Nixpkgs build custom architecture (musl soft float ABI)

README.md

Nixpkgs custom build targets

Nixpkgs provides pkgsCross with pre-defined targets that can be used for cross-compiling. We can view available targets with:

nix build nixpkgs#pkgsCross. <tab>

And then build for the target with a command like:

nix build nixpkgs#pkgsCross.aarch64-multiplatform.hello 

This would build the hello package from nixpkgs for aarch64

Custom build target flake

If the desired target platform isn't available, we can create a flake and define a custom crossSystem to build the package for. The mips-muslsf-file-flake.nix shown below builds a MIPS MUSL soft float binary on our x86_64-linux system for the file utility package. The package is also statically linked with .pkgsStatic.

We can build this flake by first setting the NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM option with:

export NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1

and then executing:

nix build --impure

From the current directory as the flake (the file must be renamed to flake.nix first).

The config parameter for the crossSystem is parsed with

nixpkgs/lib/systems/parse.nix

Which then determines the build target based on the provided system double or triple gcc compiler options can be found in:

nixpkgs/pkgs/development/compilers/gcc/common/platform-flags.nix

This will build the hello package as an ELF64 binary for aarch64 architecture. The crossSystem gcc option:

          gcc = {
            float = "soft";
          };

Will use the float option from the platform-flags to pass the --with-float option to gcc, but looking at the parse.nix file shows that:

 musleabi     = { float = "soft"; };

The musleabi could be used to achieve the same thing in the config parameter:

config = "mips-unknown-linux-musleabi";

Custom package and custom build target

What if we need to cross-compile a package that isn't in nixpkgs? We can download a local copy of nixpkgs and then add our custom package to it. First we need to clone nixpkgs.

Once we have a local copy, we can add our custom target to:

nixpkgs/lib/systems/examples.nix

I will add customTarget to the Linux block:

  customTarget = {
    config = "mips-unknown-linux-musl";
    gcc = {
      float = "soft";
    };
  };

Now when I enter the command:

nix build .#pkgsCross. <tab>

From the root of the nixpkgs directory (where the flake.nix is) I can see that customTarget is now an option to cross compile any nixpkg for!

Now we just need to add our custom package to the existing packages. We will copy our package directory containing our default.nix file to an appropriate path, in this case I will use:

nixpkgs/pkgs/applications/misc/custom-package

Now we can add it to the list of all packages by editing

nixpkgs/pkgs/top-level/all-packages.nix

Simply add

custom-package = callPackage ../applications/misc/custom-package {};

To the existing list of packages. Now from the root of the nixpkgs path we can run:

export NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1 
nix build --impure .#pkgsCross.customTarget.custom-package

And build our custom package for our custom target.

Custom package and custom build target flake

What if we just want to use nixpkgs to cross-compile our custom package, but we don't want to download and modify a local copy of nixpkgs?

We can accomplish this by creating a flake containing both our custom build target and an overlay for our custom package. The mips-muslfsf-custom-package-flake.nix file below shows an example implementation of this.

This can now be built simply running:

nix build --impure

In the directory with the flake.

Additional references

For more information on overalays see: For more information on building custom packages see: Nixpkgs itself provides 80,000 example packages that should cover just about any toolchain you want to use.

ao-flake.nix

{
  description = "Build angryoxide for MIPS MUSLSF";
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
    angryoxide = {
      url = "path:./angryoxide";
      flake = false;
    };
  };

  outputs = {
    self,
    nixpkgs,
    angryoxide,
  }: {
    defaultPackage.x86_64-linux = let
      crossPkgs = import nixpkgs {
        system = "x86_64-linux";
        crossSystem = {
          config = "mips-unknown-linux-musl";
          gcc = {
            float = "soft";
          };
        };
        overlays = [
          (self: super: {
            angryoxide = import angryoxide {
              inherit (super) lib;
              fetchFromGitHub = super.fetchFromGitHub;
              rustPlatform = super.rustPlatform;
            };
          })
        ];
      };
    in
      crossPkgs.angryoxide;
  };
}

