[RFC PATCH u-boot 01/12] build: use thin archives instead of incremental linking

[Bin Meng]

Hi Marek,

On Fri, Mar 5, 2021 at 2:17 AM Marek Behun <marek.behun at nic.cz> wrote:
>
> On Thu, 4 Mar 2021 18:57:11 +0800
> Bin Meng <bmeng.cn at gmail.com> wrote:
>
> > Hi Marek,
> >
> > On Wed, Mar 3, 2021 at 12:13 PM Marek Behún <marek.behun at nic.cz> wrote:
> > >
> > > Using thin archives instead of incremental linking
> > > - saves disk space
> > > - works better with dead code elimination
> > > - prepares for potential LTO
> >
> > The commit message is a little bit confusing. This commit actually
> > does 2 things: don't do incremental linking (using --whole-archive),
> > and use thin archive (passing T to ar). I believe they are for
> > different purposes, so we cannot say "using thin archives instead of
> > incremental linking".
> > > -       -Wl,--start-group $(patsubst $(obj)/%,%,$(u-boot-spl-main)) \
> > > -       $(patsubst $(obj)/%,%,$(u-boot-spl-platdata)) -Wl,--end-group \
> > > +       -Wl,--whole-archive $(patsubst $(obj)/%,%,$(u-boot-spl-main)) -Wl,--no-whole-archive \
> > > +       -Wl,--start-group $(patsubst $(obj)/%,%,$(u-boot-spl-platdata)) -Wl,--end-group \
> >
> > u-boot-spl-platdata is still within --start-group, --end-group, is
> > this intentional?
>
> I confess that I did not really study these options, I have made these
> changes according to old LTO patches for Linux. But you are right that
> it does not make sense. I have fixed this for the next version of this
> patch.
>
> > Is P required to make everything work?
>
> It is not. Removed in next version.

I did more investigation on this.

The Linux kernel specially added P to ar, in below commit:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9a6cfca4f4130444

So it looks like we should keep P here?

But I don't get the point of switching to thin archives. Based on my
experiment, LTO does not rely on thin archives. The Linux kernel did
not introduce thin archives for LTO.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a5967db9af51a84f

Regards,
Bin