Why should you upgrade? Well, GCC is quite similar to any other package on your system, just a bit more critical. You should upgrade GCC whenever a new version fixes some bug that annoys you, new functionality you need is introduced, or if you want to keep your system up-to-date. If none of the previous cases apply to you, you can safely postpone upgrade as long as your GCC version is supported by Gentoo developers.

If you install a newer version of GCC, the system will not switch over to use it automatically. You'll have to explicitly request the change because the migration process might require some additional steps. If you decide not to switch, Portage will continue to use older version of your compiler until you change your mind, or remove the old compiler from the system.

This guide will document the necessary steps required to perform a seamless upgrade of the compiler used by your Gentoo box. A specific section is dedicated to the upgrade from GCC 3.3 to 3.4 or greater versions and issues with libstdc++. A second specific section is for users first installing Gentoo using a stage3 tarball, after a new GCC major/minor version has been released.

Generally speaking, upgrades to bug fix releases, like from 3.3.5 to 3.3.6, should be quite safe -- just emerge new version, switch your system to use it and rebuild the only affected package, libtool. However, some GCC upgrades break binary compatibility; in such cases a rebuild of the affected packages (or even whole toolchain and system) might be required.

When we spoke about the need to switch your compiler to the newer version by hand, we said it won't happen automatically. However, there is one exception -- upgrades to bug fix releases, like from 3.3.5 to 3.3.6 in case you don't use the "multislot" feature allowing them to coexist on one system. Multislot is disabled by default as the majority of users won't benefit from it.

# emerge -uav gcc

(Please substitute "i686-pc-linux-gnu-3.4.4" with the GCC
version and CHOST settings you've upgraded to:)
# gcc-config i686-pc-linux-gnu-3.4.4
# source /etc/profile

(Rebuilding libtool)
# emerge --oneshot -av libtool

Now let's rebuild toolchain and then world so we will make use of the new compiler.

# emerge -eav system
# emerge -eav world

It is safe to remove the older GCC version at this time. If you feel the need, please issue the following command (as usual, substitute =sys-devel/gcc-3.3* with the version you want to uninstall):

# emerge -aC =sys-devel/gcc-3.3*

The upgrade from GCC-3.3 to 3.4 or greater is not seamless as the C++ ABI changed between these two versions. There is an issue with the libstdc++ library which must be taken care of, as well.

You have two possibilities on how to upgrade your system. The first method is faster and requires use of the revdep-rebuild tool from package gentoolkit while the second one rebuilds the entire system from scratch so it will make use of new GCC features. It's up to you to decide which of these two ways you will choose. In most cases, the first method is sufficient.

This method requires that you first install gentoolkit if you have not already done so. Then we will upgrade GCC and switch to the new compiler. We will also rebuild the libtool package to ensure that toolchain is in healthy state.

# emerge -an gentoolkit
# emerge -uav gcc
# gcc-config i686-pc-linux-gnu-3.4.4
# source /etc/profile

(Rebuilding libtool)
# emerge --oneshot -av libtool

Now, we want to see which packages that revdep-rebuild will want to rebuild. Then we will tell revdep-rebuild to actually rebuild the packages. This may take some time, so have some patience.

# revdep-rebuild --library libstdc++.so.5 -- -p -v
# revdep-rebuild --library libstdc++.so.5

To provide compatibility with older binary C++ applications and any packages that revdep-rebuild might have missed, sys-libs/libstdc++-v3 needs to be merged before you unmerge GCC 3.3 from your system.

# emerge --oneshot sys-libs/libstdc++-v3
# emerge -aC =sys-devel/gcc-3.3*

This method, while much slower, will rebuild your whole system to ensure that everything has been rebuilt with your new compiler, and therefore safer. At first, you will upgrade GCC and libtool and switch to your new compiler.

# emerge -uav gcc
# gcc-config i686-pc-linux-gnu-3.4.4
# source /etc/profile

(Rebuilding libtool)
# emerge --oneshot -av libtool

To provide compatibility with older binary C++ applications, sys-libs/libstdc++-v3 needs to be merged onto your system.

# emerge --oneshot sys-libs/libstdc++-v3

Now we will go about first rebuilding the system target, then the world target. This will take a very long time, depending on the number of packages that you have installed, as it will rebuild your entire toolchain and supporting system files, followed by every package on your system, including the toolchain. This is necessary to ensure that all packages have been compiled with the new toolchain, including the toolchain itself.

