Zephry LTO optimization

RE: Zephry LTO optimization

Vidar Berg — Tue, 07 May 2024 10:36:29 GMT

Experimental support for LTO was added in nRF Connect SDK v2.6.0. It can be enabled by setting CONFIG_LTO=y and CONFIG_ISR_TABLES_LOCAL_DECLARATION=y in your prj.conf file.

RE: Zephry LTO optimization

helsing — Wed, 04 Oct 2023 13:18:31 GMT

H Tamas, sorry for the delay.

[quote user="Tamas Selmeci"]I tried the statement found in the PR with more NCS-2.2.0 and NCS-2.4.1 applications, and surprisingly the final binary was usually greater than without LTO![/quote]

Our developers are experiencing the same as well.

[quote user="Tamas Selmeci"]Is there any way forcing NCS to use -lflto not only for the application code, but for the rest (Zephyr kernel) too? I'd give it a try...[/quote]

This is something currently being implemented. However, I am not able to predict at what time anything will be available.

RE: Zephry LTO optimization

Tamas Selmeci — Thu, 21 Sep 2023 04:04:21 GMT

I used to have success with LTO in my other projects during the recent years. The required technique usually was to specify -flto both during compile and linking, and at the cost of a much longer linking the resulting binary size was 3-5-7% less than without LTO. In some circumstances this can mean a lot ;)

I tried the statement found in the PR with more NCS-2.2.0 and NCS-2.4.1 applications, and surprisingly the final binary was usually greater than without LTO!

Is there any way forcing NCS to use -lflto not only for the application code, but for the rest (Zephyr kernel) too? I'd give it a try...

Thanks,

RE: Zephry LTO optimization

helsing — Mon, 11 Sep 2023 13:25:37 GMT

There is ongoing work with to enable LTO. Please see PR#11349 - asset_tracker_v2: Enable link time optimization as an example which enables LTO for the application code only. You may try the same technique to get LTO working for the application part of the code.

What is going on in the mentioned PR is that -flo is passed to the compiler when it compiles the application code. That is what this does: target_compile_options(app PRIVATE "-flto") and that is done if CONFIG_ASSET_TRACKER_V2_LTO is enabled.

More about the -flto flag can be found here: https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html .

What it really does is that it strips out unused functions from the final executable thereby saving flash space.

[quote user=""]zephyr_ld_options("-flto")[/quote]

This only adds the -flto option to the linker stage. The flag should be added to the compile stage instead, as in the PR.

RE: Zephry LTO optimization

helsing — Thu, 07 Sep 2023 13:23:30 GMT

Hello Tamas,

I am checking what can be done.