RFC: Add gcc offloading to GPUs

[viric]

Here is a dirty attempt to bring the "amdgcn-amdhsa" gcc offloading to nixpkgs. It's quite dirty but I wanted to have some 'working' base so we can translate to the structures that will handle this well. I was following the instructions in https://gcc.gnu.org/wiki/Offloading#Examples

Some of the difficulties are:

• The offload compiler needs to use its self-built newlib, by putting newlib source into gcc source tree. That isn't nicely supported today; newlib is meant as a separate derivation.
• I skipped the $lib output directory in gcc for simplicity.
• The stages do not fit well enough to the two compilers required I think.
• All is quite hardcoded to the single goal I had about gcc+amd.
• I used noSysDirs=false becasue the gcc configure/makefiles build a FLAGS_FOR_TARGET ideal for building newlib, and overriding them from our makeFlags destroys that internal mechanism. I think we should have something like a libc name "newlib-in-gcc", additional to uclibc, etc. that made gcc build newlib all in one.

With this I could build a simple "openmp target" example code in the clang page about Offloading: https://clang.llvm.org/docs/OffloadingDesign.html#offloading-example

@Ericson2314 what is your opinion how can we make these things fit? Ideally we would want AMD/NVIDIA offload in gcc, and AMD/NVIDIA/Intel offload in clang.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[wolfgangwalther]

This should target master instead, right?

[viric]

This should target master instead, right?

sure - I simply was doing the experiments on 24.11. This is not meant for merging yet. I only wanted to start the discussion on the bases of a dirty but working piece.