# emerge -e system
# emerge -e world

It is also safe to remove older GCC versions at this time:

# emerge -aC =sys-devel/gcc-3.3*

It's important to disable distcc during upgrade. Mixing compiler versions on your nodes will cause build issues. This is not required for ccache, as the cache objects will be invalidated anyway.

Always use same GCC version for your kernel and additional kernel modules. Once you rebuild your world with new GCC, external modules (like app-emulation/qemu-softmmu) will fail to load. Please rebuild your kernel with new GCC to fix that.

If your system complains about something like libtool: link: `/usr/lib/gcc-lib/i686-pc-linux-gnu/3.3.6/libstdc++.la' is not a valid libtool archive, please run /sbin/fix_libtool_files.sh 3.3.6 (substitute "3.3.6" with the version numbers from the error message).

If you see the error: /usr/bin/gcc-config: line 632: /etc/env.d/gcc/i686-pc-linux-gnu-3.3.5: No such file or directory, then try deleting /etc/env.d/gcc/config-i686-pc-linux-gnu and running gcc-config again, followed by source /etc/profile. Only do this if you do not have any cross-compilers set up, though.

If a package fails during emerge -e system or emerge -e world, you can resume operation with emerge --resume. If a package fails repeatedly, skip it with emerge --resume --skipfirst. Don't run any other instances of emerge in between or you will lose the resume information.

If you get an error message spec failure: unrecognized spec option while upgrading your compiler, try to switch back to your default compiler, unset the GCC_SPECS variable and upgrade GCC again:

# gcc-config 1
# source /etc/profile
# unset GCC_SPECS
# emerge -uav gcc

A GCC upgrade on a system after installation from a stage3 tarball is a simple affair. One advantage users of new installations have is they do not have a plethora of software installed that links against the older version of GCC. The following example is for a GCC-3.3 to 3.4 or greater upgrade. Certain parts will be different if upgrading from other versions of GCC. For example, the library names used for revdep-rebuild below are GCC 3.3 specific, as well as the need to install libstdc++-v3.

If a user has not made any customizations to their system yet, then there are very few steps to get their system upgraded to a new GCC version. As with the GCC-3.3 to 3.4 upgrade, the user has a couple options. However, unlike the GCC-3.3 to 3.4 upgrade, this one is less complicated as there are fewer differences between the methods. The first method is faster and makes use of the revdep-rebuild tool from gentoolkit, similar to the above procedure. Using revdep-rebuild causes only packages which actually link against GCC libraries to be rebuilt, while the second method causes your entire new install to be recompiled with the new GCC version and takes much longer. This second method is never required and only documented for completeness.

These first steps are common between both methods, and should be completed by everyone.

# emerge -uav gcc
# gcc-config i686-pc-linux-gnu-3.4.4
# source /etc/profile

(Rebuilding libtool)
# emerge --oneshot -av libtool

To provide compatibility with older binary C++ applications, sys-libs/libstdc++-v3 needs to be merged onto your system.

# emerge --oneshot sys-libs/libstdc++-v3

This method requires that you first install gentoolkit if you have not already done so. We will then run revdep-rebuild to actually scan the installed packages for ones we need to rebuild, then rebuild them.

# emerge -an gentoolkit
# revdep-rebuild --library libstdc++.so.5 -- -p -v
# revdep-rebuild --library libstdc++.so.5

This method, while much slower, will rebuild the system target to ensure that everything has been rebuilt with your new compiler. This is not necessary, but is valid if you are also making changes to CFLAGS or other make.conf variables that will affect the system compile.

# emerge -uav gcc
# gcc-config i686-pc-linux-gnu-3.4.4
# source /etc/profile

(Rebuilding libtool)
# emerge --oneshot -av libtool

To provide compatibility with older binary C++ applications, sys-libs/libstdc++-v3 needs to be merged onto your system.

# emerge --oneshot sys-libs/libstdc++-v3

Since we are performing these actions after an initial installation, we do not need to recompile the world target as we would when doing an upgrade on an already installed system. However, you may choose to perform a world update in place of the system update, to ensure that all packages are updated.

# emerge -e system

It is also safe to remove older GCC versions at this time. Since this is an initial installation, we are using portage's prune feature to remove all older versions of GCC.

# emerge -aP sys-devel/gcc