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Here's an updated draft. I included most of your remarks and added some |
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notes on append-flags/filter-flags. I'll probably submit it to Ulrich |
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around the end of the week. |
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|
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--- Draft BEGIN --- |
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<section> |
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<title>CFLAGS</title> |
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<body> |
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|
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<p> |
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Being able to tune the CFLAGS is part of one of the core principles of |
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Gentoo: let the user be in control. Being in control brings both |
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benefits and problems and CFLAGS tuning is not an exception. |
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</p> |
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<p> |
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The recent upgrade to gcc-4.1.1 for x86 and amd64 users changed the |
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landscape. Users that spent some time tuning their CFLAGS with gcc-3.4.6 |
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might find out that an upgrade to gcc-4.1.1 leaves them with an unstable |
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system. Example of this are : |
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<ul> |
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<li>nss_ldap stopped working with <c>-ffast-math</c> (reported to break |
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many packages changing with the actual gcc version)</li> |
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<li><c>-fvisibility-inlines-hidden</c> still breaks some code</li> |
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<li>if you used gcc-4.0, <c>-ftree-loop-linear</c> now breaks in |
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gcc-4.1(at least with mesa)</li> |
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<li>again for gcc-4.0 users, <c>-ftree-vectorize</c> is known to be |
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broken in gcc-4.1 (at least for x86 and ppc, amd64 users seem to be |
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safe)</li> |
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<li><c>-fforce-addr</c> and <c>-fweb</c> break regularly on x86 with |
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video libraries or graphic processing apps which use hand-optimised ASM</li> |
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</ul> |
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</p> |
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<p> |
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Users with unsupported CFLAGS (see the <uri |
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link='http://gentoo-wiki.com/CFLAGS_matrix'>CFLAGS matrix</uri> for |
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example) might want to return to safe CFLAGS (see <uri |
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link='http://gentoo-wiki.com/Safe_Cflags'>Safe CFLAGS</uri>) if recent |
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updates caused them stability problems. On the other hand, more |
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adventurous users might want to experiment with CFLAGS that didn't work |
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properly with gcc-3.4.6... As always, the user is in control. |
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</p> |
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<p> |
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Notes: |
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<ul> |
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<li>The gcc man page contains warnings for some unsafe optimization |
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options. You should read it carefully when you experiment with CFLAGS or |
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upgrade GCC on a CFLAGS-customized Gentoo.</li> |
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<li>Some options that are unsafe in the system-wide CFLAGS might be |
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added automatically in some ebuilds if the developper deems them safe |
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(by redefining CFLAGS or using append-flags of the flag-o-matic eclass). |
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For example <c>-ffast-math</c> is added by the xmame/xmess ebuilds on |
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most architectures even if you don't put it in your CFLAGS.</li> |
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<li>You might get an idea of the stability issues of a specific |
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optimization option by running: <c>find /usr/portage -name '*.ebuild'| |
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xargs grep -- '-<your-risky-optimization-option>'</c>. It takes quite |
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some time, but might be enlightening: look for the 'filter-flags'.</li> |
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</ul> |
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</p> |
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</body> |
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</section> |
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--- Draft END --- |
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|
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Lionel |
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-- |
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