrote:
> 
> Joel Sherrill <joel.sherrill at oarcorp.com> writes:
> > Sergei Organov wrote:
> > >
> > > Hello,
> > >
> > > I'm going to switch to new exception processing model for mpc505/509
> > > processors. In 'cpu_asm.S' in the 'new_exception_processing' subdirectory
> > > routines '_CPU_Context_save_fp' and '_CPU_Context_restore_fp' are changed
> > > compared to the version in 'old_exception_processing'. They don't contain
> > > "double" version anymore, i.e. loading/storing of FP registers is
> > > unconditionally made using 'stfs/lfs' instructions. Do I need to return double
> > > version back, or am I missing something?
> >
> > I suppose you might as well restore this double option.  I believe there
> > was some discussion that it may not have the dramatic benefits once
> > believed
> > but having it available makes it possible to enable it when it will be
> > of use.
> 
> Now I understand even less than before :-( AFAIK most PowerPC processors that
> have FP have 64 bits FP registers, i.e. they have "double" FPU and in my
> understanding for such processors using 'stfs/lfs' to save/restore FP
> registers is just wrong. Are 750 and 604 that seem to be the only two
> processors supported by the new exception processing different in this area?
> Why 'single' instructions are used in the first place? Isn't it correct to
> just replace them with 'double' version?

stfs CONVERTS the content of a double precision FPR to single precision
before storing to memory, while lfs CONVERT from single to double
precision on a load. Clearly, using stfs/lfs is wrong.

Maybe the original author can explain what we don't understand.

> 
> Thanks in advance,
> Sergei.

-- 
Charles-Antoine Gauthier
Institute for Information Technology   Institut de technologie de
l'information
National Research Council of Canada    Conseil national de recherches du
Canada