nixpkgs/doc/toolchains/llvm.chapter.md

The LLVM Toolchain

LLVM is a target-independent optimizer and code generator and serves as the basis for many compilers such as Haskell's GHC, rustc, Zig, and many others. It forms the base tools for Apple's Darwin platform.

Using LLVM

LLVM has two ways of being used. One is by using it across all of Nixpkgs and the other is to compile and build individual packages.

Building packages with LLVM

Nixpkgs supports two methods of compiling the world with LLVM. One is via setting useLLVM in crossSystem while importing. This is the recommended way when cross compiling as it is more expressive. An example of doing aarch64-linux cross compilation from x86_64-linux with LLVM on the target is the following:

import <nixpkgs> {
  localSystem = {
    system = "x86_64-linux";
  };
  crossSystem = {
    useLLVM = true;
    linker = "lld";
  };
}

Note that we set linker to lld. This is because LLVM has its own linker called "lld". By setting it, we utilize Clang and lld within this new instance of Nixpkgs. There is a shorthand method for building everything with LLVM: pkgsLLVM. This is easier to use with nix-build (or nix build):

nix-build -A pkgsLLVM.hello

This will compile the GNU hello package with LLVM and the lld linker like previously mentioned.

Using clangStdenv

Another simple way is to override the stdenv with clangStdenv. This causes a single package to be built with Clang. However, this stdenv does not override platform defaults to use compiler-rt, libc++, and libunwind. This is the preferred way to make a single package in Nixpkgs build with Clang. There are cases where just Clang isn't enough. For these situations, there is libcxxStdenv, which uses Clang with libc++ and compiler-rt.