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

[David Brown]

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.