[PATCH v2 02/13] Makefile: Allow LTO to be disabled for a build

[Simon Glass]

Hi Tom,

On Mon, 7 Mar 2022 at 07:33, Tom Rini <trini at konsulko.com> wrote:
>
> On Fri, Mar 04, 2022 at 08:42:57AM -0700, Simon Glass 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.
> >
> > Add a LTO_BUILD=n parameter to the build, so it can be disabled during
> > development if needed, for faster builds.
> >
> > Add some documentation about LTO while we are here.
> >
> > Signed-off-by: Simon Glass <sjg at chromium.org>
>
> We don't need this since you can do:
> make EXTRA_CFLAGS="-fno-lto" EXTRA_LDFLAGS="-fno-lto"
> to pass -fno-lto to compile/linking and disable lto and per
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=46905 this has been working
> for some time.

Thanks for that, it is a big pain point for me, picking up this patch
for every series I write. The incremental build time for sandbox goes
from 3 seconds to 27 seconds on my laptop with LTO, which is
intolerable.

The EXTRA_CFLAGS says it is for 'Backward compatibility' and it still
does the various LTO things (i.e. it changes the build logic). It
seems odd to me to enable the option and then disable it later in the
command line. It is therefore not quite equivalent. But it seems to
work well enough for me fom a small amount of testing. If you are
really set on not having a special option for it, I can live with it
for now. I'm also not convinced that my patch entirely removes the LTO
stuff in a consistent way.

> Not that you need to respin the series for this.

Regards,
Simon