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

[Marek Behun]

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.

> 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.

Marek