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

[Tom Rini]

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.

-- 
Tom
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