Re: [RFC]: MD_FALLBACK_FRAME_STATE_FOR macro for darwin PPC

[David Daney <ddaney@avtrex.com>]

Andrew Haley wrote:

>David Daney writes:
> > Andreas Tobler wrote:
> > 
> > > David Daney wrote:
> > >
> > >
> > >> I know next to nothing about PPC ABIs, but are any of these floating 
> > >> point registers?
> > >
> > >
> > > There are, yes.
> > >
> > >> Are there any call saved FP registers in this ABI? and if so are you 
> > >> restoring them.  Although I don't think that the unwinder uses 
> > >> floating point, it seems that restoring call saved FP registers is a 
> > >> good idea if you are not already doing it.
> > >
> > >
> > > Well, here I expect the advise from the experts, I have floats around 
> > > and I may try to restore them.
> > >
> > > But, I need some guidance here.
> > 
> > When I did the MD_FALLBACK_FRAME_STATE_FOR for mips/linux I did not 
> > handle floating point either as the problem did not occur to me until 
> > after I checked in the code.
> > 
> > However after thinking about it and posting:
> > 
> > http://gcc.gnu.org/ml/gcc/2003-10/msg00972.html
> > 
> > I learned that this is a real issue.
> > 
> > I may be about ready to do some more mips/linux work soon and may 
> > revisit MD_FALLBACK_FRAME_STATE_FOR.  Because in its current state it 
> > seems to be incomplete.
>
>You only need to restore what has been saved.  Looking at
>/usr/src/linux-2.4/arch/mips/kernel/signal.c, it seems that there is a
>call to save_fp_context().  However, this is only executed if
>(current->used_math) is set; you mustn't restore any fp registers if
>the process hasn't saved the fp state.
>
>There is a field called sc_used_math in the sigcontext struct.  I
>think this tells you what you need to know.  But I am not a kernel
>hacker...
>
>Andrew.
>  
>
Ralf,

Is this all true?

Perhaps you could shed some light on what really needs to be done in the 
MIPS/linux case.

Also what should be done in the case of mips 4Kc core where there is 
only software floating point?

David Daney.