Is it possible for mortals to rebuild gcc from scratch? Can I start with some minimal, auditable compiler (tcc?) and build up to a modern gcc? Or would it be some byzantine path where I need to compile gcc v1998, then perl, then Python 1.8, enabling you to compile gcc v2005, which lets you build Python2.3, etc.
It is a byzantine path, also because gcc switched to C++ at some point (for no good reason IMHO). But there is a project that maintains such a bootstrap path: https://www.gnu.org/software/mes/
Mh. Though, if you have deterministic builds for GCC, imagine how much of a problem some nerd in Northern Washington or Scandinavia with their own strange C build chain would be to inject something strange into these compilers into the build process.
Like, you spend millions to get that one backdoor into the compiler. And then this guy is like "Uhm. Guys. I have this largely perl-based build process reproducing a modern GCC on a Pentium with 166 Mhz swapping RAM to disk because the motherboard can't hold that much memory. But the desk fan helps cooling. It takes about 2 months or 3 to build, but that's fine. I start it and then I work in the woods. It was identical to your releases about 5 times in the last 2 years (can't build more often), and now it isn't somewhere deep in the code sections. My arduino based floppy emulator is currently moving the binaries through the network"
Sure, it's a cyberpunk hero-fantasy, but deterministic builds would make these kind of shenanigans possible.
And at the end of the day, independent validation is one of the strongest ways to fight corruption.
i.e. supply-chain safety
It doesn't entirely resolve Thompson's "Trusting Trust" problem, but it goes a long way.