diff --git a/ci.nix b/ci.nix
index 42d3ea0f3..d1509fe1c 100644
--- a/ci.nix
+++ b/ci.nix
@@ -83,6 +83,7 @@
        || (system == "aarch64-darwin" && !builtins.elem compiler-nix-name ["ghc902" "ghc928" "ghc948" "ghc966" "ghc967" "ghc96720250227" "ghc982" "ghc983" "ghc984"])
        )) {
     inherit (lib.systems.examples) ghcjs;
+    inherit (lib.systems.examples) wasi32;
   } // lib.optionalAttrs (nixpkgsName == "unstable"
       && (__match ".*llvm" compiler-nix-name == null)
       && ((system == "x86_64-linux"  && !builtins.elem compiler-nix-name ["ghc902" "ghc928" "ghc966" "ghc967" "ghc96720250227"]) # Not sure why GHC 9.6.6 TH code now wants `log1pf`
diff --git a/compiler/ghc/default.nix b/compiler/ghc/default.nix
index 418988467..aefce13cf 100644
--- a/compiler/ghc/default.nix
+++ b/compiler/ghc/default.nix
@@ -21,7 +21,7 @@ let self =
   libffi ? null
 
 , # we don't need LLVM for x86, aarch64, or ghcjs
-  useLLVM ? with stdenv.targetPlatform; !(isx86 || isAarch64 || isGhcjs)
+  useLLVM ? with stdenv.targetPlatform; !(isx86 || isAarch64 || isGhcjs || isWasm)
 , # LLVM is conceptually a run-time-only dependency, but for
   # non-x86, we need LLVM to bootstrap later stages, so it becomes a
   # build-time dependency too.
@@ -117,6 +117,34 @@ let
       INTEGER_LIBRARY = ${if enableIntegerSimple then "integer-simple" else "integer-gmp"}
     '';
 
+  libffi-wasm = buildPackages.runCommand "libffi-wasm" {
+      nativeBuildInputs = [
+        (buildPackages.haskell-nix.tool "ghc912" "libffi-wasm" {
+          src = buildPackages.haskell-nix.sources.libffi-wasm;
+        })
+        targetPackages.buildPackages.clang
+        targetPackages.buildPackages.llvm
+        targetPackages.buildPackages.binaryen
+      ];
+      outputs = ["dev" "out"];
+      NIX_NO_SELF_RPATH = true;
+    } ''
+      mkdir cbits
+      cp ${buildPackages.haskell-nix.sources.libffi-wasm}/cbits/* cbits/
+      libffi-wasm
+      wasm32-unknown-wasi-clang -Wall -Wextra -mcpu=mvp -Oz -DNDEBUG -Icbits -c cbits/ffi.c -o cbits/ffi.o
+      wasm32-unknown-wasi-clang -Wall -Wextra -mcpu=mvp -Oz -DNDEBUG -Icbits -c cbits/ffi_call.c -o cbits/ffi_call.o
+      wasm32-unknown-wasi-clang -Wall -Wextra -mcpu=mvp -Oz -DNDEBUG -Icbits -c cbits/ffi_closure.c -o cbits/ffi_closure.o
+
+      mkdir -p $dev/include
+      cp cbits/*.h $dev/include
+      mkdir -p $out/lib
+      llvm-ar -r $out/lib/libffi.a cbits/*.o
+
+      wasm32-unknown-wasi-clang -Wall -Wextra -mcpu=mvp -Oz -DNDEBUG -Icbits -fPIC -fvisibility=default -shared -Wl,--keep-section=target_features,--strip-debug cbits/*.c -o libffi.so
+      wasm-opt --low-memory-unused --converge --debuginfo --flatten --rereloop --gufa -O4 -Oz libffi.so -o $out/lib/libffi.so
+    '';
