Re: [PATCH 01/13] objtool: Rewrite hashtable sizing

[Nathan Chancellor]

On 6/10/2021 11:43 AM, Peter Zijlstra wrote:
> On Thu, Jun 10, 2021 at 11:14:51AM -0700, Nathan Chancellor wrote:
>
> > This patch as commit 25cf0d8aa2a3 ("objtool: Rewrite hashtable sizing")
> > in -tip causes a massive compile time regression with allmodconfig +
> > ThinLTO.
>
> Moo... the allyesconfig builds I used it on were much faster, but that
> was on regular GCC vmlinux.o after linking.
>
> > Adding Sami because I am not sure why this patch would have much of an impact
> > in relation to LTO. https://git.kernel.org/tip/25cf0d8aa2a3 is the patch in
> > question.
> >
> > If I can provide any further information or help debug, please let me know.
> >
> > If you are interested in reproducing this locally, you will need a
> > fairly recent LLVM stack (I used the stable release/12.x branch) and to
> > cherry-pick commit 976aac5f8829 ("kcsan: Fix debugfs initcall return
> > type") to fix an unrelated build failure. My script [2] can build a
> > self-contained toolchain fairly quickly if you cannot get one from your
> > package manager. A command like below will speed up the build a bit:
>
> Would something like llvm-13 from Debian be good enough?
>
> $ clang-13 --version
> Debian clang version 13.0.0-++20210418105309+a0898f0cecc7-1~exp1
> Target: x86_64-pc-linux-gnu
> Thread model: posix
> InstalledDir: /usr/bin

Yes, that would work. That is what we use in our CI.

Looks like Sami gave a reply that explains it.

Cheers,
Nathan