Downgrading GCC might have unwanted side effects. Refer to the troubleshooting section for some commonly reported issues.
Quick guide to GCC upgrades
Most GCC upgrades are as simple as switching the compiler version (here from 5.4.0 to 6.4.0) and rebuilding sys-devel/libtool:
emerge --ask --oneshot sys-devel/gcc
 x86_64-pc-linux-gnu-5.4.0 *  x86_64-pc-linux-gnu-6.4.0
emerge --ask --oneshot --usepkg=n sys-devel/libtool
Check the current version number and then uninstall the old version:
emerge --ask --depclean sys-devel/gcc:5.4.0
Enjoy the new compiler!
Extended guide to GCC upgrades
GCC upgrading has always been mystified, with suggestions ranging from "users do not need to do anything" to "users will need to rebuild the entire system twice". The uncertainty was caused by confusion about ABI incompatibility, something that nowadays rarely causes problems.
The reason we need to rebuild libtool after the upgrade of gcc versions is because of its main purpose: libtool is a toolset that aggregates platform-specific code in a generic interface, allowing applications to build against shared libraries without needing to deal with the platform-specific aspects of shared libraries. To fulfill its function properly, the libtool script uses various library locations that have hard-coded gcc version information in them.
wxGTK < 3.0.4 applications
If your x11-libs/wxGTK versions are < 3.0.4-r2 (slot 3.0) or < 3.0.4-r302 (slot 3.0-gtk3), this applies to you. (This bug, now obsolete, was fixed in bug #676878 in 2019.)
In such cases, first update your x11-libs/wxGTK. (And if you have dev-python/wxpython, update it too. See below.) Then update all applications that depend on wxGTK. It was necessary because wxGTK applications fixated on minor C++ ABI revisions when gcc versions increase.
ABI changes before gcc-5.1
An ABI (Application Binary Interface), is a set of conventions used by all tools that deal with binary representation of programs, including compilers, assemblers, linkers, and language runtime support (source: GCC Binary Compatibility). When the ABI used for binary applications and libraries is changed, you will risk getting linker errors or malfunctioning programs unless you rebuild all libraries that use C++ code.
If you are upgrading to GCC 4.1, or GCC 5.1, you would probably encounter ABI issues. To prevent this, the revdep-rebuild command should be run against the libstdc++.so.5 library when moving from GCC 3 to GCC 4.1, or libstdc++.so.6 when moving from GCC 4 to GCC 5.1.
revdep-rebuild --library 'libstdc++.so.6' -- --exclude gcc
This is only needed till GCC 4.1/5.1 because from that version onward, GCC uses a forward-compatible ABI, which removes the need for rebuilding applications and libraries. Of course, guarantees can never be given indefinitely, but when an incompatibility occurs again, we'll definitely document it here and release a news item. In that case, the version of the libstdc++.so library will probably be increased.
The special case C++11 (and C++14)
While GCC (or more specifically, libstdc++) goes to great lengths to guarantee stability of the ABI, this guarantee does not extend to all parts of C++ within libstdc++. Formally, with versions starting from 3.4, GCC/libstdc++ only guarantees C++98/C++03 ABI stability and not more. This is crucial for packages that depend on C++11. GCC only makes C++11 ABI stability guarantees beginning with version 5.1. This means that switching (even minor) versions of gcc (say from 4.7.3 -> 4.7.4) might cause ABI breakage for binaries built from C++11 code.
For more information and some examples, see:
- bug #513386
Which packages are known to need a rebuild?
The following table gives the packages that, if installed, need to be rebuilt and why.
|Package||Rebuild needed because ...|
|sys-devel/libtool||libtool application has hardcoded paths towards GCC internal libraries.|
|sys-devel/llvm||depends on exact gcc version, may encounter linking errors with other ebuilds making use of LLVM (e.g. media-libs/mesa) if not rebuilt.|
|sys-devel/clang||depending on exact gcc version, may be affected by sys-devel/llvm.|
|Rebuild both of wxGTK and wxpython, if you have both. If you rebuild only one of the two after a GCC upgrade, it will break things. See bug #639276.|
Be sure to rebuild the correct slot(s) of wxGTK!
emerge --ask --oneshot --usepkg=n --verbose sys-devel/libtool sys-devel/llvm sys-devel/clang dev-python/wxpython x11-libs/wxGTK
Some collections of packages need to be built with the same compiler (for example, the various qt-* packages). Such packages are usually bumped by package maintainers simultaneously, so they will always be built with the same GCC version. Cherry-picking re-installs on these packages might prove to be troublesome.
Some people swear that they need to rebuild every single package on their system when a new GCC version is made available. Of course, that doesn't make sense, since there are many applications that are not using GCC for their build and install process anyhow, so they would never be affected by such changes.
That, however, doesn't mean they are completely incorrect: newer GCC versions often include better support for the processors' instruction set, which might influence the performance of some applications in a positive way.
Apart from such "benign" benefits, rebuilding everything from scratch may be necessary in some cases to fix problems that don't seem to have any obvious cause.
Some software problems are inherently difficult to diagnose and yet could be solved by simply rebuilding one or more appropriate packages. If such a problem has arisen following a GCC upgrade and persists after using the revdep-rebuild approach described above (and after rebuilding any other obviously relevant packages), a complete system rebuild may be the answer.
emerge --ask --emptytree --usepkg=n @system
emerge --ask --emptytree --usepkg=n @world
Try this approach before reporting bugs that might have been caused by a GCC upgrade.
(Note that the commands above will cause the packages in the "system" set to be rebuilt twice, which is necessary to be absolutely certain that every package gets built in the same [presumably] "problem-free" environment. Any problems that remain after doing this are due to either "genuine bugs" that should be reported or poor system configuration.)
rebuild of boost
If dev-libs/boost needs to be rebuilt, one will get the following error message:
checking for the Boost _____ library... no configure: error: cannot find the flags to link with Boost _____
One can rebuild with:
emerge --ask --oneshot --usepkg=n --verbose dev-libs/boost
libstdc++.so.6: version `GLIBCXX_3.4.15' not found
During updates, you might encounter an error like the following:
cmake_bootstrap_28021_test: /usr/lib/gcc/i486-pc-linux-gnu/4.1.2/libstdc++.so.6: version `GLIBCXX_3.4.11' not found
This means that you are trying to build a package with an older GCC version than that with which some depending libraries were built. The C++ ABI is forward-compatible, but it ensures only that higher (or same) GCC versions can be used when building applications and linking libraries (compared to the GCC version used to build those libraries).
To rebuild all the packages depending on libstdc++, see the revdep-rebuild instructions above.
- Upgrade GCC up to 4.1, the previous version of this document
- Upgrading from gcc-4.x to gcc-5.x
- Fedora's 'Changes/GCC6' Wiki Page
- https://gcc.gnu.org/onlinedocs/gcc.pdf - Using the GNU Compiler Collection PDF.
- https://gcc.gnu.org/onlinedocs/gccint.pdf - GNU Compiler Collection Internals PDF.