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

[Marek Behun]

On Fri, 5 Mar 2021 21:34:42 +0800
Bin Meng <bmeng.cn at gmail.com> wrote:

> 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

It does not matter whether we use thin archives or real archives. But
we did not use any of this before, instead we linked the object files
in a directory into one object file in that directory. And to do this
with LTO would cause unnecessary complications.

Marek