mips-muslfsf-custom-package-flake.nix

{
  description = "Build custom-package for MIPS MUSLSF";
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
    custom-package = {
      url = "path:./custom-package";
      flake = false;
    };
  };

  outputs = {
    self,
    nixpkgs,
    custom-package,
  }: {
    defaultPackage.x86_64-linux = let
      crossPkgs = import nixpkgs {
        system = "x86_64-linux";
        crossSystem = {
          config = "mips-unknown-linux-musl";
          gcc = {
            float = "soft";
          };
        };
        overlays = [
          (self: super: {
            custom-package = import custom-package {
              inherit (super) lib;
              fetchFromGitHub = super.fetchFromGitHub;
              rustPlatform = super.rustPlatform;
            };
          })
        ];
      };
    in
      crossPkgs.custom-package;
  };
}

mips-muslsf-file-flake.nix

{
  description = "Build file utility package, statically linked, MIPS, MUSL, soft float";
  inputs.nixpkgs.url = github:NixOS/nixpkgs/nixos-23.11;
  outputs = {
    self,
    nixpkgs,
  }: {
    defaultPackage.x86_64-linux =
      (import nixpkgs {
        system = "x86_64-linux";
        crossSystem = {
          config = "mips-unknown-linux-musl";
          gcc = {
            float = "soft";
          };
        };
      })
      .pkgsStatic
      .file;
  };
}

parse.nix

# Define the list of system with their properties.
#
# See https://clang.llvm.org/docs/CrossCompilation.html and
# http://llvm.org/docs/doxygen/html/Triple_8cpp_source.html especially
# Triple::normalize. Parsing should essentially act as a more conservative
# version of that last function.
#
# Most of the types below come in "open" and "closed" pairs. The open ones
# specify what information we need to know about systems in general, and the
# closed ones are sub-types representing the whitelist of systems we support in
# practice.
#
# Code in the remainder of nixpkgs shouldn't rely on the closed ones in
# e.g. exhaustive cases. Its more a sanity check to make sure nobody defines
# systems that overlap with existing ones and won't notice something amiss.
#
{ lib }:

let
  inherit (lib)
    all
    any
    attrValues
    elem
    elemAt
    hasPrefix
    id
    length
    mapAttrs
    mergeOneOption
    optionalString
    splitString
    versionAtLeast
    ;

  inherit (lib.strings) match;

  inherit (lib.systems.inspect.predicates)
    isAarch32
    isBigEndian
    isDarwin
    isLinux
    isPower64
    isWindows
    ;

  inherit (lib.types)
    enum
    float
    isType
    mkOptionType
    number
    setType
    string
    types
    ;

  setTypes = type:
    mapAttrs (name: value:
      assert type.check value;
      setType type.name ({ inherit name; } // value));

  # gnu-config will ignore the portion of a triple matching the
  # regex `e?abi.*$` when determining the validity of a triple.  In
  # other words, `i386-linuxabichickenlips` is a valid triple.
  removeAbiSuffix = x:
    let found = match "(.*)e?abi.*" x;
    in if found == null
       then x
       else elemAt found 0;

in

rec {

  ################################################################################

  types.openSignificantByte = mkOptionType {
    name = "significant-byte";
    description = "Endianness";
    merge = mergeOneOption;
  };

  types.significantByte = enum (attrValues significantBytes);

  significantBytes = setTypes types.openSignificantByte {
    bigEndian = {};
    littleEndian = {};
  };

  ################################################################################

  # Reasonable power of 2
  types.bitWidth = enum [ 8 16 32 64 128 ];

  ################################################################################

  types.openCpuType = mkOptionType {
    name = "cpu-type";
    description = "instruction set architecture name and information";
    merge = mergeOneOption;
    check = x: types.bitWidth.check x.bits
      && (if 8 < x.bits
          then types.significantByte.check x.significantByte
          else !(x ? significantByte));
  };

  types.cpuType = enum (attrValues cpuTypes);

