Re: [avr-llvm-devel] Flash data

[Borja F. <bor...@gm...>]

Heh I knew Eric was going to get a bit mad about what you said Anton xDD

Anton, those examples you wrote work exactly the same way as you described.
However you can't make that everything that is declared with const to mean
that it comes from flash memory, because it may be a ptr to ram, it's not a
safe assumption to do. That way, __flash has to be used explicitly, so the
user is telling the compiler to change the storage of that piece of data
from the generic address space to a custom one.

Eric, about my decision of declaring variables always const, it is because
they are really read-only by hardware, so writing to them is illegal and
this way you're defining the data type as it should be, always readonly.
Another reason is that by telling the compiler the data is not going to
change throughout execution you benefit from a decent amount of
optimizations and alias analysis, avoiding lots of memory reloads and many
other goodies you get when using the const qualifier.

Even if something is declared const, with a cast the constness can be
removed and one could write to the variable, but I've added an error to
handle this sort of behavior. In theory nobody should want to do this
because it would be going against what flash memory does, so it would be
more like wanting to trick the compiler, but erroring in this case would
show it's robust and that you cant really trick it.

2012/3/18 Weddington, Eric <Eri...@at...>

>
>
> > -----Original Message-----
> > From: Anton Staaf [mailto:an...@so...]
> > Sent: Saturday, March 17, 2012 2:15 PM
> > To: Borja Ferrer
> > Cc: avr...@li...
> > Subject: Re: [avr-llvm-devel] Flash data
> >
> > Hmm, if __flash acts in the same way as const, as in it's a qualifier.
> >  Then could you make it a super set of const?  In other words __flash
> > implies const?
>
> No, no, no and no.
>
> One can have const data, but not want it in flash. And having const
> imply a particular address space really does violate the C standard.
>
> However, what I do question is if the user writes __flash, are they
> really required to use const? It seems implicit in the use of __flash.
>
> Eric Weddington
>