What, since when is GCC implemented in C++? I don't see any of that: https://github.com/gcc-mirror/gcc

Maybe in libstdc++, but what else would you use there.

I wanted to know what register allocation algorithms GCC implements, but I can't even find it. It's all files with cryptic names and hundreds in one directory. You can find _even less_ documentation, talks and blog posts about GCC internals :(