llvm: fix cross-compilation

[Mic92]

Motivation for this change

We build for both the host and target platform because we need this for rust.
I have not yet checked the build for all llvm versions.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

---

[matthewbauer]

Does LLVM_TARGETS_TO_BUILD not support the target triple?

[Mic92]

No. This is about CPU architectures not operating systems.

[dtzWill]

I'll try to get to the other bits soon, but for now a stylistic suggestion :).

[dtzWill]

Perhaps a "switch"-style instead here?

{
  mips = "Mips";
  x86 = "X86";
   ...
}."${platform.parsed.cpu.family}" or throw "Unsupported system"

(or thereabouts)

[matthewbauer]

Shoudn't LLVM_TARGETS_TO_BUILD default to all? Not sure what the rust error is though:

https://llvm.org/docs/CMake.html#llvm-specific-variables

[Ericson2314]

If it does then yes we can remove any mention of target and share llvms. Just compiler-rt would be rebuilt.

[matthewbauer]

Should we be concerned about output size/build times with this? LLVM_TARGETS_TO_BUILD is probably being overriden from the default right now when TARGET_TRIPLE is set. I kind of like that GCC's target-specific stuff doesn't give you anything for free (you have to set depsBuildBuild, nativeBuildInputs where appropriate). We might also still need DEFAULT_TARGET_TRIPLE.

[Mic92]

I did not do that because libllvm is also used for jit compiling and therefore a dependency of some applications. Do you think building all backends would have many users? Most of the time I think one picks one host platform to compile for and this is also how our cross-buildsystem is designed. So it won't be useful for many people to have all backends.

[Mic92]

@dtzWill the switch case style does not work here because of I have to use platform.parsed.cpu.name instead of family for aarch64.

[Ericson2314]

Actually, you probably won't need this is you use buildPackages.llvm and buildPackages.buildPackages.llvm. I do like the explicit target list though so we can make it a package option and then map that function over it.

[matthewbauer]

@Ericson2314 should we change "family" to use the capitalized version? Since we use LLVM as a basis for lib/systems/, it seems like we should keep them as close as possible to what LLVM accepts.

[Ericson2314]

@matthewbauer if you look at http://llvm.org/doxygen/classllvm_1_1Triple.html it does use the lower case names for those. I'm fine with changing it, but just want to be clear that LLVM may not consistently be upper-case about this either.

[Mic92]

There is one test case failing on older llvm versions: https://gist.github.com/Mic92/df4ae94fdb5071cfbe660b1c753e9b20

If someone is interested in fixing this, have a look at my old branch: https://github.com/Mic92/nixpkgs/tree/llvm-cross-targets-all

[jtojnar]

This broke mesa_noglu, see #52772