diff --git a/default.nix b/default.nix
index d7484899..1dde39a5 100644
--- a/default.nix
+++ b/default.nix
@@ -3,8 +3,9 @@
 #  buildInputs = [ pkgs.coreutils ];
 #}
 
-
 {
+  # official 24.05 release
+  #nixpkgs-path ? fetchTarball "https://github.com/NixOS/nixpkgs/archive/nixos-24.05.tar.gz",
   nixpkgs-path ? ../nixpkgs,
 
 #  pkgs ? import (fetchTarball {
@@ -18,22 +19,43 @@ let
   # since absolutely everything has to be rebuilt from source
   #
 
+  # Problem: builds *everything* with llvm18 toolchain, exposes too many compiler nits
   llvm-overlay = self: super: {
     # use 'super' when you want to override the terms of a package.
     # use 'self'  when pointing to an existing package
+    #
 
     llvmPackages = super.llvmPackages_18;
   };
 
   xo-overlay = self: super:
-    let
-      # Choose the LLVM version you want
-      llvmPackages = self.llvmPackages_18;
-    in
+    # use 'super' when you want to override the terms of a package.
+    # use 'self'  when pointing to an existing package
 
+    let
+      # note:
+      # - problems with nix llvm build-from-source w/ gcc 13.3.
+      # - building outside nix following linuxfromscratch instructions, using gcc 12.3, works fine.
+      #
+      llvmXo = self.callPackage pkgs/llvm.nix {};
+
+      # Choose the LLVM version you want
+      llvmPackages1 = super.llvmPackages_18;
+
+      llvmPackages2 = llvmPackages1 // { llvm = llvmPackages1.llvm.overrideAttrs (old: {
+        cmakeFlags = old.cmakeFlags or [] ++ [ "-DCMAKE_VERBOSE_MAKEFILE=1" ];
+        }); };
+    in
       let
-        # Rebuild stdenv to use that LLVM version
-        customStdenv = super.overrideCC super.stdenv llvmPackages.clang;
+
+        # on darwin, rebuild stdenv to use clang tied to that LLVM version.
+        # otherwise we get conflicts since darwin stdenv is using clang+llvm for gcc.
+        #
+        clangStdenv = super.overrideCC super.stdenv llvmPackages2.clang;
+
+        # stdenv to use for xo-jit
+        jitStdenv = if super.stdenv.isDarwin then clangStdenv else super.stdenv;
+
       in
 
         {
@@ -77,17 +99,18 @@ let
           xo-tokenizer      = self.callPackage pkgs/xo-tokenizer.nix      {};
           xo-reader         = self.callPackage pkgs/xo-reader.nix         {};
 
-          xo-jit            = self.callPackage pkgs/xo-jit.nix            { stdenv = customStdenv;
-                                                                            clang = llvmPackages.clang;
-                                                                            llvm = llvmPackages.llvm; };
+          xo-jit            = self.callPackage pkgs/xo-jit.nix            { #stdenv = jitStdenv;
+                                                                            #clang = llvmPackages2.clang;
+                                                                            llvm = llvmPackages2.llvm; };
           xo-pyjit          = self.callPackage pkgs/xo-pyjit.nix          {};
 
 #
           xo-userenv        = self.callPackage pkgs/xo-userenv.nix        {};
-          xo-userenv-slow   = self.callPackage pkgs/xo-userenv-slow.nix   { stdenv = customStdenv;
+          xo-userenv-slow   = self.callPackage pkgs/xo-userenv-slow.nix   { stdenv = jitStdenv;
                                                                             #clang = llvmPackages.clang;
-                                                                            llvm = llvmPackages.llvm;
+                                                                            llvm = llvmPackages2.llvm;
                                                                           };
+          llvmXo = llvmXo;
         };
 
