diff --git a/pkgs/development/compilers/llvm/15/libcxx/default.nix b/pkgs/development/compilers/llvm/15/libcxx/default.nix
index 94374c8a312df8f..cc4e9bc83058418 100644
--- a/pkgs/development/compilers/llvm/15/libcxx/default.nix
+++ b/pkgs/development/compilers/llvm/15/libcxx/default.nix
@@ -2,7 +2,7 @@
 , monorepoSrc, runCommand
 , cmake, ninja, python3, fixDarwinDylibNames, version
 , cxxabi ? if stdenv.hostPlatform.isFreeBSD then libcxxrt else libcxxabi
-, libcxxabi, libcxxrt
+, libcxxabi, libcxxrt, libunwind
 , enableShared ? !stdenv.hostPlatform.isStatic
 
 # If headersOnly is true, the resulting package would only include the headers.
@@ -62,7 +62,9 @@ stdenv.mkDerivation rec {
   nativeBuildInputs = [ cmake ninja python3 ]
     ++ lib.optional stdenv.isDarwin fixDarwinDylibNames;
 
-  buildInputs = lib.optionals (!headersOnly) [ cxxabi ];
+  buildInputs =
+    lib.optionals (!headersOnly) [ cxxabi ]
+    ++ lib.optionals (stdenv.hostPlatform.useLLVM or false) [ libunwind ];
 
   cmakeFlags = let
     # See: https://libcxx.llvm.org/BuildingLibcxx.html#cmdoption-arg-libcxx-cxx-abi-string
@@ -75,8 +77,18 @@ stdenv.mkDerivation rec {
     "-DLIBCXX_CXX_ABI=${if headersOnly then "none" else libcxx_cxx_abi_opt}"
   ] ++ lib.optional (!headersOnly && cxxabi.libName == "c++abi") "-DLIBCXX_CXX_ABI_INCLUDE_PATHS=${cxxabi.dev}/include/c++/v1"
     ++ lib.optional (stdenv.hostPlatform.isMusl || stdenv.hostPlatform.isWasi) "-DLIBCXX_HAS_MUSL_LIBC=1"
-    ++ lib.optional (stdenv.hostPlatform.useLLVM or false) "-DLIBCXX_USE_COMPILER_RT=ON"
-    ++ lib.optionals stdenv.hostPlatform.isWasm [
+    ++ lib.optionals (stdenv.hostPlatform.useLLVM or false) [
+      "-DLIBCXX_USE_COMPILER_RT=ON"
+      # (Backport fix from 16, which has LIBCXX_ADDITIONAL_LIBRARIES, but 15
+      # does not appear to)
+      # There's precedent for this in llvm-project/libcxx/cmake/caches.
+      # In a monorepo build you might do the following in the libcxxabi build:
+      #   -DLLVM_ENABLE_PROJECTS=libcxxabi;libunwind
+      #   -DLIBCXXABI_STATICALLY_LINK_UNWINDER_IN_STATIC_LIBRARY=On
+      # libcxx appears to require unwind and doesn't pull it in via other means.
+      # "-DLIBCXX_ADDITIONAL_LIBRARIES=unwind"
+      "-DCMAKE_SHARED_LINKER_FLAGS=-lunwind"
+    ] ++ lib.optionals stdenv.hostPlatform.isWasm [
       "-DLIBCXX_ENABLE_THREADS=OFF"
       "-DLIBCXX_ENABLE_FILESYSTEM=OFF"
       "-DLIBCXX_ENABLE_EXCEPTIONS=OFF"
diff --git a/pkgs/development/compilers/llvm/15/libcxxabi/default.nix b/pkgs/development/compilers/llvm/15/libcxxabi/default.nix
index cb5fa44f0700947..692a0256e901a4d 100644
--- a/pkgs/development/compilers/llvm/15/libcxxabi/default.nix
+++ b/pkgs/development/compilers/llvm/15/libcxxabi/default.nix
@@ -68,6 +68,13 @@ stdenv.mkDerivation rec {
   ] ++ lib.optionals (stdenv.hostPlatform.useLLVM or false) [
     "-DLLVM_ENABLE_LIBCXX=ON"
     "-DLIBCXXABI_USE_LLVM_UNWINDER=ON"
+  ] ++ lib.optionals ((stdenv.hostPlatform.useLLVM or false) ||
+                      (stdenv.hostPlatform.isDarwin && enableShared)) [
+    # libcxxabi's CMake looks as though it treats -nostdlib++ as implying -nostdlib,
+    # but that does not appear to be the case for example when building
+    # pkgsLLVM.libcxxabi (which uses clangNoCompilerRtWithLibc).
+    "-DCMAKE_EXE_LINKER_FLAGS=-nostdlib"
+    "-DCMAKE_SHARED_LINKER_FLAGS=-nostdlib"
   ] ++ lib.optionals stdenv.hostPlatform.isWasm [
     "-DLIBCXXABI_ENABLE_THREADS=OFF"
     "-DLIBCXXABI_ENABLE_EXCEPTIONS=OFF"
diff --git a/pkgs/development/compilers/llvm/15/libunwind/default.nix b/pkgs/development/compilers/llvm/15/libunwind/default.nix
index dd14e2acaf86927..1b677a7a2c0d7c4 100644
--- a/pkgs/development/compilers/llvm/15/libunwind/default.nix
+++ b/pkgs/development/compilers/llvm/15/libunwind/default.nix
@@ -40,6 +40,11 @@ stdenv.mkDerivation rec {
     cd ../runtimes
   '';
 
