[JoshTriplett]

Depends on the workload, but yes, codegen is a huge part of the total compilation time.

That said, that doesn't mean LLVM is always where the fixes need to be. For instance, one reason rustc spends a lot of time in LLVM is that rustc feeds more code to LLVM than it should, and relies on the LLVM optimizer to improve it. Over time, we're getting better about how much code we throw at LLVM, and that's providing performance improvements.

[Cogito]

I'm completely ignorant so forgive me if this is obvious: in the effort of the parent article - to compile rustc with gcc - will rustc still be feeding lots of code to LLVM, or would that code now be fed to gcc?

[heftig]

Which codegen backend the building compiler uses is independent of which codegen backend(s) the built compiler uses.

Similarly, you can build Clang using itself or using GCC. The resulting compiler should behave the same and produce the same machine code, even if its own machine code is somewhat different.

The produced binaries could still have artifacts from the original compiler in them, e.g. if "compiler built-in" libraries or standard libraries were compiled with the original compiler.

Both GCC and rustc use a multi-stage build process where the new compiler builds itself again, so you reach an idempotent state where no artifacts from the original compiler are left.