+
   # TODO check if this possible fix for segfaults works or not.
   targetLibffi =
     # on native platforms targetPlatform.{libffi, gmp} do not exist; thus fall back
@@ -124,7 +152,9 @@ let
     let targetLibffi = targetPackages.libffi or libffi; in
     # we need to set `dontDisableStatic` for musl for libffi to work.
     if stdenv.targetPlatform.isMusl
-    then targetLibffi.overrideAttrs (_old: { dontDisableStatic = true; })
+      then targetLibffi.overrideAttrs (_old: { dontDisableStatic = true; })
+    else if stdenv.targetPlatform.isWasm
+      then libffi-wasm
     else targetLibffi;
 
   targetGmp = targetPackages.gmp or gmp;
@@ -195,13 +225,15 @@ let
   # `--with` flags for libraries needed for RTS linker
   configureFlags = [
         "--datadir=$doc/share/doc/ghc"
-    ] ++ lib.optionals (!targetPlatform.isGhcjs && !targetPlatform.isAndroid) ["--with-curses-includes=${targetPackages.ncurses.dev}/include" "--with-curses-libraries=${targetPackages.ncurses.out}/lib"
-    ] ++ lib.optionals (targetLibffi != null && !targetPlatform.isGhcjs) ["--with-system-libffi" "--with-ffi-includes=${targetLibffi.dev}/include" "--with-ffi-libraries=${targetLibffi.out}/lib"
-    ] ++ lib.optionals (!enableIntegerSimple && !targetPlatform.isGhcjs) [
+    ] ++ lib.optionals (!targetPlatform.isGhcjs && !targetPlatform.isWasm && !targetPlatform.isAndroid) ["--with-curses-includes=${targetPackages.ncurses.dev}/include" "--with-curses-libraries=${targetPackages.ncurses.out}/lib"
+    ] ++ lib.optionals (targetLibffi != null && !targetPlatform.isGhcjs && !targetPlatform.isWasm) ["--with-system-libffi" "--with-ffi-includes=${targetLibffi.dev}/include" "--with-ffi-libraries=${targetLibffi.out}/lib"
+    ] ++ lib.optionals (targetPlatform.isWasm) [
+        "--with-system-libffi"
+    ] ++ lib.optionals (!enableIntegerSimple && !targetPlatform.isGhcjs && !targetPlatform.isWasm) [
         "--with-gmp-includes=${targetGmp.dev}/include" "--with-gmp-libraries=${targetGmp.out}/lib"
     ] ++ lib.optionals (targetPlatform == hostPlatform && hostPlatform.libc != "glibc" && !targetPlatform.isWindows) [
         "--with-iconv-includes=${libiconv}/include" "--with-iconv-libraries=${libiconv}/lib"
-    ] ++ lib.optionals (targetPlatform != hostPlatform && !targetPlatform.isGhcjs) [
+    ] ++ lib.optionals (targetPlatform != hostPlatform && !targetPlatform.isGhcjs && !targetPlatform.isWasm) [
         "--with-iconv-includes=${targetIconv}/include" "--with-iconv-libraries=${targetIconv}/lib"
     ] ++ lib.optionals (targetPlatform != hostPlatform) [
         "--enable-bootstrap-with-devel-snapshot"
@@ -229,9 +261,9 @@ let
     ;
 
   # Splicer will pull out correct variations
-  libDeps = platform: lib.optional (enableTerminfo && !targetPlatform.isGhcjs && !targetPlatform.isAndroid) [ targetPackages.ncurses targetPackages.ncurses.dev ]
+  libDeps = platform: lib.optional (enableTerminfo && !targetPlatform.isGhcjs && !targetPlatform.isWasm && !targetPlatform.isAndroid) [ targetPackages.ncurses targetPackages.ncurses.dev ]
     ++ lib.optional (!targetPlatform.isGhcjs) targetLibffi
-    ++ lib.optional (!enableIntegerSimple && !targetPlatform.isGhcjs) gmp
+    ++ lib.optional (!enableIntegerSimple && !targetPlatform.isGhcjs && !targetPlatform.isWasm) gmp
     ++ lib.optional (platform.libc != "glibc" && !targetPlatform.isWindows) libiconv
     ++ lib.optional (enableNUMA && platform.isLinux && !platform.isAarch32 && !platform.isAndroid) numactl
     ++ lib.optional enableDWARF (lib.getLib elfutils);
@@ -318,12 +350,12 @@ let
   # For build flavours and flavour transformers
   # see https://gitlab.haskell.org/ghc/ghc/blob/master/hadrian/doc/flavours.md
   hadrianArgs = "--flavour=${
-        (if targetPlatform.isGhcjs then "quick" else "default")
+        (if targetPlatform.isGhcjs || targetPlatform.isWasm then "quick" else "default")
           + lib.optionalString (!enableShared) "+no_dynamic_ghc"
           + lib.optionalString useLLVM "+llvm"
           + lib.optionalString enableDWARF "+debug_info"
-          + lib.optionalString ((enableNativeBignum && hadrianHasNativeBignumFlavour) || targetPlatform.isGhcjs) "+native_bignum"
-          + lib.optionalString targetPlatform.isGhcjs "+no_profiled_libs"
+          + lib.optionalString ((enableNativeBignum && hadrianHasNativeBignumFlavour) || targetPlatform.isGhcjs || targetPlatform.isWasm) "+native_bignum"
+          + lib.optionalString (targetPlatform.isGhcjs || targetPlatform.isWasm) "+no_profiled_libs"
       } --docs=no-sphinx -j --verbose"