+  postInstall = lib.optionalString (enableShared && !stdenv.hostPlatform.isDarwin) ''
+    # libcxxabi wants to link to libunwind_shared.so (?).
+    ln -s $out/lib/libunwind.so $out/lib/libunwind_shared.so
+  '';
+
   outputs = [ "out" "dev" ];
 
   nativeBuildInputs = [ cmake ninja python3 ];
diff --git a/pkgs/development/compilers/llvm/16/compiler-rt/default.nix b/pkgs/development/compilers/llvm/16/compiler-rt/default.nix
index aa248c0e60eba53..fc0d7533c99496c 100644
--- a/pkgs/development/compilers/llvm/16/compiler-rt/default.nix
+++ b/pkgs/development/compilers/llvm/16/compiler-rt/default.nix
@@ -32,9 +32,16 @@ stdenv.mkDerivation {
     ++ lib.optional stdenv.isDarwin xcbuild.xcrun;
   buildInputs = lib.optional stdenv.hostPlatform.isDarwin libcxxabi;
 
-  env.NIX_CFLAGS_COMPILE = toString [
+  env.NIX_CFLAGS_COMPILE = toString ([
     "-DSCUDO_DEFAULT_OPTIONS=DeleteSizeMismatch=0:DeallocationTypeMismatch=0"
-  ];
+  ] ++ lib.optionals (!haveLibc) [
+    # The compiler got stricter about this, and there is a usellvm patch below
+    # which patches out the assert include causing an implicit definition of
+    # assert. It would be nicer to understand why compiler-rt thinks it should
+    # be able to #include <assert.h> in the first place; perhaps it's in the
+    # wrong, or perhaps there is a way to provide an assert.h.
+    "-Wno-error=implicit-function-declaration"
+  ]);
 
