Re: GCC mini-summit - compiling for a particular architecture

[Jan Hubicka <hubicka at ucw dot cz>]

> > Look from what we're starting:
> > 
> > <<
> > @item -funroll-loops
> > @opindex funroll-loops
> > Unroll loops whose number of iterations can be determined at compile
> > time or upon entry to the loop.  @option{-funroll-loops} implies
> > @option{-frerun-cse-after-loop}.  This option makes code larger,
> > and may or may not make it run faster.
> > 
> > @item -funroll-all-loops
> > @opindex funroll-all-loops
> > Unroll all loops, even if their number of iterations is uncertain when
> > the loop is entered.  This usually makes programs run more slowly.
> > @option{-funroll-all-loops} implies the same options as
> > @option{-funroll-loops},
> > >>
> > 
> > It could gain a few more paragraphs written by knowledgeable people.
> > And expanding documentation doesn't introduce regressions :).
> 
> but also does not make anyone actually use the options.  Nobody reads
> the documention.  Of course, this is a bit overstatement, but with a
> few exceptions, people in general do not enable non-default flags.

I don't think this is fair.
Most people don't read the docs because they don't care about
performance, but most people who develop code that spends a lot of CPU
cycles actually read the docs at least up to loop unrolling.

BTW there is even www.funroll-loops.org ;) The content however can be
found only in wayback
http://web.archive.org/web/20060513022941/http://www.funroll-loops.org/

Honza
> 
> Zdenek