Some insights into 128 bit atomics in gcc:

https://gcc.gnu.org/legacy-ml/gcc/2018-02/msg00224.html

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84563

I'm having trouble overriding stdenv for the nix package:

{
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.05";

  outputs =
    inputs:
    let
      pkgs = import inputs.nixpkgs {
        system = "x86_64-linux";
      };
    in
    {
      packages.x86_64-linux = {
        nix_2_30_clang = pkgs.nixVersions.nix_2_30.override {
          stdenv = pkgs.clangStdenv;
        };
      };
    };
}

Building this package does not actually trigger a build but reuses the gcc artifacts while rebuilding some kind of wrapper (?)

$ nix build .#packages.x86_64-linux.nix_2_30_clang
$ readelf -p .comment ./result/bin/nix
String dump of section '.comment':
  [     1]  GCC: (GNU) 14.3.0

This should probably say something like:

  [     0]  GCC: (GNU) 14.3.0
  [    12]  clang version 18.1.8

Overriding still works for the autoconf based build (nix_2_24) but not for any of the newer package builds. It seems like the override is silently ignored and not passed down to the sub-derivations anymore.