Re: [gentoo-user] CFLAGS "...-O3 -pipe" vs "...O2 <no pipe>"

["Andrey Gerasimenko" <gak@kaluga.ru>]

On Mon, 09 Apr 2007 05:48:07 +0400, maxim wexler <blissfix@yahoo.com>  
wrote:

> Hi group,
>
> I note two schools of thought on the best CFLAGS for
> the Pentium III processor.
>
> One suggests using -O3 -pipe, the other, -O2 without
> the pipe.
>
> How much difference does this make? Is the extra level
> of optimization with pipe the equivalent of the lower
> level without?
>

Man gcc. -pipe allows to use pipes (memory) instead of temporary files  
(disk). -O controls the resulting binary sometimes making it fast, large,  
and unreliable. I have seen ebuilds that enforce -O2 even when -O3 is  
specified. This can create a faulty impression that -O3 always works fine.

-O and -pipe interact through the size of temporary files. Theoretically,  
-pipes may slow down compiles if there is not enough memory. Another  
factor that should be taken into account is the number of threads for  
make. For a PIII machine the generally recommended number is 2, but when  
the RAM is limited, say 384 megs, 1 thread with pipes works better.

Ideally you should measure compile times for large projects like Open  
Office and find out what works best on your system. I would start with 1  
thread -O2 and -pipe.

-- 
Andrei Gerasimenko
-- 
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