Upgrading GCC/ja

この文書では、GCC のアップグレード手順を案内しています.

はじめに
この記事は、GCCの"アップグレード"についてです. 反対にもしもGCCをダウングレードしたら、望まぬ弊害が起こる危険性があります. また、一般的に報告されている問題のいくつかについては、トラブルシューティングの節を参照してください.

次のセクションでは、GCCのアップグレード（とどのように簡単に彼らは）への迅速なプライマーを提供します. あなたはGCCのアップグレードの背後に長い理由を読みたい場合は、GCCの説明のアップグレードに進んで下さい.

Short version
GCC をアップグレードしたときには、コンパイラのバージョンを変更することと、libtool を再ビルドすること以外には必要なことはありません：

ただし、GCC を 3.4.0 (3.x 系) や 4.1 より前のバージョンからアップグレードしたときには、 も実行する必要があります：

現在のバージョンを確認し、旧バージョンをアンインストールしましょう

以上です. 新しいコンパイラで楽しんでください！

はじめに
GCC upgrading has always been mystified, with suggestions ranging from "You do not need to do anything" up to "You will need to rebuild your entire system twice". Most of this FUD comes from the confusion surrounding ABI incompatibility. But first a quick pointer towards.

libtool と fix_libtool_files.sh
Earlier installments of GCC on Gentoo required you to run a specific command called. Some time ago, the execution of this command has been integrated in the package deployments itself (through the toolchain eclass) so there is no need for users to call this themselves anymore.

The reason we need to rebuild libtool after the upgrade of 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 script uses various library locations that have hard-coded  version information in them.

ABI の変化
An ABI, or 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.

Yes, C++, since most incompatibilities occur within the C++ ABI. If you are upgrading to GCC 4.1, or GCC 5.1, you would probably encounter ABI issues. This is also why we use the command against the  (from GCC 3 to GCC 4.1), or  (from GCC 4 to GCC 5.1).

So why is this only needed up to GCC 3.4.0/4.1/5.1? That's 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. In that case, the version of the library will probably be increased.

C++11 (および C++14) 特有の事象
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:


 * https://bugs.gentoo.org/show_bug.cgi?id=513386
 * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61758
 * https://blogs.gentoo.org/blueness/2015/03/10/the-c11-abi-incompatibility-problem-in-gentoo/
 * http://stackoverflow.com/a/16196475

すべてをリビルドすべきか
「GCC の新たなバージョンを入れたときには、各パッケージのすべてを再ビルドせねばならない」と心に誓っている人もいます. もちろん、そうする意味はありません. なぜなら、ビルドやインストールに GCC を一切使わないアプリケーションソフトもたくさんあり、そうしたソフトウェアには影響がないからです.

そうはいっても、彼らが完全に間違えているかというと、そうでもありません： 新しいバージョンの GCC ではしばしば、プロセッサの命令セットのサポートが向上していることがあります. そうすると、より良い性能が得られるアプリケーションソフトもあるかもしれません. そうした性能の向上は一般的にたかが知れている範囲ですが、ときには（特に CPU に大きく頼っているソフトウェアでは）目立った向上が生み出されることもあります.

There are also known cases where packages need to be built with the same compiler. Although these packages are usually bumped by Gentoo simultaneously (so that they are always built with the same GCC version) cherry-picking re-installs on these packages might prove to be troublesome. The various packages are a nice example on this matter.

libstdc++.so.6: version `GLIBCXX_3.4.15' not found
During updates, you might encounter an error like the following:

This means that you are trying to build a package with an older GCC version than with which some depending libraries were built. Remember when we told that the C++ ABI is forward-compatible? That is true, 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 previous revdep-rebuild section.

Which packages are known to need a rebuild?
The following table gives the packages that, if installed, need to be rebuilt and why.