Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

[Sedat Dilek]

On Wed, Mar 10, 2021 at 10:47 PM Nicolas Pitre <nico@xxxxxxxxxxx> wrote:
...
> > With CONFIG_TRIM_UNUSED_KSYMS=y I see a 3x-loops of building .version
> > and folowing steps - got no answer if this is intended.
>
> Yes it is intended. I explained it here:
>
> https://lkml.org/lkml/2021/3/9/1099
>

Ah, cool.
Thanks for that link.

> With CONFIG_TRIM_UNUSED_KSYMS some EXPORT_SYMBOL() are removed, which
> allows for optimizing away the corresponding code, which in turn opens
> the possibility for more EXPORT_SYMBOL() to be removed, etc. The process
> eventually converge to a stable build. Normally only 2 passes are needed
> to converge, but LTO opens the possibilities for extra passes.
>
> > Means longer build-time.
>
> Oh, absolutely.  LTO (at least when I played with it) is slow. Add the
> multi-pass from CONFIG_TRIM_UNUSED_KSYMS on top of that and your kernel
> build becomes agonizingly slow. This is not something you want when
> doing kernel development.
>

Thanks for the feedback.

> > I did not follow this anymore as both Kconfigs with Clang-LTO consume
> > more build-time and the resulting vmlinux is some MiB bigger than with
> > Clang-CFI.
>
> That's rather strange. At least with gcc LTO I always obtained smaller
> kernels.
>

I cannot say much to GCC-LTO - I never used it.

If you are interested in Clang-CFI (see [1]) - which requires
Clang-LTO enabled and LLVM/Clang >= 12.
Some hours ago version 12.0.0-rc3 was released, see [2].

- Sedat -

[1] https://github.com/samitolvanen/linux/commits/clang-cfi
[2] https://github.com/ClangBuiltLinux/linux/issues/1259