[PATCH u-boot 16/39] build: use thin archives instead of incremental linking

[Bin Meng]

On Mon, Mar 8, 2021 at 6:11 PM Marek Behun <marek.behun at nic.cz> wrote:
>
> On Mon, 8 Mar 2021 17:16:03 +0800
> Bin Meng <bmeng.cn at gmail.com> wrote:
>
> > Hi Marek,
> >
> > On Sun, Mar 7, 2021 at 12:26 PM Marek Behún <marek.behun at nic.cz> wrote:
> > >
> > > Currently we use incremental linking (ld -r) to link several object
> > > files from one directory into one built-in.o object file containing the
> > > linked code from that directory (and its subdirectories).
> > >
> > > Linux has, some time ago, moved to thin archives instead.
> > >
> > > Thin archives are archives (.a) that do not really contain the object
> > > files, only references to them.
> > >
> > > Using thin archives instead of incremental linking
> > > - saves disk space
> > > - apparently works better with dead code elimination
> > > - makes things easier for LTO
> > >
> > > The third point is the important one for us. With incremental linking
> > > there are several options how to do LTO, and that would unnecessarily
> > > complicate things.
> > >
> > > On the other hand, by using thin archives we can make (via the
> > > --whole-archive use flag) the final linking behave as if we passed all
> > > the object files from the archives to the linking program as arguments.
> >
> > I don't think --whole-archive is required for LTO to work.
>
> It is. Linking fails if it is not used, for example for
> nokia_rx51_defconfig.

Could you investigate why? Is this due to missing marking some
variables/functions as __used?

>
> > Switching to --whole-archive should be made conditionally when LTO is on,
> > otherwise for targets that don't have
> > -ffunction-sections/data-sections/--gc-sections specified, it will
> > create unnecessary bloat.
>
> OK I will push into CI without this flag for non-LTO and if it passes
> all tests I shall remove this flag for non-LTO.
>

Regards,
Bin