gcc - why multiple passes for building Linux From Scratch (LFS)?

Question

I am trying to understand the concept of Linux From Scratch and would like to know why there are multiple passes for building binutils, gcc etc.

Why do we need pass1 and pass2 separately? Why can't we build the tools in pass 1 and then use them to build gcc , glibc, libstdc++ , etc.

Answer 1

The goal is to ensure that your build is consistent, no matter which compiler you're using to compile your compiler (and thus which bugs that compiler has).

Let's say you're building gcc 4.1 with gcc 3.2 (I'm going to call that gcc 3.2 "stage-0"). The folks who did QA for gcc 4.1 didn't test it to work correctly when built with any compiler other than gcc 4.1 -- hence, the need to first build a stage-1 gcc, and then use that stage-1 to compile a stage-2 compiler, to prevent any bugs in the stage-0 compiler from impacting the final result.

Then, the default compile process for gcc uses the stage-2 compiler to build a stage-3 compiler, and compares the two binaries: Any difference between them can be used as proof of presence of a bug.

(Of course, this is only an effective mechanism to avoid unintended bugs; see the classic Ken Thompson paper Reflections on Trusting Trust for a discussion of how intended bugs can survive this kind of measure).

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This goes beyond gcc into the entire toolchain because the same principles apply throughout: If you have any differences in the result between building glibc-x.y on a system running glibc-x.y and a system running glibc-x.(y-1) and you don't do an extra pass to ensure that you're building in a match for your target environment, then reproducing those bugs (and testing proposed fixes) is made far more difficult than would otherwise be the case: Nobody who doesn't have your (typically undisclosed) build environment can necessarily recreate the bug!