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

[Bin Meng]

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. 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.

>
> We also need to use the P flag for ar, otherwise final linking may fail.
>
> Signed-off-by: Marek Behún <marek.behun at nic.cz>
> ---
>  Makefile               |  4 ++--
>  arch/sandbox/config.mk | 10 +++++++---
>  scripts/Makefile.build | 16 ++++++++--------
>  scripts/Makefile.spl   |  4 ++--
>  4 files changed, 19 insertions(+), 15 deletions(-)
>

Regards,
Bin