[avr-libc-dev] Re: AW: Register allocator / -fnew-ra issue

[E. Weddington]

Björn Haase wrote:

> > > P.P.S.: Two further examples of compilation results (all with -Os
> > >      
> and -fnew-ra)
>  
>
> > IIRC, the new-ra work on GCC is pretty much on hold due to a bunch of
> >    
> issues. I would highly
>  
>
> > recommend that you stay away from it for the time being.
> >
> > Eric
> >    
>
> I have come to the conclusion, that the main possible improvement of avr-gcc
> code
> efficiency now is the register allocator. I have seen really *weird* code
> when using the
> default. ... seems mainly to be related to SI objects ...
> "-morder1" sometimes.
> My experience is that -fnew-ra generates much
> better results, in the rare case that it does not make gcc crash :-|, of
> course.
> I have observed this with both, 4.0 and older versions.
>
> I have some ideas on how to help the register allocator to use the resources
> more efficiently. Unfortunately, that would require more substantial changes
> in the
> compiler ABI.
>  

I'm assuming you mean helping the *current* register allocator? (not 
-fnew-ra)

> <good stuff snipped>
>
> 4.)
> Adapt the library functions that are written in assembler such that they are
> compliant
> with the modified ABI.
>
> In my opinion, this way, we could possibly help the RA to make much better
> use of the register
> file.
>  

*If* it can be shown with good test results, that this would work, then 
I would be willing to help out with changes to avr-libc. The other 
developers would have to offer their opinions as well. If is is good 
enough, then perhaps some of avr-libc could be translated back into C 
from assembly. IMO, the big issue with changing avr-libc would be the 
math / floating point library. We could really use someone who has 
expertise in this area.

Eric