       # This is needed to prevent $GCC from emitting out of line atomics.
       # Those would then result in __aarch64_ldadd1_sync and others being referenced, which
@@ -335,8 +367,12 @@ let
       + lib.optionalString (!hostPlatform.isAarch64 && targetPlatform.isLinux && targetPlatform.isAarch64)
         " '*.rts.ghc.c.opts += -optc-mno-outline-atomics'"
       # PIC breaks GHC annotations on windows (see test/annotations for a test case)
-      + lib.optionalString (enableRelocatedStaticLibs && !targetPlatform.isWindows)
+      + lib.optionalString (enableRelocatedStaticLibs && !targetPlatform.isWindows && !targetPlatform.isWasm)
         " '*.*.ghc.*.opts += -fPIC' '*.*.cc.*.opts += -fPIC'"
+      # C options for wasm
+      + lib.optionalString targetPlatform.isWasm (
+          " 'stage1.*.ghc.*.opts += -optc-Wno-error=int-conversion -optc-O3 -optc-mcpu=lime1 -optc-mreference-types -optc-msimd128 -optc-mtail-call -optc-DXXH_NO_XXH3'"
+        + " 'stage1.*.ghc.cpp.opts += -optc-fno-exceptions'")
       # `-fexternal-dynamic-refs` causes `undefined reference` errors when building GHC cross compiler for windows
       + lib.optionalString (enableRelocatedStaticLibs && targetPlatform.isx86_64 && !targetPlatform.isWindows)
         " '*.*.ghc.*.opts += -fexternal-dynamic-refs'"
@@ -437,9 +473,23 @@ haskell-nix.haskellLib.makeCompilerDeps (stdenv.mkDerivation (rec {
         export EM_CACHE=$(mktemp -d)
         mv config.sub.ghcjs config.sub
     '')
+    + lib.optionalString (targetPlatform.isWasm) ''
+        export CC="${targetCC}/bin/${targetCC.targetPrefix}cc"
+        export CXX="${targetCC}/bin/${targetCC.targetPrefix}c++"
+        export LD="${buildPackages.llvmPackages.lld}/bin/wasm-ld"
+        export AS="${targetCC.bintools.bintools}/bin/${targetCC.bintools.targetPrefix}as"
+        export AR="${targetCC.bintools.bintools}/bin/${targetCC.bintools.targetPrefix}ar"
+        export NM="${targetCC.bintools.bintools}/bin/${targetCC.bintools.targetPrefix}nm"
+        export RANLIB="${targetCC.bintools.bintools}/bin/${targetCC.bintools.targetPrefix}ranlib"
+        export READELF="${targetCC.bintools.bintools}/bin/${targetCC.bintools.targetPrefix}readelf"
+        export STRIP="${bintoolsFor.strip}/bin/${bintoolsFor.strip.targetPrefix}strip"
+        export NIX_CFLAGS_COMPILE_FOR_BUILD+=" -I${libffi.dev}/include -L${libffi.out}/lib"
+        export NIX_CFLAGS_COMPILE_FOR_TARGET+=" -I${targetLibffi.dev}/include -L${targetLibffi.out}/lib"
+        substituteInPlace compiler/GHC.hs --replace-fail "panic \"corrupted wasi-sdk installation\"" "pure \"${targetPackages.wasilibc}\""
+    ''
