Re: [avr-llvm-devel] Load / Store

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

 

> -----Original Message-----
> From: John Myers [mailto:ato...@gm...] 
> Sent: Thursday, November 04, 2010 5:43 PM
> To: Weddington, Eric
> Cc: Borja Ferrer; avr...@li...
> Subject: Re: [avr-llvm-devel] Load / Store
> 
> 
> 	>       Another thing, i dont know how feasible is doing this,
> 	> but should we make program and data space addrs transparent
> 	> to the user?
> 	>
> 	>
> 	
> 
>  On Mon, Nov 1, 2010 at 9:10 PM, Weddington, Eric 
> <Eri...@at...> wrote:
> 
> 	>
> 	> Yes I believe that would be a useful feature.
> 	> I think we should try as much as possible without, of course,
> 	> violating any language standards.
> 	> It would allow users to write more portable/generic code.
> 	> We probable should have a command line option to
> 	> enable/disable this so users have more control.
> 	> LLVM has an address space qualifier mechanism already so I
> 	> don't think it will take too much work.
> 	
> 	
> 	I definitely agree with the above, except for the part 
> about a command line option: I would suggest that there be no 
> command line option for this. It will just be extra work for 
> something that I doubt anyone will ever use (a command-line 
> option). Just implement the feature.
> 	
> 
> 
> Eric,
> 
> I think transparently placing, for example, const qualified 
> objects in flash will only take creating an LLVM 
> transformation Pass. Which means the extra work has already 
> been done by the LLVM Pass Framework. Now if we have to do 
> work in the front end then I can see your point.
> 
> --John

Oh, I'm sorry, I didn't fully grasp what we're talking about here.

It really is not appropriate to automatically put const qualified objects in flash. The keyword 'const' has a specific meaning (read-only value) which does not necessarily mean that it should be put into a specific memory space, even if it is a read-only value. We will need to have an additional memory space qualifier on the object to put it in a different address space. That way it is the least surprise to users who are used to using GCC and IAR (and other compilers).

Eric