Q powerpc new exception processing.

[Sergei Organov]

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?

Thanks in advance,
Sergei.