  cpuTypes = let inherit (significantBytes) bigEndian littleEndian; in setTypes types.openCpuType {
    arm      = { bits = 32; significantByte = littleEndian; family = "arm"; };
    armv5tel = { bits = 32; significantByte = littleEndian; family = "arm"; version = "5"; arch = "armv5t"; };
    armv6m   = { bits = 32; significantByte = littleEndian; family = "arm"; version = "6"; arch = "armv6-m"; };
    armv6l   = { bits = 32; significantByte = littleEndian; family = "arm"; version = "6"; arch = "armv6"; };
    armv7a   = { bits = 32; significantByte = littleEndian; family = "arm"; version = "7"; arch = "armv7-a"; };
    armv7r   = { bits = 32; significantByte = littleEndian; family = "arm"; version = "7"; arch = "armv7-r"; };
    armv7m   = { bits = 32; significantByte = littleEndian; family = "arm"; version = "7"; arch = "armv7-m"; };
    armv7l   = { bits = 32; significantByte = littleEndian; family = "arm"; version = "7"; arch = "armv7"; };
    armv8a   = { bits = 32; significantByte = littleEndian; family = "arm"; version = "8"; arch = "armv8-a"; };
    armv8r   = { bits = 32; significantByte = littleEndian; family = "arm"; version = "8"; arch = "armv8-a"; };
    armv8m   = { bits = 32; significantByte = littleEndian; family = "arm"; version = "8"; arch = "armv8-m"; };
    aarch64  = { bits = 64; significantByte = littleEndian; family = "arm"; version = "8"; arch = "armv8-a"; };
    aarch64_be = { bits = 64; significantByte = bigEndian; family = "arm"; version = "8";  arch = "armv8-a"; };

    i386     = { bits = 32; significantByte = littleEndian; family = "x86"; arch = "i386"; };
    i486     = { bits = 32; significantByte = littleEndian; family = "x86"; arch = "i486"; };
    i586     = { bits = 32; significantByte = littleEndian; family = "x86"; arch = "i586"; };
    i686     = { bits = 32; significantByte = littleEndian; family = "x86"; arch = "i686"; };
    x86_64   = { bits = 64; significantByte = littleEndian; family = "x86"; arch = "x86-64"; };

    microblaze   = { bits = 32; significantByte = bigEndian;    family = "microblaze"; };
    microblazeel = { bits = 32; significantByte = littleEndian; family = "microblaze"; };

    mips     = { bits = 32; significantByte = bigEndian;    family = "mips"; };
    mipsel   = { bits = 32; significantByte = littleEndian; family = "mips"; };
    mips64   = { bits = 64; significantByte = bigEndian;    family = "mips"; };
    mips64el = { bits = 64; significantByte = littleEndian; family = "mips"; };

    mmix     = { bits = 64; significantByte = bigEndian;    family = "mmix"; };

    m68k     = { bits = 32; significantByte = bigEndian; family = "m68k"; };

    powerpc  = { bits = 32; significantByte = bigEndian;    family = "power"; };
    powerpc64 = { bits = 64; significantByte = bigEndian; family = "power"; };
    powerpc64le = { bits = 64; significantByte = littleEndian; family = "power"; };
    powerpcle = { bits = 32; significantByte = littleEndian; family = "power"; };

    riscv32  = { bits = 32; significantByte = littleEndian; family = "riscv"; };
    riscv64  = { bits = 64; significantByte = littleEndian; family = "riscv"; };

    s390     = { bits = 32; significantByte = bigEndian; family = "s390"; };
    s390x    = { bits = 64; significantByte = bigEndian; family = "s390"; };

    sparc    = { bits = 32; significantByte = bigEndian;    family = "sparc"; };
    sparc64  = { bits = 64; significantByte = bigEndian;    family = "sparc"; };

    wasm32   = { bits = 32; significantByte = littleEndian; family = "wasm"; };
    wasm64   = { bits = 64; significantByte = littleEndian; family = "wasm"; };

    alpha    = { bits = 64; significantByte = littleEndian; family = "alpha"; };

    rx       = { bits = 32; significantByte = littleEndian; family = "rx"; };
    msp430   = { bits = 16; significantByte = littleEndian; family = "msp430"; };
    avr      = { bits = 8; family = "avr"; };

    vc4      = { bits = 32; significantByte = littleEndian; family = "vc4"; };

    or1k     = { bits = 32; significantByte = bigEndian; family = "or1k"; };

    loongarch64 = { bits = 64; significantByte = littleEndian; family = "loongarch"; };

    javascript = { bits = 32; significantByte = littleEndian; family = "javascript"; };
  };

