.incbin, LTO, and incremental compilation

[nickdesaulniers]

I haven't had time to triage yet, but there was a reported issue with multiple configs where .incbin asm directives are causing incremental compilation surprises with LTO:

https://lore.kernel.org/lkml/164847778869.3060675.8115416881394543419.stgit@devnote2/
https://lore.kernel.org/all/20220321183500.GA4065@pswork/T/#u
https://lore.kernel.org/all/20220327115526.cc4b0ff55fc53c97683c3e4d@kernel.org/

Making note of this so we don't lose it.

[mhiramat]

I confirmed this happens only if we choose CONFIG_LTO_CLANG_THIN.
Fortunately, the bootconfig could avoid this issue by separating the inline asm as an independent asm file.

https://lore.kernel.org/all/164870615889.127053.9055569952366814752.stgit@devnote2/T/#u

However, since the .incbin directive is used also for /proc/config.gz and /proc/kheaders.tar.xz, those also be affected by this issue. These can not separate the asm code because those can be a module. Thus I recommend to disable LTO for those files.

[nathanchance]

I wonder if this upstream LLVM issue is relevant to this issue?

[nathanchance]

This came up again in #2021. That issue has more information, so I am going to duplicate this one in favor of that one.

I've sent a patch removing use of the ThinLTO cache due to this, as the correctness is more important than the performance improvement that we get with caching: https://lore.kernel.org/20240501-kbuild-llvm-drop-thinlto-cache-v1-1-c117cc50a24b@kernel.org/