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List:       gentoo-user-fr
Subject:    Re: [gentoo-user-fr] le systeme ne boote pas..
From:       Guillaume Morin <webmaster () lejay-lagoute ! fr>
Date:       2003-07-30 12:25:26
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Montier Jacques a écrit :

>Merci aussi pour l'info sur l'erreur de frappe du commutateur -o3 ;
>cela a-t-il eu une incidence sur le résultat de la compilation ??
>
>Jacques
>  
>
Oui bien sûr cela a une influence. L'option "-O" et ses variantes "-Ox" 
permettent d'optimiser le code. "-O" ou "-O1" est la plus faible 
optimisation, "-O3" en théorie la plus performante (mais c'est à voir, 
en effet dans de nombreux cas "-O2" est plus efficace). Un des gros 
intérêts de Gentoo est d'optimiser la compilation pour sa machine, et ça 
mérite donc un petit peu de temps de réflexion. Tu trouveras sur la page 
suivante la présentation des meilleurs options de compilation en 
fonction de ton processeur :
http://www.freehackers.org/gentoo/gccflags/flag_gcc3.html

Tout ceci est décrit dans le manuel de gcc (man gcc) :

Options That Control Optimization

       These options control various sorts of optimizations:

       -O
       -O1 Optimize.  Optimizing compilation takes somewhat more time, and a
           lot more memory for a large function.

           Without -O, the compiler's goal is to reduce the cost of compila-
           tion and to make debugging produce the expected results.  State-
           ments are independent: if you stop the program with a breakpoint
           between statements, you can then assign a new value to any 
variable
           or change the program counter to any other statement in the func-
           tion and get exactly the results you would expect from the source
           code.

           With -O, the compiler tries to reduce code size and execution 
time,
           without performing any optimizations that take a great deal 
of com-
           pilation time.

       -O2 Optimize even more.  GCC performs nearly all supported optimiza-
           tions that do not involve a space-speed tradeoff.  The compiler
           does not perform loop unrolling or function inlining when you 
spec-
           ify -O2.  As compared to -O, this option increases both 
compilation
           time and the performance of the generated code.

           -O2 turns on all optional optimizations except for loop 
unrolling,
           function inlining, and register renaming.  It also turns on the
           -fforce-mem option on all machines and frame pointer 
elimination on
           machines where doing so does not interfere with debugging.

           Please note the warning under -fgcse about invoking -O2 on 
programs
           that use computed gotos.

       -O3 Optimize yet more.  -O3 turns on all optimizations specified 
by -O2
           and also turns on the -finline-functions and -frename-registers
           options.

       -O0 Do not optimize.

       -Os Optimize for size.  -Os enables all -O2 optimizations that do not
           typically increase code size.  It also performs further optimiza-
           tions designed to reduce code size.

           If you use multiple -O options, with or without level 
numbers, the
           last such option is the one that is effective.




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