  # GNU build systems assume that older NetBSD architectures are using a.out.
  gnuNetBSDDefaultExecFormat = cpu:
    if (cpu.family == "arm" && cpu.bits == 32) ||
       (cpu.family == "sparc" && cpu.bits == 32) ||
       (cpu.family == "m68k" && cpu.bits == 32) ||
       (cpu.family == "x86" && cpu.bits == 32)
    then execFormats.aout
    else execFormats.elf;

  # Determine when two CPUs are compatible with each other. That is,
  # can code built for system B run on system A? For that to happen,
  # the programs that system B accepts must be a subset of the
  # programs that system A accepts.
  #
  # We have the following properties of the compatibility relation,
  # which must be preserved when adding compatibility information for
  # additional CPUs.
  # - (reflexivity)
  #   Every CPU is compatible with itself.
  # - (transitivity)
  #   If A is compatible with B and B is compatible with C then A is compatible with C.
  #
  # Note: Since 22.11 the archs of a mode switching CPU are no longer considered
  # pairwise compatible. Mode switching implies that binaries built for A
  # and B respectively can't be executed at the same time.
  isCompatible = with cpuTypes; a: b: any id [
    # x86
    (b == i386 && isCompatible a i486)
    (b == i486 && isCompatible a i586)
    (b == i586 && isCompatible a i686)

    # XXX: Not true in some cases. Like in WSL mode.
    (b == i686 && isCompatible a x86_64)

    # ARMv4
    (b == arm && isCompatible a armv5tel)

    # ARMv5
    (b == armv5tel && isCompatible a armv6l)

    # ARMv6
    (b == armv6l && isCompatible a armv6m)
    (b == armv6m && isCompatible a armv7l)

    # ARMv7
    (b == armv7l && isCompatible a armv7a)
    (b == armv7l && isCompatible a armv7r)
    (b == armv7l && isCompatible a armv7m)

    # ARMv8
    (b == aarch64 && a == armv8a)
    (b == armv8a && isCompatible a aarch64)
    (b == armv8r && isCompatible a armv8a)
    (b == armv8m && isCompatible a armv8a)

    # PowerPC
    (b == powerpc && isCompatible a powerpc64)
    (b == powerpcle && isCompatible a powerpc64le)

    # MIPS
    (b == mips && isCompatible a mips64)
    (b == mipsel && isCompatible a mips64el)

    # RISCV
    (b == riscv32 && isCompatible a riscv64)

    # SPARC
    (b == sparc && isCompatible a sparc64)

    # WASM
    (b == wasm32 && isCompatible a wasm64)

    # identity
    (b == a)
  ];

  ################################################################################

  types.openVendor = mkOptionType {
    name = "vendor";
    description = "vendor for the platform";
    merge = mergeOneOption;
  };

  types.vendor = enum (attrValues vendors);

  vendors = setTypes types.openVendor {
    apple = {};
    pc = {};
    knuth = {};

    # Actually matters, unlocking some MinGW-w64-specific options in GCC. See
    # bottom of https://sourceforge.net/p/mingw-w64/wiki2/Unicode%20apps/
    w64 = {};

    none = {};
    unknown = {};
  };

  ################################################################################

  types.openExecFormat = mkOptionType {
    name = "exec-format";
    description = "executable container used by the kernel";
    merge = mergeOneOption;
  };

  types.execFormat = enum (attrValues execFormats);

  execFormats = setTypes types.openExecFormat {
    aout = {}; # a.out
    elf = {};
    macho = {};
    pe = {};
    wasm = {};

    unknown = {};
  };

