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

[Marek Behún]

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.

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

Marek Behún (12):
  build: use thin archives instead of incremental linking
  sandbox: errno: avoid conflict with libc's errno
  linker_lists: declare entries and lists externally visible
  efi_loader: fix warning when linking with LTO
  binman: declare symbols externally visible
  build: support building with Link Time Optimizations
  arch: sandbox: make LTO available
  sandbox: build with LTO
  ARM: make gd a function for LTO
  string: make memcpy() visible to fix LTO linking errors
  arch: ARM: make LTO available
  Nokia RX-51: build with LTO

 Kbuild                             |  2 ++
 Kconfig                            | 19 +++++++++++++++++++
 Makefile                           | 28 +++++++++++++++++++++++++++-
 arch/Kconfig                       |  2 ++
 arch/arm/include/asm/global_data.h |  2 +-
 arch/arm/lib/Makefile              |  2 ++
 arch/sandbox/config.mk             | 10 +++++-----
 configs/nokia_rx51_defconfig       |  1 +
 configs/sandbox64_defconfig        |  1 +
 configs/sandbox_defconfig          |  1 +
 configs/sandbox_flattree_defconfig |  1 +
 configs/sandbox_spl_defconfig      |  1 +
 include/binman.h                   |  1 +
 include/binman_sym.h               |  4 ++--
 include/efi_loader.h               |  4 ++--
 include/errno.h                    |  8 +++++++-
 include/linker_lists.h             |  6 ++++--
 lib/efi_loader/Makefile            |  2 +-
 lib/errno.c                        |  4 +++-
 lib/string.c                       |  3 ++-
 scripts/Makefile.build             |  5 ++---
 scripts/Makefile.lib               |  3 +++
 scripts/Makefile.spl               | 20 ++++++++++++++++----
 23 files changed, 106 insertions(+), 24 deletions(-)

-- 
2.26.2