[gentoo-dev] RFC: factoring logic for selection of "safe" gcc -O flags in ebuilds (uclibc users take note)

[Gavin]

Problem
===========
Many ebuilds force gcc to use "-O2" instead of other, possibly viable values.  For example, "-Os" does work in some circumstances for certain ebuilds that currently force the use of "-O2".  Those using uclibc and/or CPUs with small caches might prefer "-Os".  However, maintaining a clone of an ebuild in a portage overlay directory for the sole purpose of changing "replace-flags -O? -O2" to "replace-flags -O? -Os" present users with an unecessary hassle.

Proposed Solution
====================
Are ebuilds the best place to make the decision regarding which optimization is "safe" for the target system?  Perhaps we can factor and encapsulate the logic into flag-o-matic?

As a first step, what if we add 'SAFE_GCC_O_FLAG="-O2"' to /etc/make.globals?  It might be a long time before we could expect everyone using new ebuilds to have a make.global with SAFE_GCC_O_FLAG defined.  Ideally, an appropriate default might be automatically determined, but I want a super simple, first step solution.

/usr/portage/eclass/flag-o-matic.eclass:
# credits to Martin Schlemmer's input (see http://bugs.gentoo.org/show_bug.cgi?id=57223)
replace-with-safe-gcc-o-flag() {
  [ -z "${SAFE_GCC_O_FLAG}" ] && SAFE_GCC_O_FLAG="-O2"
  CFLAGS="${CFLAGS//-O?/ } ${SAFE_GCC_O_FLAG}"
  CXXFLAGS="${CXXFLAGS//-O?/ } ${SAFE_GCC_O_FLAG}"
  export CFLAGS CXXFLAGS
  return 0
}

Alternative (less efficient, but less code):
replace-with-safe-gcc-o-flag() {
  [ -z "${SAFE_GCC_O_FLAG}" ] && SAFE_GCC_O_FLAG="-O2"
  replace-flags -O? ${SAFE_GCC_O_FLAG}"
  return 0
}

# appends SAFE_GCC_O_FLAG to "emerge info" results:
--- /usr/lib/portage/bin/emerge.orig    2004-07-17 09:13:48.690078831 -0700
+++ /usr/lib/portage/bin/emerge 2004-07-17 09:14:08.067093698 -0700
@@ -2314,7 +2314,7 @@
                myvars=['GENTOO_MIRRORS', 'CONFIG_PROTECT', 'CONFIG_PROTECT_MASK',
                                'PORTDIR', 'DISTDIR', 'PKGDIR', 'PORTAGE_TMPDIR', 'PORTDIR_OVERLAY',
                                'USE', 'COMPILER', 'CHOST', 'CFLAGS', 'CXXFLAGS','ACCEPT_KEYWORDS',
-                               'MAKEOPTS', 'AUTOCLEAN', 'SYNC', 'FEATURES']
+                               'MAKEOPTS', 'AUTOCLEAN', 'SYNC', 'FEATURES', 'SAFE_GCC_O_FLAG']
        myvars.sort()
        for x in myvars:
                print x+'="'+portage.settings[x]+'"'


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