[Nix-dev] Replace default gcc through overlays?

[Nicolas Pierron]

Hi,

You can indeed replace gcc by a newer version of it, but the overlay
system does not allow you to do so inside the early stages of the
bootstrapping.  If you have to replace the early stages of the
bootstrapping, then I suggest you look at `pkgs/stdenv/default.nix`
and `pkgs/stdenv/linux/default.nix` and give it as argument of
`pkgs/top-level/default.nix` which is currently loading this file to
bootstrap the standard environment.

Otherwise, an overlay should help you with what you are looking for by
replacing the `stdenv` attribute as well as all the packages used for
bootstrapping.  You have to replace these packages as well because you
would otherwise have an inifnite loop as you try to add the new
compiler to the stdenv, which would be needed for compiling the
compiler it-self, due to `callPackage` taking stdenv from `self.`

```nix
self: super:

let
  ...
  stdenv = super.stdenvAdapters.overrideCC super.stdenv ...; # see [1]
in

{ inherit stdenv; } // stdenv.overrides self super
```

I did something similar a while ago, to use an old version of gcc, in
order to reproduce a bug which was on a different CI.  You can find
the detail here[1].
Note that, you might have to override the stdenv argument (and maybe
others) used for building the compiler:

```nix
cc.override { stdenv = super.stdenv }
```

In addition to changing the source of the derivation, otherwise you
might have an infinite loop, as the compiler is by default compiled
with the `callPackage` function which takes its inputs from the result
of the fix-point, which is not what you want here for bootstrapping
this new stdenv for every packages.

About making it a cross compiler, I do not know enough, but I can only
give you some pointers to `makeStdenvCross` stdenv adapter
function[2].

I have not tested this for replacing the stdenv globally, but I think
this will bring you closer to the final solution.
Good luck.

[1] https://github.com/mozilla/nixpkgs-mozilla/blob/master/release.nix#L96
[2] https://github.com/NixOS/nixpkgs/blob/master/pkgs/stdenv/adapters.nix#L59


On Wed, May 24, 2017 at 12:15 AM, Mateusz Czaplinski
<czapkofan at gmail.com> wrote:
> I'm writing a local (non-nixpkgs) derivation, and I'd like to replace the
> default "gcc" to be a patched gcc6 - also for all implicit dependencies in
> nixpkgs. Is it possible to do that (I assume via overlays and resulting
> fixpoint)? If yes, how should I write this to work?
>
> For background: I want to cross-compile the Linux kernel (actually, a fork -
> L4Linux) to a new target platform. But setting `crossSystem` seems to
> trigger rebuilding of gcc, which crashes on me because of OOM killer on
> genattrtab. This I believe could be mitigated by a Nov'2016 patch to gcc
> (https://patchwork.ozlabs.org/patch/695293/), so I believe I need to have it
> incorporated into default gcc. This I hope would let me overcome the "out of
> memory" obstacle, so that I could go back to hacking on my main task...
>
> When fixing this, I would much prefer not to have to fork whole nixpkgs, if
> feasible. I suppose maintaining a nixpkgs fork is harder, and also sharing
> sounds not so simple then as in the basic case of a single .nix file?
>
> I'd be grateful for any help.
> Thanks,
> /Mateusz.
>
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-- 
Nicolas Pierron
http://www.linkedin.com/in/nicolasbpierron - http://nbp.name/