[RFC PATCH u-boot 00/12] U-Boot LTO (Sandbox + ARM Nokia RX-51)

[Tom Rini]

On Wed, Mar 03, 2021 at 05:41:57PM +0100, Marek Behun wrote:
> On Wed, 3 Mar 2021 11:11:59 -0500
> Tom Rini <trini at konsulko.com> wrote:
> 
> > On Wed, Mar 03, 2021 at 05:11:59AM +0100, Marek Behún wrote:
> > 
> > > Hello,
> > > 
> > > I have managed to add support for building U-Boot with LTO (with GCC)
> > > in a rather sane way (in LOC changed).
> > > 
> > > This series and its follows will also be available at
> > > https://github.com/elkablo/u-boot branch lto.
> > > 
> > > I have tested these builds on Turris Omnia, Turris MOX and on Nokia N900
> > > (via the test/nokia_rx51_test.sh script). For other tests I have created
> > > a pull-request on github to trigger CI (https://github.com/u-boot/u-boot/pull/57)
> > > For some reason it is waiting now, maybe Azure is not working or
> > > something.  
> > 
> > As we're on the free tier with Azure it sometimes just queues us up for
> > a long time, this job finally started running recently.
> > 
> > > My tests on Omnia and MOX show that U-Boot boots sucessfully, and basic
> > > commands seem to work. But of course something broken due to LTO may be
> > > found later.
> > > 
> > > So for all of you that are interested and have an ARM board, please test
> > > this on your boards by enabling CONFIG_LTO option. Also please report
> > > code size reductions. (Chris Packham reports an error related to
> > > jobserver, so if `make -jN` produces an error, please try without the
> > > `-jN` flag.)
> > > 
> > > I have only tested with gcc-10. There are still some warnings printed,
> > > like:
> > >   bfd plugin: invalid symbol type found
> > > but these don't seem to matter. I will look into this later.
> > > 
> > > Here are some results by how much code size reduced. Note that SPL
> > > binary seems to gain more code reduction (15.4 % on average) than main
> > > binary (4.5 % on average).
> > > 
> > > I guess this is because of how drivers are written. The optimizer cannot
> > > know which code paths won't be used, since it does not see the device
> > > tree. Maybe this could be somehow integrated with Simon's work on
> > > OF_PLATDATA_INST in the future, to make the compiler optimize out unused
> > > code paths in drivers by understanding the device tree.
> > > 
> > >                         u-boot.bin       u-boot-spl.bin
> > > 
> > >          clearfog    4.34 %  19.0 KB    13.55 %  16.8 KB
> > >   controlcenterdc    4.79 %  24.2 KB    16.27 %  21.9 KB
> > >    db-88f6820-amc    4.23 %  25.0 KB    16.17 %  22.9 KB
> > >     db-88f6820-gp    4.42 %  22.1 KB    17.00 %  23.8 KB
> > >           helios4    4.32 %  18.9 KB    13.70 %  16.8 KB
> > >        nokia_rx51    6.11 %  16.5 KB
> > >        turris_mox    4.17 %  31.8 KB
> > >      turris_omnia    4.32 %  30.2 KB    14.91 %  16.6 KB
> > >              x530    3.93 %  30.0 KB    16.26 %  23.4 KB
> > > 
> > > Marek  
> > 
> > Thanks for starting on this!  It's been on my list for a long time,
> > especially since it does give overall better reduction than
> > function/data-sections/discard.  It does seem like clang fails to build
> > with this series.  One thing I want to try locally, and I'll fire off
> > the results once I do it, is moving to LTO by default for ARM.
> > 
> 
> Yes, it seems clang is the last thing I need to look at. I did not even
> try, really, my first priority was gcc. I will look into this tomorrow.
> 
> All in all I am happy with this since it seems to be running for
> several different boards without issue.

Great.  I'll give it a spin on my platforms as well, but I suspect
things are good.

> If you want to enable LTO by default for ARM, we probably need to
> determine which gcc version should be minimal for this. Because older
> gcc versions may have problems with LTO. What is the current minimal
> version of gcc for U-Boot?

We, funny enough, have a check for gcc-4.0.4 on ARM, followed by
gcc-6.0.  That 4.0.4 check should be dropped, and gcc-6.0 is the
minimum.  I think gcc-7.2 or so is going to be important to keep working
but given the age of the Linux Kernel LTO support, we should be fine in
that regard.

-- 
Tom
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