I have to compile 3 versions of gcc, say 9, 10 and 11.
My system gcc is version 8 (let's say).
Question : do I have any advantage on compiling gcc-v9 with gcc-v8, gcc-10 with gcc-v9 and gcc-v11 with gcc-v10 ?
Or I don't have any advantage and I can compile them all the gcc-v8 ?
Thank you for pointing out some directions for further research.
GCC has a "bootstrap" build process. So when you try to build gcc-11 with only gcc-8 installed, it will build a temporary "stage 1" version of gcc-11 using gcc-8, then compile gcc-11 again using gcc-11-stage1. Thus no matter what you start with, the version of gcc-11 that comes out of the build process was effectively compiled with itself.
So all that matters is that gcc-8, or whatever "system compiler" was previously installed, is able to build a stage1 version of gcc-11 that runs well enough to compile the stage 2 version. It doesn't matter whether your system compiler is good at optimizing, and gcc's source code is deliberately written to use a fairly minimal set of language feature (at least for stage 1), so you are not likely to run into trouble with your system compiler having missing or buggy support for obscure corners of the language. Historically, the "system compiler" was often not gcc at all, but some compiler provided by the computer vendor or an unrelated third party, and so one couldn't rely much on its quality; gcc was designed with that in mind.
Theoretically your system compiler could have a bug which miscompiles gcc-11-stage1 in such a way that it appears to work, but itself miscompiles stage2. This is unlikely, and it's even less likely that it would happen in a way that wasn't obvious (e.g. the stage2 compiler simply segfaulting). If worried, there's an option to have stage2 build a stage3 compiler, and then check that both versions are identical. So as long as the build completes, you can be pretty confident that the final installed compiler is fine and unaffected by bugs in the original system compiler. (All that said, a reference to Ken Thompson's "Reflections on Trusting Trust" is obligatory here.)
So in practice, you don't need to worry about the version of gcc used to build a new version. Whatever you happen to have installed already, within reason, will be fine.
