`gcc -march=native` work incorrectly in devshell

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1

gcc -march=native does not enable __AVX2__ and __SSE__ inside devshell.

When I run gcc -march=native -dM -E - </dev/null | grep AVX outside devshell, it outputs:

#define __AVX__ 1
#define __AVX2__ 1

However when I run nix develop and then run gcc -march=native -dM -E - </dev/null | grep AVX again, it outputs nothing.

__AES__ is the same.

my devshell flake.nix:

{
  description = "C/C++ development environment";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";

    utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, ... }@inputs: inputs.utils.lib.eachSystem [
    "x86_64-linux"
    "i686-linux"
    "aarch64-linux"
    "x86_64-darwin"
  ]
    (system:
      let
        pkgs = import nixpkgs {
          inherit system;
        };
      in
      {
        devShells.default = pkgs.mkShell {
          packages = with pkgs; [
            autoconf
            automake
            pkg-config
            cmake
            bear
            clang-tools_16
          ];
        };
      });
}

Now I am building a c++ project that needs AVX and AES, so this is very important for me.

2

gcc wrapper from nixpkgs tries to guard you against impurity (dependence on the build server CPU) imposed by -march=native:

$ cat `which gcc` | grep -C10 native
...
if [ "$NIX_ENFORCE_NO_NATIVE_x86_64_unknown_linux_gnu" = 1 ]; then
    kept=()
    # Old bash empty array hack
    for p in ${params+"${params[@]}"}; do
        if [[ "$p" = -m*=native ]]; then
            skip "$p"
        else
            kept+=("$p")
        fi
    done
    # Old bash empty array hack
    params=(${kept+"${kept[@]}"})
fi

If you still want the impurity you can disable it:

$ export NIX_ENFORCE_NO_NATIVE=0
$ gcc -march=native -dM -E - </dev/null | grep AVX
#define __AVX__ 1
#define __AVX2__ 1

Otherwise it might be better to explicitly set -march= or -mavx2 via NIX_CFLAGS_COMPILE as part of a package you need.

3

Thank you very much!