That's not what's going on though. Clang's not slow because LLVM is using additional optimizatons, it's slow because Clang is spending a ton of time in the frontend mostly lexing and keeping track of source locations for macro tokens.

(I literally just looked at a perf record of a whole build of the linux kernel with Clang; and know more about the subject than...anyone).

ToT LLVM, linux-next x86_64 defconfig: https://photos.app.goo.gl/hNwwy2K1Gh7Uc2D67 (yes, I know about disabling kptr_restrict)

Our next big project is to look into lazy parsing.