[jira] [Assigned] (ARROW-12533) [C++] Random real generator is slow on Arm64 Linux when built with clang

["Yibo Cai (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ARROW-12533?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Yibo Cai reassigned ARROW-12533:
--------------------------------

    Assignee: Yibo Cai

> [C++] Random real generator is slow on Arm64 Linux when built with clang
> ------------------------------------------------------------------------
>
>                 Key: ARROW-12533
>                 URL: https://issues.apache.org/jira/browse/ARROW-12533
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: C++
>            Reporter: Yibo Cai
>            Assignee: Yibo Cai
>            Priority: Minor
>
> Many benchmarks run very slow on Arm64 Linux when built with clang.
>  Most time is spent in preparing test data, not the test itself.
> Per my investigation, it boils down to poor performance of `std::uniform_real_distribution`, which uses software emulated `long double` arithmetic on Arm64 [1].
> Apple M1 doesn't have this issue. Clang aarch64 sets `long double` size to 64 bits on MacOS, but 128 on Linux [2].
> Gcc aarch64 doesn't have this issue. It doesn't use `long double` to generate random reals [1].
> clang `-ffast-math` option removes the `long double` arithmetic (and adds other simplifications to floating point arithmetic), it improves speed 100x on Arm64 in generating random reals.
> It may deserve some effort to study if `long double` is really necessary, and if `-ffast-math` is acceptable for generating test bits.
> [1] [https://godbolt.org/z/Y3Tc6MTME]
>  [2] [https://en.wikipedia.org/wiki/Long_double]



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