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

[Tom Rini]

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.

-- 
Tom
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