   cmakeFlags = [
     "-DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON"
@@ -102,7 +109,7 @@ stdenv.mkDerivation {
   '' + lib.optionalString stdenv.isDarwin ''
     substituteInPlace cmake/config-ix.cmake \
       --replace 'set(COMPILER_RT_HAS_TSAN TRUE)' 'set(COMPILER_RT_HAS_TSAN FALSE)'
-  '' + lib.optionalString (useLLVM) ''
+  '' + lib.optionalString (useLLVM && !haveLibc) ''
     substituteInPlace lib/builtins/int_util.c \
       --replace "#include <stdlib.h>" ""
     substituteInPlace lib/builtins/clear_cache.c \
@@ -117,6 +124,12 @@ stdenv.mkDerivation {
   '' + lib.optionalString (useLLVM) ''
     ln -s $out/lib/*/clang_rt.crtbegin-*.o $out/lib/crtbegin.o
     ln -s $out/lib/*/clang_rt.crtend-*.o $out/lib/crtend.o
+    # Note the history of crt{begin,end}S in previous versions of llvm in nixpkg:
+    # The presence of crtbegin_shared has been added and removed; it's possible
+    # people have added/removed it to get it working on their platforms.
+    # Try each in turn for now.
+    ln -s $out/lib/*/clang_rt.crtbegin-*.o $out/lib/crtbeginS.o
+    ln -s $out/lib/*/clang_rt.crtend-*.o $out/lib/crtendS.o
     ln -s $out/lib/*/clang_rt.crtbegin_shared-*.o $out/lib/crtbeginS.o
     ln -s $out/lib/*/clang_rt.crtend_shared-*.o $out/lib/crtendS.o
   '' + lib.optionalString doFakeLibgcc ''
diff --git a/pkgs/development/compilers/llvm/16/default.nix b/pkgs/development/compilers/llvm/16/default.nix
index 0c0ff44e155f8fb..d9a363037a92038 100644
--- a/pkgs/development/compilers/llvm/16/default.nix
+++ b/pkgs/development/compilers/llvm/16/default.nix
@@ -254,6 +254,14 @@ in let
         [ "-rtlib=compiler-rt"
           "-Wno-unused-command-line-argument"
           "-B${targetLlvmLibraries.compiler-rt}/lib"
+
+          # Combat "__cxxabi_config.h not found". Maybe this could be fixed by
+          # copying these headers into libcxx? Note that building libcxx
+          # outside of monorepo isn't supported anymore, might be related to
+          # https://github.com/llvm/llvm-project/issues/55632
+          # ("16.0.3 libcxx, libcxxabi: circular build dependencies")
+          # Looks like the machinery changed in https://reviews.llvm.org/D120727.
+          "-I${lib.getDev targetLlvmLibraries.libcxx.cxxabi}/include/c++/v1"
         ]
         ++ lib.optional (!stdenv.targetPlatform.isWasm) "--unwindlib=libunwind"
         ++ lib.optional
diff --git a/pkgs/development/compilers/llvm/16/libcxx/default.nix b/pkgs/development/compilers/llvm/16/libcxx/default.nix
index 94374c8a312df8f..c387764b4f65066 100644
--- a/pkgs/development/compilers/llvm/16/libcxx/default.nix
+++ b/pkgs/development/compilers/llvm/16/libcxx/default.nix
@@ -2,7 +2,7 @@
 , monorepoSrc, runCommand
 , cmake, ninja, python3, fixDarwinDylibNames, version
 , cxxabi ? if stdenv.hostPlatform.isFreeBSD then libcxxrt else libcxxabi
-, libcxxabi, libcxxrt
+, libcxxabi, libcxxrt, libunwind
 , enableShared ? !stdenv.hostPlatform.isStatic
 
 # If headersOnly is true, the resulting package would only include the headers.
@@ -47,8 +47,6 @@ stdenv.mkDerivation rec {
 
   patches = [
     ./gnu-install-dirs.patch
-  ] ++ lib.optionals stdenv.hostPlatform.isMusl [
-    ../../libcxx-0001-musl-hacks.patch
   ];
 
   postPatch = ''
@@ -62,7 +60,9 @@ stdenv.mkDerivation rec {
   nativeBuildInputs = [ cmake ninja python3 ]
     ++ lib.optional stdenv.isDarwin fixDarwinDylibNames;
 
