Re: C strict aliasing rules

[Eric Norum <norume@aps.anl.gov>]

On Nov 28, 2006, at 9:23 AM, Marty Kraimer wrote:

> Kay-Uwe Kasemir wrote:
>
> > Hi:
> >
> > Do I understand correctly that the most likely impact for EPICS  
> > might  be this type of code:
> > struct xyz_regs
> > {
> >     Word control;
> >     Word status;
> >     ...
> > }
> >
> > void xyz_operate(char *vme_base)
> > {
> >     struct xyz_regs *xyz = (struct xyz_regs *) vme_base;
> >
> >     xyz->control = 0x1234;
> >     if (xyz->status & 0x8000)
> >       ....
> > }
> >
> > The mapping of C structures to hardware registers?
>
> volatile should be used for all access to hardware registers.
> volatile prevents compiler optimization.
Let's not get off on a tangent here.  The problem Kay was describing  
was one of structure padding, alignment, endianness when using a  
structure to define the layout of hardware registers.
The issue on the table is that of compilers imposing their strict  
aliasing rules, and what to do about that.
--
Eric Norum <norume@aps.anl.gov>
Advanced Photon Source
Argonne National Laboratory
(630) 252-4793