     # GHC is a bit confused on its cross terminology, as these would normally be
     # the *host* tools.
-    + lib.optionalString (!targetPlatform.isGhcjs) (''
+    + lib.optionalString (!targetPlatform.isGhcjs && !targetPlatform.isWasm) (''
         export CC="${targetCC}/bin/${targetCC.targetPrefix}cc"
         export CXX="${targetCC}/bin/${targetCC.targetPrefix}c++"
     ''
diff --git a/flake.lock b/flake.lock
index 1239e4d14..ca62bc708 100644
--- a/flake.lock
+++ b/flake.lock
@@ -100,23 +100,6 @@
         "type": "github"
       }
     },
-    "ghc-8.6.5-iohk": {
-      "flake": false,
-      "locked": {
-        "lastModified": 1600920045,
-        "narHash": "sha256-DO6kxJz248djebZLpSzTGD6s8WRpNI9BTwUeOf5RwY8=",
-        "owner": "input-output-hk",
-        "repo": "ghc",
-        "rev": "95713a6ecce4551240da7c96b6176f980af75cae",
-        "type": "github"
-      },
-      "original": {
-        "owner": "input-output-hk",
-        "ref": "release/8.6.5-iohk",
-        "repo": "ghc",
-        "type": "github"
-      }
-    },
     "hackage": {
       "flake": false,
       "locked": {
@@ -491,7 +474,6 @@
         "cabal-36": "cabal-36",
         "cardano-shell": "cardano-shell",
         "flake-compat": "flake-compat",
-        "ghc-8.6.5-iohk": "ghc-8.6.5-iohk",
         "hackage": "hackage",
         "hackage-for-stackage": "hackage-for-stackage",
         "hls": "hls",
diff --git a/flake.nix b/flake.nix
index 017d69555..bc701ef98 100644
--- a/flake.nix
+++ b/flake.nix
@@ -49,13 +49,6 @@
       url = "github:input-output-hk/cardano-shell";
       flake = false;
     };
-    "ghc-8.6.5-iohk" = {
-      type = "github";
-      owner = "input-output-hk";
-      repo = "ghc";
-      ref = "release/8.6.5-iohk";
-      flake = false;
-    };
     hpc-coveralls = {
       url = "github:sevanspowell/hpc-coveralls";
       flake = false;
diff --git a/lazy-inputs/default.nix b/lazy-inputs/default.nix
index 9552c8468..1fe105262 100644
--- a/lazy-inputs/default.nix
+++ b/lazy-inputs/default.nix
@@ -41,6 +41,7 @@ in {
       inherit ((callFlake { pkgs = final; src = ./ghc9122; }).defaultNix) ghc9122;
       inherit ((callFlake { pkgs = final; src = ./ghc912X; }).defaultNix) ghc912X;
       inherit ((callFlake { pkgs = final; src = ./ghc913; }).defaultNix) ghc913;
+      inherit ((callFlake { pkgs = final; src = ./libffi-wasm; }).defaultNix) libffi-wasm;
     } // prev.haskell-nix.sources;
   };
 }
diff --git a/lazy-inputs/libffi-wasm/flake.lock b/lazy-inputs/libffi-wasm/flake.lock
new file mode 100644
index 000000000..1240847b6
--- /dev/null
+++ b/lazy-inputs/libffi-wasm/flake.lock
@@ -0,0 +1,29 @@
+{
+  "nodes": {
+    "libffi-wasm": {
+      "flake": false,
+      "locked": {
+        "host": "gitlab.haskell.org",
+        "lastModified": 1741286406,
+        "narHash": "sha256-GxC9G+LYRHwKArwXNeU9Enq7u4arvPbFfE4kQj1cHyc=",
+        "owner": "haskell-wasm",
+        "repo": "libffi-wasm",
+        "rev": "e2dbdd8ded953e7a4df42c192da65ab36b9f0345",
+        "type": "gitlab"
+      },
+      "original": {
+        "host": "gitlab.haskell.org",
+        "owner": "haskell-wasm",
+        "repo": "libffi-wasm",
+        "type": "gitlab"
+      }
+    },
+    "root": {
+      "inputs": {
+        "libffi-wasm": "libffi-wasm"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}
diff --git a/lazy-inputs/libffi-wasm/flake.nix b/lazy-inputs/libffi-wasm/flake.nix
new file mode 100644
index 000000000..447a162af
--- /dev/null
+++ b/lazy-inputs/libffi-wasm/flake.nix
@@ -0,0 +1,12 @@
+{