-  buildInputs = lib.optionals (!headersOnly) [ cxxabi ];
+  buildInputs =
+    lib.optionals (!headersOnly) [ cxxabi ]
+    ++ lib.optionals (stdenv.hostPlatform.useLLVM or false) [ libunwind ];
 
   cmakeFlags = let
     # See: https://libcxx.llvm.org/BuildingLibcxx.html#cmdoption-arg-libcxx-cxx-abi-string
@@ -75,8 +75,15 @@ stdenv.mkDerivation rec {
     "-DLIBCXX_CXX_ABI=${if headersOnly then "none" else libcxx_cxx_abi_opt}"
   ] ++ lib.optional (!headersOnly && cxxabi.libName == "c++abi") "-DLIBCXX_CXX_ABI_INCLUDE_PATHS=${cxxabi.dev}/include/c++/v1"
     ++ lib.optional (stdenv.hostPlatform.isMusl || stdenv.hostPlatform.isWasi) "-DLIBCXX_HAS_MUSL_LIBC=1"
-    ++ lib.optional (stdenv.hostPlatform.useLLVM or false) "-DLIBCXX_USE_COMPILER_RT=ON"
-    ++ lib.optionals stdenv.hostPlatform.isWasm [
+    ++ lib.optionals (stdenv.hostPlatform.useLLVM or false) [
+      "-DLIBCXX_USE_COMPILER_RT=ON"
+      # There's precedent for this in llvm-project/libcxx/cmake/caches.
+      # In a monorepo build you might do the following in the libcxxabi build:
+      #   -DLLVM_ENABLE_PROJECTS=libcxxabi;libunwinder
+      #   -DLIBCXXABI_STATICALLY_LINK_UNWINDER_IN_STATIC_LIBRARY=On
+      # libcxx appears to require unwind and doesn't pull it in via other means.
+      "-DLIBCXX_ADDITIONAL_LIBRARIES=unwind"
+    ] ++ lib.optionals stdenv.hostPlatform.isWasm [
       "-DLIBCXX_ENABLE_THREADS=OFF"
       "-DLIBCXX_ENABLE_FILESYSTEM=OFF"
       "-DLIBCXX_ENABLE_EXCEPTIONS=OFF"
diff --git a/pkgs/development/compilers/llvm/16/libcxxabi/default.nix b/pkgs/development/compilers/llvm/16/libcxxabi/default.nix
index cb5fa44f0700947..692a0256e901a4d 100644
--- a/pkgs/development/compilers/llvm/16/libcxxabi/default.nix
+++ b/pkgs/development/compilers/llvm/16/libcxxabi/default.nix
@@ -68,6 +68,13 @@ stdenv.mkDerivation rec {
   ] ++ lib.optionals (stdenv.hostPlatform.useLLVM or false) [
     "-DLLVM_ENABLE_LIBCXX=ON"
     "-DLIBCXXABI_USE_LLVM_UNWINDER=ON"
+  ] ++ lib.optionals ((stdenv.hostPlatform.useLLVM or false) ||
+                      (stdenv.hostPlatform.isDarwin && enableShared)) [
+    # libcxxabi's CMake looks as though it treats -nostdlib++ as implying -nostdlib,
+    # but that does not appear to be the case for example when building
+    # pkgsLLVM.libcxxabi (which uses clangNoCompilerRtWithLibc).
+    "-DCMAKE_EXE_LINKER_FLAGS=-nostdlib"
+    "-DCMAKE_SHARED_LINKER_FLAGS=-nostdlib"
   ] ++ lib.optionals stdenv.hostPlatform.isWasm [
     "-DLIBCXXABI_ENABLE_THREADS=OFF"
     "-DLIBCXXABI_ENABLE_EXCEPTIONS=OFF"
diff --git a/pkgs/development/compilers/llvm/16/libunwind/default.nix b/pkgs/development/compilers/llvm/16/libunwind/default.nix
index dd14e2acaf86927..1b677a7a2c0d7c4 100644
--- a/pkgs/development/compilers/llvm/16/libunwind/default.nix
+++ b/pkgs/development/compilers/llvm/16/libunwind/default.nix
@@ -40,6 +40,11 @@ stdenv.mkDerivation rec {
     cd ../runtimes
   '';
 