  ################################################################################

  types.openKernelFamily = mkOptionType {
    name = "exec-format";
    description = "executable container used by the kernel";
    merge = mergeOneOption;
  };

  types.kernelFamily = enum (attrValues kernelFamilies);

  kernelFamilies = setTypes types.openKernelFamily {
    bsd = {};
    darwin = {};
  };

  ################################################################################

  types.openKernel = mkOptionType {
    name = "kernel";
    description = "kernel name and information";
    merge = mergeOneOption;
    check = x: types.execFormat.check x.execFormat
        && all types.kernelFamily.check (attrValues x.families);
  };

  types.kernel = enum (attrValues kernels);

  kernels = let
    inherit (execFormats) elf pe wasm unknown macho;
    inherit (kernelFamilies) bsd darwin;
  in setTypes types.openKernel {
    # TODO(@Ericson2314): Don't want to mass-rebuild yet to keeping 'darwin' as
    # the normalized name for macOS.
    macos    = { execFormat = macho;   families = { inherit darwin; }; name = "darwin"; };
    ios      = { execFormat = macho;   families = { inherit darwin; }; };
    # A tricky thing about FreeBSD is that there is no stable ABI across
    # versions. That means that putting in the version as part of the
    # config string is paramount.
    freebsd12 = { execFormat = elf;     families = { inherit bsd; }; name = "freebsd"; version = 12; };
    freebsd13 = { execFormat = elf;     families = { inherit bsd; }; name = "freebsd"; version = 13; };
    linux    = { execFormat = elf;     families = { }; };
    netbsd   = { execFormat = elf;     families = { inherit bsd; }; };
    none     = { execFormat = unknown; families = { }; };
    openbsd  = { execFormat = elf;     families = { inherit bsd; }; };
    solaris  = { execFormat = elf;     families = { }; };
    wasi     = { execFormat = wasm;    families = { }; };
    redox    = { execFormat = elf;     families = { }; };
    windows  = { execFormat = pe;      families = { }; };
    ghcjs    = { execFormat = unknown; families = { }; };
    genode   = { execFormat = elf;     families = { }; };
    mmixware = { execFormat = unknown; families = { }; };
  } // { # aliases
    # 'darwin' is the kernel for all of them. We choose macOS by default.
    darwin = kernels.macos;
    watchos = kernels.ios;
    tvos = kernels.ios;
    win32 = kernels.windows;
  };

  ################################################################################

  types.openAbi = mkOptionType {
    name = "abi";
    description = "binary interface for compiled code and syscalls";
    merge = mergeOneOption;
  };

  types.abi = enum (attrValues abis);

  abis = setTypes types.openAbi {
    cygnus       = {};
    msvc         = {};

    # Note: eabi is specific to ARM and PowerPC.
    # On PowerPC, this corresponds to PPCEABI.
    # On ARM, this corresponds to ARMEABI.
    eabi         = { float = "soft"; };
    eabihf       = { float = "hard"; };

    # Other architectures should use ELF in embedded situations.
    elf          = {};

    androideabi  = {};
    android      = {
      assertions = [
        { assertion = platform: !platform.isAarch32;
          message = ''
            The "android" ABI is not for 32-bit ARM. Use "androideabi" instead.
          '';
        }
      ];
    };

    gnueabi      = { float = "soft"; };
    gnueabihf    = { float = "hard"; };
    gnu          = {
      assertions = [
        { assertion = platform: !platform.isAarch32;
          message = ''
            The "gnu" ABI is ambiguous on 32-bit ARM. Use "gnueabi" or "gnueabihf" instead.
          '';
        }
        { assertion = platform: !(platform.isPower64 && platform.isBigEndian);
          message = ''
            The "gnu" ABI is ambiguous on big-endian 64-bit PowerPC. Use "gnuabielfv2" or "gnuabielfv1" instead.
          '';
        }
      ];
    };
    gnuabi64     = { abi = "64"; };
    muslabi64    = { abi = "64"; };

