Re: [avr-llvm-devel] Flash data

[Anton S. <an...@so...>]

On Sun, Mar 18, 2012 at 11:48 AM, Borja Ferrer <bor...@gm...> wrote:
> Ohhh, I misuderstood you Anton, I read the opposite thing xD
>
> Ok, so you and Eric are saying the same thing. As I said above, we wouldn't
> benefit from the constantness optimizations, unless we set every flash
> variable as const internally for the frontend, but that doesn't feel very
> good to me. So doing __flash const int and __flash int would be the same
> thing introducing redundancy in the language which is not standard at all. I
> personally like the idea of making the user add the const so he doesn't
> assign values accidentally to variables even if then we have a check for
> erroring when this happens because he will get the typical error of "can't
> assign a value to const variables" which is language specific and standard
> and not a custom error added because the flash space cant be written which
> is not language standard.
>
> So what do you think?

Yes, I can see the reason for wanting to preserve the standard error
messages over adding __flash specific error messages in as many cases
as possible.  Perhaps making __flash a type qualifier and having a
single check that every instance of __flash is associated with a const
qualifier would work?

-Anton

>
> 2012/3/18 Anton Staaf <an...@so...>
>>
>> I'm confused, or have miss-communicated.  If you look again at what I
>> said, I explicitly said that __flash would imply const, not the other
>> way around.  Of coarse you want to be able to have const types that
>> don't imply flash storage.  And the examples I gave all used __flash,
>> not const.  No where did I suggest that const should imply __flash.
>>
>> Thanks,
>>    Anton
>>
>> On Sun, Mar 18, 2012 at 8:35 AM, Borja Ferrer <bor...@gm...>
>> wrote:
>> > 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
>> >
>> >
>
>