Re: [avr-libc-dev] PSTR() and duplicate strings

[David Brown]

On 05/04/2011 20:30, Benoît Ryder wrote:
> Thanks for these answers, it is now completely clear for me.
> Indeed, I checked only the (ROM) size of the final binary, forgetting
> about .data.
>
> > There is a "-fmerge-all-constants" flag - it is possible that this will give
> > you the merging you want (though it doesn't seem to have the effect when I
> > tested on a single file).
> Unfortunately, it does not seem to work. According to gcc's manual
> -fmerge-all-constants is only a more aggressive version of
> -fmerge-constants, which is responsible for the optimizations
> mentioned previously; so it not suprising.

Yes.  The big problem with doing such merging is that it is in conflict 
with a fundamental C requirement - different objects shall always have 
different addresses.  gcc can do quite well at getting around this if 
you don't look at the addresses of the objects, but in the case of the 
PSTR macros you /do/ take the addresses of the char arrays.  The 
compiler can only merge the constants if it can guarantee that your code 
doesn't rely on them being at different addresses, which is not an easy 
task.