8 ms·

I wonder to what extent compilers even emit avx512 instructions apart from the common ones (load, store, shuffle, arithmetic) in case you don’t want to manually

I wonder to what extent compilers even emit avx512 instructions apart from the common ones (load, store, shuffle, arithmetic) in case you don’t want to manually optimize for sse / avx / avx2 / avx512.

celrod 4y ago
How much code is compiled with `-march=native` or function multiversioning? I would guess the percentage is relatively small, at least when it comes to distributed binaries. Compiler autovectorizers also aren't very good at producing fast AVX512 code, so most of the benefit would probably come from using optimized libraries like Intel's MKL or simdjson.
- derefr 4y ago
  > How much code is compiled with `-march=native` Any installation of Gentoo is, presumably. (Otherwise, what's the point of compiling it all yourself?) More interestingly, possibly all OEM firmware-installed copies of ChromeOS are -march=native builds as well, given that ChromeOS is based off of a Gentoo upstream.
  - celrod 4y ago
    True. I have never gone down the Gentoo rabbit hole. Might be fun to try sometime, but I'd seriously doubt that the time spent compiling would be won back from better performance. Clear Linux is probably a more practical alternative. I used it a couple years ago, and found that they had a lot of avx2 and avx512 versions of random libraries built, with the appropriate ones presumably being loaded based on the hardware. Random glibc math function calls, for example, were much faster on Clear Linux than Arch or Fedora. But development of Clear seems to have stopped, libraries like llvm aren't being updated anymore so the toolchains are outdated. I'd wanted to avoid the blood and sweat of managing my own toolchains, and ironically being on bleeding edge distros (Arch,Fedora,etc) was the way to keep that to a minimum. Next time I reinstall an OS, I'll look at Clear again. Or maybe Guix or Nix. Or maybe use spack for package management on top of some other distro.
    - Thev00d00 4y ago
      When I ran Gentoo I just had the builds running in the background with a really high niceness.
ryao 4y ago
Even if you use the GNU C vector extension to explicitly give the compiler ways of optimizing C, it is not very good at generating good vector code: https://github.com/openzfs/zfs/pull/14234#issuecomment-1334583224 https://github.com/openzfs/zfs/pull/14234#issuecomment-13345... A bug report has been filed with GCC for one of the issues. LLVM is much better here, but not perfect, or at least that has been my experience when trying to have the compiler generate assembly for an explicitly vectorized fletcher4 implementation.

celrod 4y ago

How much code is compiled with `-march=native` or function multiversioning? I would guess the percentage is relatively small, at least when it comes to distributed binaries. Compiler autovectorizers also aren't very good at producing fast AVX512 code, so most of the benefit would probably come from using optimized libraries like Intel's MKL or simdjson.

derefr 4y ago
> How much code is compiled with `-march=native` Any installation of Gentoo is, presumably. (Otherwise, what's the point of compiling it all yourself?) More interestingly, possibly all OEM firmware-installed copies of ChromeOS are -march=native builds as well, given that ChromeOS is based off of a Gentoo upstream.
- celrod 4y ago
  True. I have never gone down the Gentoo rabbit hole. Might be fun to try sometime, but I'd seriously doubt that the time spent compiling would be won back from better performance. Clear Linux is probably a more practical alternative. I used it a couple years ago, and found that they had a lot of avx2 and avx512 versions of random libraries built, with the appropriate ones presumably being loaded based on the hardware. Random glibc math function calls, for example, were much faster on Clear Linux than Arch or Fedora. But development of Clear seems to have stopped, libraries like llvm aren't being updated anymore so the toolchains are outdated. I'd wanted to avoid the blood and sweat of managing my own toolchains, and ironically being on bleeding edge distros (Arch,Fedora,etc) was the way to keep that to a minimum. Next time I reinstall an OS, I'll look at Clear again. Or maybe Guix or Nix. Or maybe use spack for package management on top of some other distro.
  - Thev00d00 4y ago
    When I ran Gentoo I just had the builds running in the background with a really high niceness.

ryao 4y ago

Even if you use the GNU C vector extension to explicitly give the compiler ways of optimizing C, it is not very good at generating good vector code: https://github.com/openzfs/zfs/pull/14234#issuecomment-1334583224 https://github.com/openzfs/zfs/pull/14234#issuecomment-13345... A bug report has been filed with GCC for one of the issues. LLVM is much better here, but not perfect, or at least that has been my experience when trying to have the compiler generate assembly for an explicitly vectorized fletcher4 implementation.