Re: [avr-llvm-devel] Flash data

[Weddington, E. <Eri...@at...>]

> -----Original Message-----
> From: Borja Ferrer [mailto:bor...@gm...]
> Sent: Sunday, March 18, 2012 9:35 AM
> To: Weddington, Eric
> Cc: Anton Staaf; avr...@li...
> Subject: Re: [avr-llvm-devel] Flash data
> 
> 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.
> 

Ok, it sounds reasonable to me. :-)

Thanks for the explanation!

Eric