Go deeper. You're right on the cusp of it since all of your questions are
fantastic. Even digging into one of your questions will bring up highly relevant material for understanding how a Linux system functions.
> The sed command is not explained at all. Even the description is non-sense if you don't know already what they are talking about. What is this "default directory", why do you need to set it, why there and only there, why that command works? Even the command itself is something, which I need to check the manual what the heck it does, because it's not a simple one.
By "default directory" they just mean that the upstream GCC source code has a file with default variables and they are modifying those variables with the sed command to use $prefix/lib and $prefix/usr/lib instead of $prefix/lib64 and $prefix/usr/lib64, e.g. lines that contain "m64=" and replacing "lib64" to be "lib". This is what sed is used for: To make string substitutions based on pattern matching. Think through ways of writing the sed command and testing on your own file to see how it behaves. This will lead you to more tools like diff, grep and awk.
> > -enable-default-pie and --enable-default-ssp
> The description is almost unusable. So we don't need it, but it's "cleaner", which in this context means exactly nothing. So what happens if I left out? Nothing? Then why should I care?
Go back and re-read the sections up to this point. Make note that you're in Chapter 5, which is the
bootstrapping phase for the toolchain cross-compilers. Then look into the features that are mentioned. You can see descriptions by running the ./configure --help most times or looking up the GCC documentation. Those features are for security purposes and if you put that in perspective of the bootstrap phase they aren't needed if the only purpose of the temporary GCC binaries is to compile the final GCC in a later phase. To your point, perform an experiment and enable those features to see if there really
is a difference other than time spent to compile. GCC incrementally adds security features like this and they are a
big thing in hardened distributions.
> > --disable-multilib
> Okay, it doesn't support "something". I have no idea what is multilib, or why I should care. Basically the next arguments' description tells me that because it wouldn't work the compilation otherwise. And then..
A great feature to look up! Check out
https://gcc.gnu.org/install/configure.html and search for the option and you'll find that it has to do with supporting a variety of target calling conventions. I can see how that'd be pretty confusing. It has to do with the underlying hardware support for application binary interfaces that GCC can utilize and it turns out you probably only need to support your native hardware (e.g. x86_64). That is, you're compiling your system from scratch and it'll only run on your native hardware (x86_64) but if you were a C/C++ programmer maybe you'd want to have support for other hardware (64-bit ARM systems are pretty common today as an example). So you can save time/space by disabling the defaults and honestly the defaults it includes are just not all that relevant on most systems today.
> > --disable-threads, --disable-libatomic, --disable-libgomp, --disable-libquadmath, --disable-libssp, --disable-libvtv, --disable-libstdcxx
> But why would they fail? I want to understand what's happening here, and I need to blindly trust the manual because they just tell me, that "they won't work, believe us".
Try it and find out. I would expect that they would fail due to reliance on other dependencies that may not have been installed or included in this bootstrapped build. Or maybe be/c those components don't behaved well with the LFS-based bootstrap methodology and ultimately aren't needed to bootstrap. Sure trust the LFSers but also think through a way to test your own assertions of the build process and try it out!
> > --enable-languages=c,c++
> Why are these the only languages which we need? What are the other languages?
GCC supports many language front-ends. See
https://gcc.gnu.org/frontends.html. Only C/C++ is needed be/c you're bootstrapping only C and C++ based package sources. You can validate this as you build the remaining sections. It's conceivable that if you needed other languages in the bootstrap you could include them.
> So at the end, descriptions are not really helping to understand what's happening, if you don't know already. The last time when I started LFS (about 10 years ago), that was my main problem. That you already need to know almost everything to understand what's really happening, and why, or reading manuals, or trying to find basically unsearchable information (like why libatomic compiling would fail at this step). So after a while, I started the blind copy-pasting, because I didn't have the patience of literary months, and when I realized that this was pointless, I gave up.
It's a steep learning curve, especially of a bootstrap which by its nature is circular! tbh, that's sort of the utility of LFS, it can take you up to a certain point but there are so many options and pitfalls in building a Linux system (or really any complex piece of software) and the real usefulness is pushing through, picking something and learning about it. Then using what you learned to apply to the unknown. GCC is one of the most important packages too, so there's a lot to unpack in understanding anything about it, but the impact is large.