Mention reproduciblity

[joebonrichie]

Distributions often won't PGO their packages as it can hurt reproducibility, this is mainly due the compiler potentially producing different run to run profiling results for the given workload affecting the optimized build. This is less of an issue with FDO as the profile provided is static.

For distributions like Arch Linux this is especially problematic as the binary packages are currently compiled the maintainer's personal machines then uploaded to the repository server AFAIU. Therefore the reproducibility requirements are high.

GCC has a flag to mitigate this https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html#index-fprofile-reproducible. E.g. the GCC auto PGO build will enable fprofile-reproducible=parallel-runs by default.

I currently don't know if clang provides any flags that affect profile reproducibility

Any toolchain resources compiled here mentioning reproducible profiles would be handy.

[zamazan4ik]

Yeah, I know the pain about PGO and reproducibility - it's probably the main blocker for distributions about enabling PGO for the packages (as far as I can see from multiple discussions with OS maintainers).

This is less of an issue with FDO as the profile provided is static.

Could you please clarify what do you mean here under the "FDO" term? The Sampling PGO (Clang docs) or something else?

For distributions like Arch Linux this is especially problematic as the binary packages are currently compiled the maintainer's personal machines then uploaded to the repository server AFAIU. Therefore the reproducibility requirements are high.

Wow, I didn't know about such a, khm-khm, tricky detail about Arch Linux packages. I agree, that in such cases the importance of reproducibility for packages is higher.

GCC has a flag to mitigate this https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html#index-fprofile-reproducible. E.g. the GCC auto PGO build will enable fprofile-reproducible=parallel-runs by default.

I didn't know about such a flag in GCC - thank you! I definitely need to spend more time with the GCC toolchain, huh. Most of my experience is about LLVM due to various reasons (I have nothing against GCC - it's just a personal preference, no more).

I currently don't know if clang provides any flags that affect profile reproducibility

That's a good question! I also didn't know about such a flag or any alternative in LLVM. There is a similar flag -fprofile-update=<option> flag in Clang (docs) but I don't know does it guarantee or not the "full" profile reproducibility or not. I will ask this question on LLVM forums, and update the article accordingly.

Any toolchain resources compiled here mentioning reproducible profiles would be handy.

Sure! I think I will extend the current "Reproducibilty" section in the article with such information.

If you have more information to share about PGO - you are welcome! I really appreciate contributions about this topic.

[zamazan4ik]

The question about profile reproducibility in LLVM: https://discourse.llvm.org/t/pgo-profile-reproducibility/82861