+  description = "Lazy Input for Haskell.nix";
+
+  inputs = {
+    libffi-wasm = {
+      flake = false;
+      url = "gitlab:haskell-wasm/libffi-wasm?host=gitlab.haskell.org";
+    };
+  };
+
+  outputs = inputs: inputs;
+}
diff --git a/lib/call-cabal-project-to-nix.nix b/lib/call-cabal-project-to-nix.nix
index 96da9715d..4e9350f19 100644
--- a/lib/call-cabal-project-to-nix.nix
+++ b/lib/call-cabal-project-to-nix.nix
@@ -331,6 +331,8 @@ let
                 then "OSMinGW32"
               else if pkgs.stdenv.targetPlatform.isGhcjs
                 then "OSGhcjs"
+              else if pkgs.stdenv.targetPlatform.isWasi
+                then "OSWasi"
               else throw "Unknown target os ${pkgs.stdenv.targetPlatform.config}"
             }")'
           echo ',("target arch","${
@@ -346,6 +348,8 @@ let
                 then "ArchAArch32"
               else if pkgs.stdenv.targetPlatform.isJavaScript
                 then "ArchJavaScript"
+              else if pkgs.stdenv.targetPlatform.isWasm
+                then "ArchWasm32"
               else throw "Unknown target arch ${pkgs.stdenv.targetPlatform.config}"
           }")'
           echo ',("target platform string","${platformString pkgs.stdenv.targetPlatform}")'
diff --git a/lib/host-map.nix b/lib/host-map.nix
index b4a529f03..d91970d2b 100644
--- a/lib/host-map.nix
+++ b/lib/host-map.nix
@@ -18,6 +18,7 @@ with stdenv.hostPlatform; {
        if isiOS     then "Ios"     else
        if isAndroid then "Android" else
        if isGhcjs   then "Ghcjs"   else
+       if isWasi    then "Wasi"    else
        if isAsterius then "Asterius" else
        throw "Unknown OS";
   arch = if isx86     && is32bit then "I386"    else
diff --git a/overlays/default.nix b/overlays/default.nix
index bb5a18f65..a74dca70c 100644
--- a/overlays/default.nix
+++ b/overlays/default.nix
@@ -92,6 +92,7 @@ let
     cacheCompilerDeps = import ./cache-compiler-deps.nix;
     lazy-inputs = import ../lazy-inputs;
     rcodesign = import ./rcodesign.nix;
+    wasm = import ./wasm.nix;
   };
 
   composeExtensions = f: g: final: prev:
@@ -126,6 +127,7 @@ let
     cabalPkgConfig
     gobject-introspection
     hix
+    wasm
     # Restore nixpkgs haskell and haskellPackages
     (_: prev: { inherit (prev.haskell-nix-prev) haskell haskellPackages; })
     cacheCompilerDeps
diff --git a/overlays/patches/wasm/llvm/gnu-install-dirs.patch b/overlays/patches/wasm/llvm/gnu-install-dirs.patch
new file mode 100644
index 000000000..482325452
--- /dev/null
+++ b/overlays/patches/wasm/llvm/gnu-install-dirs.patch
@@ -0,0 +1,152 @@
+diff --git a/CMakeLists.txt b/CMakeLists.txt
+index c9ff3696e22d..bd96aab5e237 100644
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -1133,9 +1133,9 @@ if (NOT TENSORFLOW_AOT_PATH STREQUAL "")
+   add_subdirectory(${TENSORFLOW_AOT_PATH}/xla_aot_runtime_src
+     ${CMAKE_ARCHIVE_OUTPUT_DIRECTORY}/tf_runtime)
+   install(TARGETS tf_xla_runtime EXPORT LLVMExports
+-    ARCHIVE DESTINATION lib${LLVM_LIBDIR_SUFFIX} COMPONENT tf_xla_runtime)
++    ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}${LLVM_LIBDIR_SUFFIX} COMPONENT tf_xla_runtime)
+   install(TARGETS tf_xla_runtime EXPORT LLVMDevelopmentExports
+-    ARCHIVE DESTINATION lib${LLVM_LIBDIR_SUFFIX} COMPONENT tf_xla_runtime)
++    ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}${LLVM_LIBDIR_SUFFIX} COMPONENT tf_xla_runtime)
+   set_property(GLOBAL APPEND PROPERTY LLVM_EXPORTS tf_xla_runtime)
+   # Once we add more modules, we should handle this more automatically.
