Re: [avr-libc-dev] Considerable Compiler improvement neeeds testing

[Rolf Ebert]

On Sun, 30 Jan 2005 17:48:16 +0100, Björn Haase <address@hidden>  
wrote:

> Hi,
>
> find enclosed a mail posted by Andy Hutchinson on the address@hidden  
> list. I
> forward this mail to this list since I think that this is the better  
> forum
> for discussing this issue.
>
> The story is:
> It seems that the gcc internals documentation was kind of misleading and  
> for
> this reason avr-gcc was not able to take benefit of a couple of  
> optimizations
> that avoid useless moves across registers when dealing with variables of
> different byte sizes.
> The change suggested by Andy Hutchinson affects one single line in
> the gcc/config/avr/avr.h that presently reads
> #define MODES_TIEABLE_P 0
> to
> #define MODES_TIEABLE_P 1
> . My tests have shown that *with* this change, about 15 testcases in the  
> gcc
> testsuite work that did not beforehand and the code becomes considerably
> tighter.
>
> Yours,
>
> Björn

I applied the proposed patch to the current gcc-3.4 branch of AVR-Ada, the  
Ada cross compiler for AVR. I tested it with some example programs and  
also my main application (a data logger based on Butterfly (mega169)). No  
use of floating point vars.

 - I did not detect any functional regression,
 - most functions (~95%) do not change size at all,
 - a few functions shrinked by 1 - 7% in code size,
 - a single function increased from 224 to 232 bytes,
 - the main application shrinked from xx to 1866 bytes.
 - I did not check RAM usage.

Non-Ada-specific compile options: -Os -ffunction-sections

function-sections was very helpful here as avr-size -A shows the code size  
for every function separately.

HTH

    Rolf