On Mon, 2014-02-24 at 16:40 +0100, David Brown wrote:
> On 24/02/14 16:35, Sebastian Huber wrote:
> > On 2014-02-24 16:07, David Brown wrote:
> >>> extern const int const_var;
> >>> >
> >>> >GCC will read the value of const_var again after a compiler memory
> >>> >barrier (e.g. __asm__ volatile("" ::: "memory")).
> >> That's the rules - a "memory clobber" says that/any/ memory may change,
> >> and things read from memory could change and must therefore be re-read.
> >> Specifying the extern var as "const" does not tell the compiler that
> >> the value is constant - it simply tells the compiler that/you/ promise
> >> not to change it. (It would be nice if C, or at least gcc, had a way to
> >> say that the value is never changed, but it does not.)
> >
> > These variables go into the .rodata section. It seems a bit over
> > paranoid to assume that they change. In my case the .rodata section is
> > a read-only region covering a NOR flash, so its unlikely to change.
> >
>
> C does not have any way to express this - so there is no way for the
> compiler to know that the value cannot change. (It might be able to do
> so if you use LTO or whole-program optimisation, or if the constant were
> static rather than extern.)
>
> As an embedded programmer, I would like some way to say "this value will
> /never/ change", as that would suit many common uses - but there is no
> way (AFAIK) to do so.
On some targets you can specify variable attributes to accomplish
exactly this. See also
http://gcc.gnu.org/onlinedocs/gcc/Variable-Attributes.html#AVR-Variable-Attributes
and
http://gcc.gnu.org/onlinedocs/gcc/Named-Address-Spaces.html#AVR%20Named%20Address%20Spaces
Of course it would be great if there was a target independent way of
doing these kind of things...
Cheers,
Oleg
