[PATCH 01/10] Makefile: v2 Allow LTO to be disabled for a build

[Simon Glass]

Hi Andrew,

On Thu, 31 Mar 2022 at 04:29, Andrew Scull <ascull at google.com> wrote:
>
> On Sun, 27 Mar 2022 at 21:27, Simon Glass <sjg at chromium.org> wrote:
> >
> > LTO (Link-Time Optimisation) is an very useful feature which can
> > significantly reduce the size of U-Boot binaries. So far it has been
> > made available for selected ARM boards and sandbox.
> >
> > However, incremental builds are much slower when LTO is used. For example,
> > an incremental build of sandbox takes 2.1 seconds on my machine, but 6.7
> > seconds with LTO enabled.
>
> This is something that has been bothering me too. I'd resorted to adding
> `# CONFIG_LTO is not set` to sandbox_defconfig because I couldn't work
> out how to stop .config being regenerated each build. But then the
> diff gets in the way and is difficult to manage.
>
> > Add a LTO_BUILD=n parameter to the build, so it can be disabled during
> > development if needed, for faster builds.
>
> If you go the build parameter route rather than config route to
> address this, could the flag be consistent e.g. with NO_SDL=1, which
> would mean NO_LTO=1?

That seems OK to me. Would you like to do that and send a new version?

Here is a better version of the patch to use as a starting point:

https://source.denx.de/u-boot/custodians/u-boot-dm/-/commit/2d4ebfb0e8e86a1b347e6cbd4dbec9a4eb745bd5

Tom, please consider dropping your objection to this. I have already
shown that it makes a dramatic difference to incremental build time
for any board...

>
> > Add some documentation about LTO while we are here.
> >
> > Signed-off-by: Simon Glass <sjg at chromium.org>
> > ---
> >
> >  Makefile                           | 18 +++++++++++++-----
> >  arch/arm/config.mk                 |  4 ++--
> >  arch/arm/include/asm/global_data.h |  2 +-
> >  doc/build/gcc.rst                  | 17 +++++++++++++++++
> >  scripts/Makefile.spl               |  2 +-
> >  5 files changed, 34 insertions(+), 9 deletions(-)
> >

Regards,
Simon