RE: [avr-gcc-list] Re: Code optimistaion in AVR Tiny13

[Weddington, Eric]

> -----Original Message-----
> From: 
> address@hidden 
> [mailto:address@hidden
> org] On Behalf Of David Brown
> Sent: Monday, February 16, 2009 5:07 PM
> To: address@hidden
> Subject: [avr-gcc-list] Re: Code optimistaion in AVR Tiny13
> 
> Weddington, Eric wrote:
> > 
> > 
> >> -----Original Message----- From: 
> >> address@hidden 
> >> [mailto:address@hidden org]
> >> On Behalf Of David Brown Sent: Monday, February 16, 2009 2:31 PM 
> >> To: address@hidden Subject: [avr-gcc-list] Re: Code
> >> optimistaion in AVR Tiny13
> >> 
> >> Another important point in getting code as small as possible is to
> >> use -combine and -fwhole-program - it can significantly improve
> >> code, especially for functions that are called only once but live 
> >> in different modules.
> > 
> > In my tests on real world programs, it have decreased the code by up
> > to 25%. And it has also *increased* the code by up to 25%!
> > Unfortunately I don't know what about the code that causes it to
> > increase. But clearly it is not always a win and can be a detriment.
> > So be sure to check your code sizes when you use this.
> 
> If these flags *increase* code size, there's a bug somewhere - or at 
> least some badly tuned parameters (such as -finline-limit, or 
> something 
> similar).  They give the compiler increased knowledge and more 
> opportunity to combine code and do inter-procedural 
> optimisations across 
> compilation units.  I've only used it myself on a couple of programs 
> (boot programs, which are particularly space-concious), and it helped 
> significantly.

Oh, I definitely agree that it could help significantly. The problem, again, 
has to do with maintenance. I spoke with one of the guys who works on gcc (for 
Google) and it seems that no one is really maintaining -fwhole-program. Most 
targets on gcc optimize for speed, not size. So there's no one to go to with 
problems with this switch. It's one of those: if we want it fixed, we have to 
do it ourselves because no one else is working on it. :-/