[PATCH v2 02/41] Makefile: Allow LTO to be disabled for a build

[Simon Glass]

Hi Tom,

On Wed, 27 Oct 2021 at 07:13, Tom Rini <trini at konsulko.com> wrote:
>
> On Wed, Oct 27, 2021 at 02:21:17PM +0200, Heinrich Schuchardt wrote:
> >
> >
> > On 10/27/21 10:50, Ilias Apalodimas wrote:
> > > Hi Simon
> > >
> > > How does this patch related to the standard boot series? Shouldn't
> > > this be a completely separate patch?
> > >
> > > Thanks
> > > /Ilias
> > >
> > > On Sun, 24 Oct 2021 at 02:26, 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.
> > > >
> > > > 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>
> > > > ---
> > > >
> > > > (no changes since v1)
> > > >
> > > >   Makefile                           | 18 +++++++++++++-----
> > > >   arch/arm/config.mk                 |  4 ++--
> > > >   arch/arm/include/asm/global_data.h |  2 +-
> > > >   doc/build/gcc.rst                  | 17 +++++++++++++++++
> > > >   4 files changed, 33 insertions(+), 8 deletions(-)
> > > >
> > > > diff --git a/Makefile b/Makefile
> > > > index b79b2319ff6..7057723e046 100644
> > > > --- a/Makefile
> > > > +++ b/Makefile
> > > > @@ -434,6 +434,9 @@ KBUILD_CFLAGS       += -fshort-wchar -fno-strict-aliasing
> > > >   KBUILD_AFLAGS   := -D__ASSEMBLY__
> > > >   KBUILD_LDFLAGS  :=
> > > >
> > > > +# Set this to "n" use of LTO for this build, e.g. LTO_BUILD=n
> > > > +LTO_BUILD      ?= y
> >
> > This does not allow LTO_BUILD=y to enable LTO for CONFIG_LTO=n.
>
> I don't understand why we need this patch at all.  If you want to
> disable LTO, disable LTO.  Yes, LTO makes linking take longer which can
> be annoying on iterative development.  I have a few different "HACK: DO
> NOT PUSH:  ..." things I git am at the start of a branch, depending on
> needs.  You can just do that to drop "imply LTO" from the SANDBOX stanza
> in arch/Kconfig.  We do not need a whole thing around a CONFIG option
> that can be disabled in the defconfig, or local .config file even.
>

Cranky time.

Of course we don't *need* it. I could just buy a slower build machine
and type with two fingers. There are lots of ways to slow things down
and LTO is one of them. I change branches at least a dozen times a day
and am always trying things out from patchwork. I am sure others do
too. LTO dramatically slows down builds. Having a way to easily do
this from the build system saves time.

Regards,
Simon