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Nix Cross Compiling

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18 Dec 2025
@matthewcroughan:defenestrate.itmatthewcroughanwhereas an out-of-date unmaintained thing is less relevant for nixpkgs inclusion13:33:30
@matthewcroughan:defenestrate.itmatthewcroughanstill shows how you can provide all that infra just fine outside of nixpkgs though13:33:46
@matthewcroughan:defenestrate.itmatthewcroughanLike, overlays, overrideAttrs, override, all provide you with ways of achieving what you want to do, you can't keep everything forever in the tree13:35:37
@bake.monorail:matrix.orgbake.monorail I feel you're being a bit unnecessarily adversarial. this said, having an overlay is quite different story, since it does not enable you to reuse "private" logic from nixpkgs.
for instance, in the GCC package it's not possible to override the versions without forking (there's a "private" variable determining the versions).
13:35:42
@bake.monorail:matrix.orgbake.monorail * I feel you're being a bit unnecessarily adversarial. this said, having an overlay is a quite different story, since it does not enable you to reuse "private" logic from nixpkgs.
for instance, in the GCC package it's not possible to override the versions without forking (there's a "private" variable determining the versions).
13:36:13
@matthewcroughan:defenestrate.itmatthewcroughanNot at all trying to be adversarial, just stating that if you want something like you're asking for, you should maintain it in your own tree, and this is what other people do too13:36:18
@matthewcroughan:defenestrate.itmatthewcroughanAnd that there's nothing too bad about providing toolchains in overlays13:36:41
@matthewcroughan:defenestrate.itmatthewcroughanlike providing toolchains as overlays in nix works well13:37:04
@emilazy:matrix.orgemily(as the person who dropped the most recent batch of GCCs and LLVMs:) yes, it is a meaningful burden on LLVM and GCC maintenance13:38:48
@matthewcroughan:defenestrate.itmatthewcroughan The fact that it's in a file means it's not private, you could just versions = import ./pkgs.path + "pkgs/development/compilers/gcc/versions.nix"; no? 13:39:19
@matthewcroughan:defenestrate.itmatthewcroughan * The fact that it's in a file means it's not private, you could just versions = (import ./pkgs.path + "pkgs/development/compilers/gcc/versions.nix"); no? 13:39:34
@bake.monorail:matrix.orgbake.monorail even without taking cc-wrapper into account? 13:39:41
@matthewcroughan:defenestrate.itmatthewcroughan * The fact that it's in a file means it's not private, you could just versions = (import ./pkgs.path + "/pkgs/development/compilers/gcc/versions.nix"); no? 13:39:44
@emilazy:matrix.orgemilyyes13:39:45
@emilazy:matrix.orgemily

e.g.

  • having to non-trivially patch old compilers to keep working with newer versions of everything else (e.g. newer Darwin SDK incompatibilites with with older compiler versions)
  • having to backport patches from new compilers to old compilers to work around bugs
13:40:11
@emilazy:matrix.orgemilyoften in tandem13:40:14
@emilazy:matrix.orgemilyalso, dealing with divergence in build systems over time13:40:25
@emilazy:matrix.orgemilyall of these add non-trivial complexity to the compiler derivations and backporting work for patches13:40:38
@emilazy:matrix.orgemilyhaving old compilers available also means that things will inevitably use them, so e.g. LLVM 12 was load-bearing for way too long on AArch64 because of GHC13:41:11
@emilazy:matrix.orgemilywhich exacerbated the amount of work that had to be done to keep it working with manual backports13:41:27
@emilazy:matrix.orgemilyanother example of the former is that we have had multiple cases where old compilers would not build with new compilers13:41:56
@emilazy:matrix.orgemilydue to them getting more strict13:41:59
@emilazy:matrix.orgemily so since everything roots to a new compiler in the stdenv bootstrap, we had to backport patches to fix that as well 13:42:18
@bake.monorail:matrix.orgbake.monorailI can understand the others, but in my experience this is not something tragic to do and it's worthwhile, at least with LLVM.13:43:45
@bake.monorail:matrix.orgbake.monorail but yeah, I was thinking of making a PR to enable manually overriding compiler versions (see the versions.nix thing above), letting whoever hacks with handle it, but at least in a way that does not require forking. 13:44:32
@emilazy:matrix.orgemilyit is maintenance burden however you slice it, and I can attest that it was a very frustrating way to spend our limited maintenance resources for very little benefit13:44:37
@bake.monorail:matrix.orgbake.monorailfair fair13:45:51
@emilazy:matrix.orgemilyI considered https://github.com/NixOS/nixpkgs/pull/440273, https://github.com/NixOS/nixpkgs/pull/435019, and all the PRs linked from them to be worth the effort to do compared to what carrying many EOL compilers has been like, despite them taking like 2 months in total13:46:00
@emilazy:matrix.orgemily(you can see the thousands of lines deleted from the compiler derivations in those and the vastly reduced conditional complexity of the derivations, but can't see the burden over time of when stuff breaks)13:46:35
@artturin:matrix.orgArtturin
In reply to @bake.monorail:matrix.org
I'm rather sad that end-of-life'd compilers are dropped from nixpkgs. old compilers are sometimes very useful. is it such a large burden to keep them alive?
Luckily it's easy to use a old nixpkgs revision
13:46:34

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