Gentoo Archives: gentoo-user-fr

From: Guillaume Morin <webmaster@×××××××××××××.fr>
To: gentoo-user-fr@g.o
Subject: Re: [gentoo-user-fr] le systeme ne boote pas..
Date: Wed, 30 Jul 2003 12:25:28
Message-Id: 3F27B936.6070005@lejay-lagoute.fr
In Reply to: Re: [gentoo-user-fr] le systeme ne boote pas.. by Montier Jacques
1 Montier Jacques a écrit :
2
3 >Merci aussi pour l'info sur l'erreur de frappe du commutateur -o3 ;
4 >cela a-t-il eu une incidence sur le résultat de la compilation ??
5 >
6 >Jacques
7 >
8 >
9 Oui bien sûr cela a une influence. L'option "-O" et ses variantes "-Ox"
10 permettent d'optimiser le code. "-O" ou "-O1" est la plus faible
11 optimisation, "-O3" en théorie la plus performante (mais c'est à voir,
12 en effet dans de nombreux cas "-O2" est plus efficace). Un des gros
13 intérêts de Gentoo est d'optimiser la compilation pour sa machine, et ça
14 mérite donc un petit peu de temps de réflexion. Tu trouveras sur la page
15 suivante la présentation des meilleurs options de compilation en
16 fonction de ton processeur :
17 http://www.freehackers.org/gentoo/gccflags/flag_gcc3.html
18
19 Tout ceci est décrit dans le manuel de gcc (man gcc) :
20
21 Options That Control Optimization
22
23 These options control various sorts of optimizations:
24
25 -O
26 -O1 Optimize. Optimizing compilation takes somewhat more time, and a
27 lot more memory for a large function.
28
29 Without -O, the compiler's goal is to reduce the cost of compila-
30 tion and to make debugging produce the expected results. State-
31 ments are independent: if you stop the program with a breakpoint
32 between statements, you can then assign a new value to any
33 variable
34 or change the program counter to any other statement in the func-
35 tion and get exactly the results you would expect from the source
36 code.
37
38 With -O, the compiler tries to reduce code size and execution
39 time,
40 without performing any optimizations that take a great deal
41 of com-
42 pilation time.
43
44 -O2 Optimize even more. GCC performs nearly all supported optimiza-
45 tions that do not involve a space-speed tradeoff. The compiler
46 does not perform loop unrolling or function inlining when you
47 spec-
48 ify -O2. As compared to -O, this option increases both
49 compilation
50 time and the performance of the generated code.
51
52 -O2 turns on all optional optimizations except for loop
53 unrolling,
54 function inlining, and register renaming. It also turns on the
55 -fforce-mem option on all machines and frame pointer
56 elimination on
57 machines where doing so does not interfere with debugging.
58
59 Please note the warning under -fgcse about invoking -O2 on
60 programs
61 that use computed gotos.
62
63 -O3 Optimize yet more. -O3 turns on all optimizations specified
64 by -O2
65 and also turns on the -finline-functions and -frename-registers
66 options.
67
68 -O0 Do not optimize.
69
70 -Os Optimize for size. -Os enables all -O2 optimizations that do not
71 typically increase code size. It also performs further optimiza-
72 tions designed to reduce code size.
73
74 If you use multiple -O options, with or without level
75 numbers, the
76 last such option is the one that is effective.
77
78
79
80
81 --
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