    # NOTE: abi=n32 requires a 64-bit MIPS chip!  That is not a typo.
    # It is basically the 64-bit abi with 32-bit pointers.  Details:
    # https://www.linux-mips.org/pub/linux/mips/doc/ABI/MIPS-N32-ABI-Handbook.pdf
    gnuabin32    = { abi = "n32"; };
    muslabin32   = { abi = "n32"; };

    gnuabielfv2  = { abi = "elfv2"; };
    gnuabielfv1  = { abi = "elfv1"; };

    musleabi     = { float = "soft"; };
    musleabihf   = { float = "hard"; };
    musl         = {};

    uclibceabi   = { float = "soft"; };
    uclibceabihf = { float = "hard"; };
    uclibc       = {};

    unknown = {};
  };

  ################################################################################

  types.parsedPlatform = mkOptionType {
    name = "system";
    description = "fully parsed representation of llvm- or nix-style platform tuple";
    merge = mergeOneOption;
    check = { cpu, vendor, kernel, abi }:
           types.cpuType.check cpu
        && types.vendor.check vendor
        && types.kernel.check kernel
        && types.abi.check abi;
  };

  isSystem = isType "system";

  mkSystem = components:
    assert types.parsedPlatform.check components;
    setType "system" components;

  mkSkeletonFromList = l: {
    "1" = if elemAt l 0 == "avr"
      then { cpu = elemAt l 0; kernel = "none"; abi = "unknown"; }
      else throw "Target specification with 1 components is ambiguous";
    "2" = # We only do 2-part hacks for things Nix already supports
      if elemAt l 1 == "cygwin"
        then { cpu = elemAt l 0;                      kernel = "windows";  abi = "cygnus";   }
      # MSVC ought to be the default ABI so this case isn't needed. But then it
      # becomes difficult to handle the gnu* variants for Aarch32 correctly for
      # minGW. So it's easier to make gnu* the default for the MinGW, but
      # hack-in MSVC for the non-MinGW case right here.
      else if elemAt l 1 == "windows"
        then { cpu = elemAt l 0;                      kernel = "windows";  abi = "msvc";     }
      else if (elemAt l 1) == "elf"
        then { cpu = elemAt l 0; vendor = "unknown";  kernel = "none";     abi = elemAt l 1; }
      else   { cpu = elemAt l 0;                      kernel = elemAt l 1;                   };
    "3" =
      # cpu-kernel-environment
      if elemAt l 1 == "linux" ||
         elem (elemAt l 2) ["eabi" "eabihf" "elf" "gnu"]
      then {
        cpu    = elemAt l 0;
        kernel = elemAt l 1;
        abi    = elemAt l 2;
        vendor = "unknown";
      }
      # cpu-vendor-os
      else if elemAt l 1 == "apple" ||
              elem (elemAt l 2) [ "wasi" "redox" "mmixware" "ghcjs" "mingw32" ] ||
              hasPrefix "freebsd" (elemAt l 2) ||
              hasPrefix "netbsd" (elemAt l 2) ||
              hasPrefix "genode" (elemAt l 2)
      then {
        cpu    = elemAt l 0;
        vendor = elemAt l 1;
        kernel = if elemAt l 2 == "mingw32"
                 then "windows"  # autotools breaks on -gnu for window
                 else elemAt l 2;
      }
      else throw "Target specification with 3 components is ambiguous";
    "4" =    { cpu = elemAt l 0; vendor = elemAt l 1; kernel = elemAt l 2; abi = elemAt l 3; };
  }.${toString (length l)}
    or (throw "system string has invalid number of hyphen-separated components");