+   if (DEFINED LLVM_OVERRIDE_MODEL_HEADER_INLINERSIZEMODEL)
+diff --git a/cmake/modules/AddLLVM.cmake b/cmake/modules/AddLLVM.cmake
+index baf47677b247..81954240a9bf 100644
+--- a/cmake/modules/AddLLVM.cmake
++++ b/cmake/modules/AddLLVM.cmake
+@@ -974,8 +974,8 @@ macro(add_llvm_library name)
+       endif()
+       install(TARGETS ${name}
+               ${export_to_llvmexports}
+-              LIBRARY DESTINATION lib${LLVM_LIBDIR_SUFFIX} COMPONENT ${name}
+-              ARCHIVE DESTINATION lib${LLVM_LIBDIR_SUFFIX} COMPONENT ${name}
++              LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}${LLVM_LIBDIR_SUFFIX} COMPONENT ${name}
++              ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}${LLVM_LIBDIR_SUFFIX} COMPONENT ${name}
+               RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}" COMPONENT ${name})
+ 
+       if (NOT LLVM_ENABLE_IDE)
+@@ -2240,7 +2240,7 @@ function(llvm_install_library_symlink name dest type)
+     set(LLVM_LINK_OR_COPY copy)
+   endif()
+ 
+-  set(output_dir lib${LLVM_LIBDIR_SUFFIX})
++  set(output_dir ${CMAKE_INSTALL_FULL_LIBDIR}${LLVM_LIBDIR_SUFFIX})
+   if(WIN32 AND "${type}" STREQUAL "SHARED")
+     set(output_dir "${CMAKE_INSTALL_BINDIR}")
+   endif()
+@@ -2516,15 +2516,37 @@ function(llvm_setup_rpath name)
+
+   if (APPLE)
+     set(_install_name_dir INSTALL_NAME_DIR "@rpath")
+-    set(_install_rpath "@loader_path/../lib${LLVM_LIBDIR_SUFFIX}" ${extra_libdir})
++    set(_install_rpath ${extra_libdir})
+   elseif(${CMAKE_SYSTEM_NAME} MATCHES "AIX" AND BUILD_SHARED_LIBS)
+     # $ORIGIN is not interpreted at link time by aix ld.
+     # Since BUILD_SHARED_LIBS is only recommended for use by developers,
+     # hardcode the rpath to build/install lib dir first in this mode.
+     # FIXME: update this when there is better solution.
+-    set(_install_rpath "${LLVM_LIBRARY_OUTPUT_INTDIR}" "${CMAKE_INSTALL_PREFIX}/lib${LLVM_LIBDIR_SUFFIX}" ${extra_libdir})
++    set(_install_rpath "${LLVM_LIBRARY_OUTPUT_INTDIR}" "${CMAKE_INSTALL_FULL_LIBDIR}${LLVM_LIBDIR_SUFFIX}" ${extra_libdir})
+   elseif(UNIX)
+-    set(_install_rpath "\$ORIGIN/../lib${LLVM_LIBDIR_SUFFIX}" ${extra_libdir})
++    # Note that we add `extra_libdir` (aka `LLVM_LIBRARY_DIR` in our case) back
++    # to `_install_rpath` here.
++    #
++    # In nixpkgs we do not build and install LLVM alongside rdeps of LLVM (i.e.
++    # clang); instead LLVM is its own package and thus lands at its own nix
++    # store path. This makes it so that the default relative rpath (`../lib/`)
++    # does not point at the LLVM shared objects.
++    #
++    # More discussion here:
++    #   - https://github.com/NixOS/nixpkgs/pull/235624#discussion_r1220150329
++    #   - https://reviews.llvm.org/D146918 (16.0.5+)
++    #
++    # Note that we leave `extra_libdir` in `_build_rpath`: without FHS there is
++    # no potential that this will result in us pulling in the "wrong" LLVM.