 in
diff --git a/pkgs/llvm.nix b/pkgs/llvm.nix
new file mode 100644
index 00000000..e3173547
--- /dev/null
+++ b/pkgs/llvm.nix
@@ -0,0 +1,113 @@
+{
+  lib,
+
+  stdenv, cmake, python3,
+
+  enableSharedLibraries ? !stdenv.hostPlatform.isStatic,
+
+  libxml2, libffi, libedit, ncurses, zlib,
+
+  fetchFromGitHub,
+} :
+
+let
+  version = "18.1.7";
+
+  src = fetchFromGitHub {
+    name = "llvm-${version}-source";
+    owner = "llvm";
+    repo = "llvm-project";
+    rev = "llvmorg-${version}";
+    hash = "sha256-qBL/1zh2YFabiPAyHehvzDSDfnwnCvyH6nY/pzG757A=";
+  };
+
+  defaultTargets =
+    if stdenv.hostPlatform.isAarch64 then "AArch64"
+    else if stdenv.hostPlatform.isx86 then "X86"
+    else "all";
+
+in {
+  # running out of memory during linking!!
+
+  llvm = stdenv.mkDerivation (finalAttrs:
+    {
+      pname = "llvm-xo";
+      inherit version src;
+
+      # only taking llvm subdir from upstream repo
+      sourceRoot = "${src.name}/llvm";
+
+      outputs = [ "out" ];
+
+      nativeBuildInputs = [ cmake python3 ];
+
+      #NIX_CFLAGS_LINK = "-Wl,--no-keep-memory, -Wl,--reduce-memory-overheads";
+      #NIX_CFLAGS_LINK = "-fuse-ld=gold"; # or -fuse-ld=lld
+
+      buildInputs = [ libxml2
+                      libffi
+                      zlib
+                      ncurses
+                      libedit
+      ];
+
+      propagatedBuildInputs = [ ncurses zlib ];
+
+      cmakeFlags = [
+        "-DCMAKE_BUILD_TYPE=Release"
+
+        "-DLLVM_TARGETS_TO_BUILD=${defaultTargets}"
+
+        "-DLLVM_BUILD_LLVM_DYLIB=${lib.boolToString enableSharedLibraries}"
+        "-DLLVM_LINK_LLVM_DYLIB=${lib.boolToString enableSharedLibraries}"
+
+        # host configuration
+        "-DLLVM_INSTALL_CMAKE_DIR=${placeholder "bin"}/lib/cmake/llvm"
+        "-DLLVM_INSTALL_BINUTILS_SYMLINKS=OFF"
+
+      ];
+
+      buildFlags = [
+        "VERBOSE=1"
+        "-j 12"
+      ];
+
+      # running out of memory!
+      enableParallelBuilding = true;
+
+      # for llvm keep debug symbols
+      separateDebugInfo = stdenv.isLinux;
+
+      postInstall = ''
+        # create compatibility symlinks
+        mkdir -p $out/bin
+        for tool in llvm-config llvm-nm llvm-objdump llvm-readobj llvm-ar; do
+          ln -sf $out/bin/$tool $out/bin/${stdenv.hostPlatform.config}-$tool
+        done
+
+#        # dev related files to llvm.dev
+#        moveToOutput "lib/cmake" "$dev"
+#        moveToOutput "include" "$dev"
+#        moveToOutput "lib/*.a" "$dev"
+
+#        # move any shared libraries to llvm.lib
+#        mkdir -p $lib/lib
+#        mv $out/lib/*.so* $lib/lib/ || true
+      '';
+
+      passthru = {
+        inherit src;
+        isClang = false;
+        hardeningUnsupportedFlags = [ "fortify3" ];
+      };
+
+      meta = with lib; {
+        description = "A collection of modular and reusable compiler and toolchain technologies";
+        homepage = "https://llvm.org/";
+        license = lib.licenses.ncsa;
+        maintainers = with maintainers; [];
+        platforms = platforms.all;
+        broken = stdenv.hostPlatform.is32bit;
+      };
+    });
+}