+  postInstall = lib.optionalString (enableShared && !stdenv.hostPlatform.isDarwin) ''
+    # libcxxabi wants to link to libunwind_shared.so (?).
+    ln -s $out/lib/libunwind.so $out/lib/libunwind_shared.so
+  '';
+
   outputs = [ "out" "dev" ];
 
   nativeBuildInputs = [ cmake ninja python3 ];
diff --git a/pkgs/development/compilers/llvm/git/default.nix b/pkgs/development/compilers/llvm/git/default.nix
index 8cf311e9b23ace4..5ec5d35dcec517a 100644
--- a/pkgs/development/compilers/llvm/git/default.nix
+++ b/pkgs/development/compilers/llvm/git/default.nix
@@ -313,6 +313,19 @@ in let
       # what stdenv we use here, as long as CMake is happy.
       cxx-headers = callPackage ./libcxx {
         inherit llvm_meta;
+        # Note that if we use the regular stdenv here we'll get cycle errors
+        # when attempting to use this compiler in the stdenv.
+        #
+        # The final stdenv pulls `cxx-headers` from the package set where
+        # hostPlatform *is* the target platform which means that `stdenv` at
+        # that point attempts to use this toolchain.
+        #
+        # So, we use `stdenv_` (the stdenv containing `clang` from this package
+        # set, defined below) to sidestep this issue.
+        #
+        # Because we only use `cxx-headers` in `libcxxabi` (which depends on the
+        # clang stdenv _anyways_), this is okay.
+        stdenv = stdenv_;
         headersOnly = true;
       };
 
diff --git a/pkgs/development/compilers/llvm/git/libcxx/default.nix b/pkgs/development/compilers/llvm/git/libcxx/default.nix
index 94374c8a312df8f..f267b753b24ab5e 100644
--- a/pkgs/development/compilers/llvm/git/libcxx/default.nix
+++ b/pkgs/development/compilers/llvm/git/libcxx/default.nix
@@ -2,7 +2,7 @@
 , monorepoSrc, runCommand
 , cmake, ninja, python3, fixDarwinDylibNames, version
 , cxxabi ? if stdenv.hostPlatform.isFreeBSD then libcxxrt else libcxxabi
-, libcxxabi, libcxxrt
+, libcxxabi, libcxxrt, libunwind
 , enableShared ? !stdenv.hostPlatform.isStatic
 
 # If headersOnly is true, the resulting package would only include the headers.
@@ -62,7 +62,10 @@ stdenv.mkDerivation rec {
   nativeBuildInputs = [ cmake ninja python3 ]
     ++ lib.optional stdenv.isDarwin fixDarwinDylibNames;
 
-  buildInputs = lib.optionals (!headersOnly) [ cxxabi ];
+  buildInputs =
+    lib.optionals (!headersOnly) [ cxxabi ]
+    ++ lib.optionals (stdenv.hostPlatform.useLLVM or false) [ libunwind ];
+
 