++    # Adding this to the build rpath means we aren't forced to use
++    # `installCheckPhase` instead of `checkPhase` (i.e. binaries in the build
++    # dir, pre-install, will have the right rpath for LLVM).
++    #
++    # As noted in the differential above, an alternative solution is to have
++    # all rdeps of nixpkgs' LLVM (that use the AddLLVM.cmake machinery) set
++    # `CMAKE_INSTALL_RPATH`.
++    set(_build_rpath "\$ORIGIN/../lib${LLVM_LIBDIR_SUFFIX}" ${extra_libdir})
++    set(_install_rpath ${extra_libdir})
+     if(${CMAKE_SYSTEM_NAME} MATCHES "(FreeBSD|DragonFly)")
+       set_property(TARGET ${name} APPEND_STRING PROPERTY
+                    LINK_FLAGS " -Wl,-z,origin ")
+@@ -2539,9 +2561,9 @@ function(llvm_setup_rpath name)
+   endif()
+
+   # Enable BUILD_WITH_INSTALL_RPATH unless CMAKE_BUILD_RPATH is set.
+-  if("${CMAKE_BUILD_RPATH}" STREQUAL "")
+-    set_property(TARGET ${name} PROPERTY BUILD_WITH_INSTALL_RPATH ON)
+-  endif()
++  #if("${CMAKE_BUILD_RPATH}" STREQUAL "")
++  #  set_property(TARGET ${name} PROPERTY BUILD_WITH_INSTALL_RPATH ON)
++  #endif()
+
+   set_target_properties(${name} PROPERTIES
+                         INSTALL_RPATH "${_install_rpath}"
+diff --git a/cmake/modules/AddOCaml.cmake b/cmake/modules/AddOCaml.cmake
+index 2d9116b08a52..2dd7cad4ec66 100644
+--- a/cmake/modules/AddOCaml.cmake
++++ b/cmake/modules/AddOCaml.cmake
+@@ -147,9 +147,9 @@ function(add_ocaml_library name)
+   endforeach()
+ 
+   if( APPLE )
+-    set(ocaml_rpath "@executable_path/../../../lib${LLVM_LIBDIR_SUFFIX}")
++    set(ocaml_rpath ${LLVM_LIBRARY_DIR})
+   elseif( UNIX )
+-    set(ocaml_rpath "\\$ORIGIN/../../../lib${LLVM_LIBDIR_SUFFIX}")
++    set(ocaml_rpath ${LLVM_LIBRARY_DIR})
+   endif()
+   list(APPEND ocaml_flags "-ldopt" "-Wl,-rpath,${ocaml_rpath}")
+ 
+diff --git a/cmake/modules/CMakeLists.txt b/cmake/modules/CMakeLists.txt
+index ef4cfa3acdb5..7478e157a7c2 100644
+--- a/cmake/modules/CMakeLists.txt
++++ b/cmake/modules/CMakeLists.txt
+@@ -130,7 +130,7 @@ set(LLVM_CONFIG_INCLUDE_DIRS
+   )
+ list(REMOVE_DUPLICATES LLVM_CONFIG_INCLUDE_DIRS)
+ 
+-extend_path(LLVM_CONFIG_LIBRARY_DIR "\${LLVM_INSTALL_PREFIX}" "lib\${LLVM_LIBDIR_SUFFIX}")
++extend_path(LLVM_CONFIG_LIBRARY_DIR "\${LLVM_INSTALL_PREFIX}" "${CMAKE_INSTALL_LIBDIR}\${LLVM_LIBDIR_SUFFIX}")
+ set(LLVM_CONFIG_LIBRARY_DIRS
+   "${LLVM_CONFIG_LIBRARY_DIR}"
+   # FIXME: Should there be other entries here?