  # This should revert the job done by config.guess from the gcc compiler.
  mkSystemFromSkeleton = { cpu
                         , # Optional, but fallback too complex for here.
                           # Inferred below instead.
                           vendor ? assert false; null
                         , kernel
                         , # Also inferred below
                           abi    ? assert false; null
                         } @ args: let
    getCpu    = name: cpuTypes.${name} or (throw "Unknown CPU type: ${name}");
    getVendor = name:  vendors.${name} or (throw "Unknown vendor: ${name}");
    getKernel = name:  kernels.${name} or (throw "Unknown kernel: ${name}");
    getAbi    = name:     abis.${name} or (throw "Unknown ABI: ${name}");

    parsed = {
      cpu = getCpu args.cpu;
      vendor =
        /**/ if args ? vendor    then getVendor args.vendor
        else if isDarwin  parsed then vendors.apple
        else if isWindows parsed then vendors.pc
        else                     vendors.unknown;
      kernel = if hasPrefix "darwin" args.kernel      then getKernel "darwin"
               else if hasPrefix "netbsd" args.kernel then getKernel "netbsd"
               else                                   getKernel (removeAbiSuffix args.kernel);
      abi =
        /**/ if args ? abi       then getAbi args.abi
        else if isLinux parsed || isWindows parsed then
          if isAarch32 parsed then
            if versionAtLeast (parsed.cpu.version or "0") "6"
            then abis.gnueabihf
            else abis.gnueabi
          # Default ppc64 BE to ELFv2
          else if isPower64 parsed && isBigEndian parsed then abis.gnuabielfv2
          else abis.gnu
        else                     abis.unknown;
    };

  in mkSystem parsed;

  mkSystemFromString = s: mkSystemFromSkeleton (mkSkeletonFromList (splitString "-" s));

  kernelName = kernel:
    kernel.name + toString (kernel.version or "");

  doubleFromSystem = { cpu, kernel, abi, ... }:
    /**/ if abi == abis.cygnus       then "${cpu.name}-cygwin"
    else if kernel.families ? darwin then "${cpu.name}-darwin"
    else "${cpu.name}-${kernelName kernel}";

  tripleFromSystem = { cpu, vendor, kernel, abi, ... } @ sys: assert isSystem sys; let
    optExecFormat =
      optionalString (kernel.name == "netbsd" &&
                          gnuNetBSDDefaultExecFormat cpu != kernel.execFormat)
        kernel.execFormat.name;
    optAbi = optionalString (abi != abis.unknown) "-${abi.name}";
  in "${cpu.name}-${vendor.name}-${kernelName kernel}${optExecFormat}${optAbi}";

  ################################################################################

}

platform-flags.nix

{ lib, targetPlatform }:

let
  isAarch64Darwin = targetPlatform.isDarwin && targetPlatform.isAarch64;
  gcc = targetPlatform.gcc or {};
  p =  gcc
    // targetPlatform.parsed.abi;
in lib.concatLists [
  # --with-arch= is unknown flag on x86_64 and aarch64-darwin.
  (lib.optional (!targetPlatform.isx86_64 && !isAarch64Darwin && p ? arch) "--with-arch=${p.arch}")
  # --with-cpu on aarch64-darwin fails with "Unknown cpu used in --with-cpu=apple-a13".
  (lib.optional (!isAarch64Darwin && p ? cpu) "--with-cpu=${p.cpu}")
  (lib.optional (p ? abi) "--with-abi=${p.abi}")
  (lib.optional (p ? fpu) "--with-fpu=${p.fpu}")
  (lib.optional (p ? float) "--with-float=${p.float}")
  (lib.optional (p ? mode) "--with-mode=${p.mode}")
  (lib.optionals targetPlatform.isPower64
    # musl explicitly rejects 128-bit long double on
    # powerpc64; see musl/arch/powerpc64/bits/float.h
    (lib.optionals
      (!targetPlatform.isMusl
       && (targetPlatform.isLittleEndian ||
           # "... --with-long-double-format is only supported if the default cpu is power7 or newer"
           #  https://github.com/NixOS/nixpkgs/pull/170215#issuecomment-1202164709
           (lib.lists.elem
             (lib.strings.substring 0 6 (p.cpu or ""))
             [ "power7" "power8" "power9" "power1"/*0, 11, etc*/ ]))) [
      "--with-long-double-128"
      "--with-long-double-format=${gcc.long-double-format or "ieee"}"
    ]))
  (lib.optional targetPlatform.isMips64n32 "--disable-libsanitizer") # libsanitizer does not compile on mips64n32
]