   cmakeFlags = let
     # See: https://libcxx.llvm.org/BuildingLibcxx.html#cmdoption-arg-libcxx-cxx-abi-string
@@ -75,8 +78,18 @@ stdenv.mkDerivation rec {
     "-DLIBCXX_CXX_ABI=${if headersOnly then "none" else libcxx_cxx_abi_opt}"
   ] ++ lib.optional (!headersOnly && cxxabi.libName == "c++abi") "-DLIBCXX_CXX_ABI_INCLUDE_PATHS=${cxxabi.dev}/include/c++/v1"
     ++ lib.optional (stdenv.hostPlatform.isMusl || stdenv.hostPlatform.isWasi) "-DLIBCXX_HAS_MUSL_LIBC=1"
-    ++ lib.optional (stdenv.hostPlatform.useLLVM or false) "-DLIBCXX_USE_COMPILER_RT=ON"
-    ++ lib.optionals stdenv.hostPlatform.isWasm [
+    ++ lib.optionals (stdenv.hostPlatform.useLLVM or false) [
+      "-DLIBCXX_USE_COMPILER_RT=ON"
+      # (Backport fix from 16, which has LIBCXX_ADDITIONAL_LIBRARIES, but 15
+      # does not appear to)
+      # There's precedent for this in llvm-project/libcxx/cmake/caches.
+      # In a monorepo build you might do the following in the libcxxabi build:
+      #   -DLLVM_ENABLE_PROJECTS=libcxxabi;libunwinder
+      #   -DLIBCXXABI_STATICALLY_LINK_UNWINDER_IN_STATIC_LIBRARY=On
+      # libcxx appears to require unwind and doesn't pull it in via other means.
+      # "-DLIBCXX_ADDITIONAL_LIBRARIES=unwind"
+      "-DCMAKE_SHARED_LINKER_FLAGS=-lunwind"
+    ] ++ lib.optionals stdenv.hostPlatform.isWasm [
       "-DLIBCXX_ENABLE_THREADS=OFF"
       "-DLIBCXX_ENABLE_FILESYSTEM=OFF"
       "-DLIBCXX_ENABLE_EXCEPTIONS=OFF"
diff --git a/pkgs/development/compilers/llvm/git/libcxxabi/default.nix b/pkgs/development/compilers/llvm/git/libcxxabi/default.nix
index 65c585181a7028f..51020fcdb0caf05 100644
--- a/pkgs/development/compilers/llvm/git/libcxxabi/default.nix
+++ b/pkgs/development/compilers/llvm/git/libcxxabi/default.nix
@@ -68,6 +68,13 @@ stdenv.mkDerivation rec {
   ] ++ lib.optionals (stdenv.hostPlatform.useLLVM or false) [
     "-DLLVM_ENABLE_LIBCXX=ON"
     "-DLIBCXXABI_USE_LLVM_UNWINDER=ON"
+  ] ++ lib.optionals ((stdenv.hostPlatform.useLLVM or false) ||
+                      (stdenv.hostPlatform.isDarwin && enableShared)) [
+    # libcxxabi's CMake looks as though it treats -nostdlib++ as implying -nostdlib,
+    # but that does not appear to be the case for example when building
+    # pkgsLLVM.libcxxabi (which uses clangNoCompilerRtWithLibc).
+    "-DCMAKE_EXE_LINKER_FLAGS=-nostdlib"
+    "-DCMAKE_SHARED_LINKER_FLAGS=-nostdlib"
   ] ++ lib.optionals stdenv.hostPlatform.isWasm [
     "-DLIBCXXABI_ENABLE_THREADS=OFF"
     "-DLIBCXXABI_ENABLE_EXCEPTIONS=OFF"
diff --git a/pkgs/development/compilers/llvm/git/libunwind/default.nix b/pkgs/development/compilers/llvm/git/libunwind/default.nix
index dd14e2acaf86927..1b677a7a2c0d7c4 100644
--- a/pkgs/development/compilers/llvm/git/libunwind/default.nix
+++ b/pkgs/development/compilers/llvm/git/libunwind/default.nix
@@ -40,6 +40,11 @@ stdenv.mkDerivation rec {
     cd ../runtimes
   '';
 
+  postInstall = lib.optionalString (enableShared && !stdenv.hostPlatform.isDarwin) ''
+    # libcxxabi wants to link to libunwind_shared.so (?).
+    ln -s $out/lib/libunwind.so $out/lib/libunwind_shared.so
+  '';
+
   outputs = [ "out" "dev" ];
 
   nativeBuildInputs = [ cmake ninja python3 ];