+diff --git a/tools/llvm-config/BuildVariables.inc.in b/tools/llvm-config/BuildVariables.inc.in
+index 370005cd8d7d..7e790bc52111 100644
+--- a/tools/llvm-config/BuildVariables.inc.in
++++ b/tools/llvm-config/BuildVariables.inc.in
+@@ -23,6 +23,7 @@
+ #define LLVM_CXXFLAGS "@LLVM_CXXFLAGS@"
+ #define LLVM_BUILDMODE "@LLVM_BUILDMODE@"
+ #define LLVM_LIBDIR_SUFFIX "@LLVM_LIBDIR_SUFFIX@"
++#define LLVM_INSTALL_LIBDIR "@CMAKE_INSTALL_LIBDIR@"
+ #define LLVM_INSTALL_INCLUDEDIR "@CMAKE_INSTALL_INCLUDEDIR@"
+ #define LLVM_INSTALL_PACKAGE_DIR "@LLVM_INSTALL_PACKAGE_DIR@"
+ #define LLVM_TARGETS_BUILT "@LLVM_TARGETS_BUILT@"
+diff --git a/tools/llvm-config/llvm-config.cpp b/tools/llvm-config/llvm-config.cpp
+index d5b76b1bb6c1..1dbdb2a8f10d 100644
+--- a/tools/llvm-config/llvm-config.cpp
++++ b/tools/llvm-config/llvm-config.cpp
+@@ -366,7 +366,11 @@ int main(int argc, char **argv) {
+       sys::fs::make_absolute(ActivePrefix, Path);
+       ActiveBinDir = std::string(Path);
+     }
+-    ActiveLibDir = ActivePrefix + "/lib" + LLVM_LIBDIR_SUFFIX;
++    {
++      SmallString<256> Path(LLVM_INSTALL_LIBDIR LLVM_LIBDIR_SUFFIX);
++      sys::fs::make_absolute(ActivePrefix, Path);
++      ActiveLibDir = std::string(Path);
++    }
+     {
+       SmallString<256> Path(LLVM_INSTALL_PACKAGE_DIR);
+       sys::fs::make_absolute(ActivePrefix, Path);
diff --git a/overlays/wasm.nix b/overlays/wasm.nix
new file mode 100644
index 000000000..707321b7f
--- /dev/null
+++ b/overlays/wasm.nix
@@ -0,0 +1,43 @@
+final: prev: prev.lib.optionalAttrs prev.stdenv.targetPlatform.isWasm {
+  llvmPackages = final.llvmPackages_20.override {
+    version = "20.1.0-haskel-wasm";
+    gitRelease.rev-version = "20.1.0-haskell-wasm";
+    officialRelease = null;
+    patchesFn = p: p // { "llvm/gnu-install-dirs.patch" = [{path = ./patches/wasm;}]; };
+    monorepoSrc = final.buildPackages.fetchFromGitLab {
+      domain = "gitlab.haskell.org";
+      owner = "haskell-wasm";
+      repo = "llvm-project";
+      rev = "3af5c33f0010c300d23adff0c576c637ba381580";
+      hash = "sha256-GnOP0tpyk+cWjMCJtxGi7FT78Wckl8fb6eibfdFWAJk=";
+      fetchSubmodules = true;
+    };
+  };
+  wasilibc = prev.wasilibc.overrideAttrs (old: {
+    version = "25";
+    src = final.buildPackages.fetchFromGitLab {
+      domain = "gitlab.haskell.org";
+      owner = "haskell-wasm";
+      repo = "wasi-libc";
+      rev = "f8f0d3101e02aa3aaf37c5e31db23de34963053d";
+      hash = "sha256-EvqbvVP9EH63C7KUmN4QaYjYbc4yGPU7vNev9u6a46o=";
+      fetchSubmodules = true;
+    };
+    preBuild = ''
+      patchShebangs ./scripts
+      makeFlagsArray+=(
+        "default"
+        "libc_so"
+        ) 
+    '';
+    postBuild = ''
+      mkdir -p ${builtins.placeholder "out"}
+      mkdir -p ${builtins.placeholder "dev"}
+      mkdir -p ${builtins.placeholder "share"}
+      cp -r sysroot/lib/wasm32-wasi ${builtins.placeholder "out"}/lib
+      cp -r sysroot/include/wasm32-wasi ${builtins.placeholder "dev"}/include
+      cp -r sysroot/share/wasm32-wasi ${builtins.placeholder "share"}/share
+    '';
+    nativeBuildInputs = old.nativeBuildInputs or [] ++ [ final.buildPackages.lld ];
+  });
+}