Targeting particular x86_64 generation (haswell/ivybridge/skylake/...)

21

Yes.
And you do not have to disable access to the binary cache, the packages just have other hashes.
So it does not even break purity :slight_smile:

1 Like
22

@volth Thank you so much! Learning to use NixOS is a fun exercise (that I’m falling in love with).

I’m still stumped on overriding default makeopts like CFLAGS, CXXFLAGS, etc. With Gentoo it is so easy… I found this in the nixpkgs manual:

makeFlagsArray

A shell array containing additional arguments passed to make . You must use this instead of makeFlags if the arguments contain spaces, e.g.

preBuild = 
'' makeFlagsArray+=(CFLAGS="-O0 -g" LDFLAGS="-lfoo -lbar")
 '';

Note that shell arrays cannot be passed through environment variables, so you cannot set makeFlagsArray in a derivation attribute (because those are passed through environment variables): you have to define them in shell code.

So how exactly can I set these for every package? It feels like this should be as easy as changing gcc.arch or gcc.tune!

Edit: Could I do it like this (copied from a Nix cheatsheet site)?
stdenv.userHook = '' NIX_CFLAGS_COMPILE+=" -march=native" '';

Or is something like this required and an overridden stdenv?

23

Probably, you cannot without patching nixpkgs.

In the initial proposal there was gcc.extraFlags (akin to gcc.arch) (https://github.com/NixOS/nixpkgs/search?q=gcc.extraFlags&type=Issues) but it did not get to the mainline

cc @matthewbauer

24

I think the best way to do this now is like this:

let
  pkgs = import <nixpkgs> {
    crossOverlays = [
      (self: super: {
        stdenv = super.stdenvAdapters.impureUseNativeOptimizations super.stdenv;
      })
    ];
  };
in pkgs.hello

You can use “overlays” or “crossOverlays”, but overlays requires you to rebuild the entire stdenv, while you probably don’t need that. On the other hand, this will make sure that everything is built with -march=native.

Other “adapters” are possible, see https://github.com/NixOS/nixpkgs/blob/c7b4b3bb1677d4b50251605a428016c6b5ac1d60/pkgs/stdenv/adapters.nix

We probably should add some more for things like this.

2 Likes
25

Thank you, I can try this on a new install.

Instead of adding more adapters, why not just create one where the user specifies the desired CFLAGS, CXXFLAGS, LDFLAGS, jobs (-j4 for ex.)? Maybe this is just a Gentoo thing, but I don’t know why these shouldn’t be easily specified by the user if desired (I understand not allowing -march=native for preserving purity but everything else like -O3 is host independent).

If I do use a stdenvAdapter, is setting gcc.arch/gcc.tune still necessary?

26

Hello I’m having a problem with my NixOS 19.09
When specifying

nixpkgs.localSystem.system   = builtins.currentSystem;
  nixpkgs.localSystem.platform = pkgs.lib.systems.platforms.pc64 // {
    gcc.arch = "bdver2";
    gcc.tune = "bdver2";
  };

Also tried setting …system = “x86_64-linux” to no avail (nothing changed)
I can not rebuild system, and get errors:

$ nixos-rebuild test
error: infinite recursion encountered, at /nix/var/nix/profiles/per-user/root/channels/nixos/lib/modules.nix:336:9
(use '--show-trace' to show detailed location information)
building Nix...
error: infinite recursion encountered, at /nix/var/nix/profiles/per-user/root/channels/nixos/lib/modules.nix:336:9
(use '--show-trace' to show detailed location information)
building the system configuration...
error: infinite recursion encountered, at /nix/var/nix/profiles/per-user/root/channels/nixos/lib/modules.nix:336:9
(use '--show-trace' to show detailed location information)

Tried --show-trace but don’t get what broke from it.
Is this deprecated?

27

No, AMD support is not merged yet (https://github.com/NixOS/nixpkgs/pull/61019)

28

Is there information about when it can be merged?
I would like to help if i can. What i could do?

29

Looks like localSystem.platform is not supported anywhere since 22.05 ( discussion on nixos.wiki.

The error message displayed on 23.05:

building Nix...
building the system configuration...
error:
       Failed assertions:
       - Your system configures nixpkgs with the platform parameter:
       nixpkgs.hostPlatform, with values defined in:
         - /etc/nixos/hardware-configuration.nix

       However, it also defines the legacy options:
       nixpkgs.system, with values defined in:
         - /nix/var/nix/profiles/per-user/root/channels/nixos/nixos/lib/eval-config.nix
       nixpkgs.localSystem, with values defined in:
         - /etc/nixos/configuration.nix

       For a future proof system configuration, we recommend to remove
       the legacy definitions.
(use '--show-trace' to show detailed location information)

Any idea how to target a particular CPU generation on recent NixOS releases ?