Re: compiler bug in gcc version 2.8.1 (Tornado1) for PPC corrupts floationg point constants?

["J. Frederick Bartlett" <bartlett@fnal.gov>]

Jeff,

  Several years ago I encountered this problem when I was writing an 
EPICS driver for a MIL/STD1553B bus controller under the vxWorks OS. 
Another process on the IOC, which did use the FP registers and which had 
not set the FP save option in the task status, was throwing FP 
exceptions. Although the problem was corrected by setting the 
save-option bit, we did discover during the investigation that a FP 
register's contents was being altered during the execution of the 
driver's interrupt service routine (ISR) and that, even though no FP 
operations were being performed in the ISR, the compiler optimization 
level that we were using caused the FP registers to be used to store 
temporary values. As I recall, changing the compiler optimization level 
also eliminated this "hidden" use of the FP registers.

                                                                        
                                                                       
        Fritz

Jeff Hill wrote:

>  
>
> > -----Original Message-----
> > From: Ralph Lange [mailto:Ralph.Lange@bessy.de]
> >
> > is that true for all target architectures or just for
> > the PPC arch that
> > Dirk is seeing this on?
> >
> >    
>
> The WRS warnings concerning this issue are in an
> architecture independent section of the documentation,
> and I don't see anything specific on this issue in the
> PPC specific section of the vxWorks 5.4 documentation.
>
> A potential pitfall might be that the PPC compiler used
> the FP registers to pass integer parameters, and that
> the user's ISR didn't anticipate that scenario.
>
> Jeff
>
>  
>
> > Jeff Hill wrote:
> >
> >    
> >
> > > Dirk,
> > >
> > > I don't know if anyone has made this suggestion, but
> > > watch out for a driver that uses floating point in
> > >      
> > its
> >    
> >
> > > interrupt service routines as, on vxWorks, the FP
> > > register contents are not saved / restored when
> > >      
> > entering
> >    
> >
> > > / exiting interrupt context. The driver must do this
> > > manually if it finds a need to use FP in its ISR.
> > >
> > > Jeff
> > >
> > >
> > >      
>
>