diff --git a/pkgs/development/compilers/llvm/12/default.nix b/pkgs/development/compilers/llvm/12/default.nix
deleted file mode 100644
index 352cf8313d67..000000000000
--- a/pkgs/development/compilers/llvm/12/default.nix
+++ /dev/null
@@ -1,445 +0,0 @@
-{ lowPrio, newScope, pkgs, lib, stdenv
-, preLibcCrossHeaders
-, substitute, substituteAll, fetchFromGitHub, fetchpatch, fetchurl
-, overrideCC, wrapCCWith, wrapBintoolsWith
-, libxcrypt
-, buildLlvmTools # tools, but from the previous stage, for cross
-, targetLlvmLibraries # libraries, but from the next stage, for cross
-, targetLlvm
-# This is the default binutils, but with *this* version of LLD rather
-# than the default LLVM version's, if LLD is the choice. We use these for
-# the `useLLVM` bootstrapping below.
-, bootBintoolsNoLibc ?
-    if stdenv.targetPlatform.linker == "lld"
-    then null
-    else pkgs.bintoolsNoLibc
-, bootBintools ?
-    if stdenv.targetPlatform.linker == "lld"
-    then null
-    else pkgs.bintools
-, darwin
-# Allows passthrough to packages via newScope. This makes it possible to
-# do `(llvmPackages.override { <someLlvmDependency> = bar; }).clang` and get
-# an llvmPackages whose packages are overridden in an internally consistent way.
-, ...
-}@args:
-
-let
-  candidate = ""; # empty or "rcN"
-  dash-candidate = lib.optionalString (candidate != "") "-${candidate}";
-
-  metadata = rec {
-    release_version = "12.0.1";
-    version = "${release_version}${dash-candidate}"; # differentiating these (variables) is important for RCs
-    inherit (import ../common/common-let.nix { inherit lib release_version; }) llvm_meta;
-    fetch = name: sha256: fetchurl {
-      url = "https://github.com/llvm/llvm-project/releases/download/llvmorg-${metadata.version}/${name}-${metadata.release_version}${candidate}.src.tar.xz";
-      inherit sha256;
-    };
-    clang-tools-extra_src = fetch "clang-tools-extra" "1r9a4fdz9ci58b5z2inwvm4z4cdp6scrivnaw05dggkxz7yrwrb5";
-  };
-
-  inherit (metadata) fetch;
-
-
-  tools = lib.makeExtensible (tools: let
-    callPackage = newScope (tools // args // metadata);
-    mkExtraBuildCommands0 = cc: ''
-      rsrc="$out/resource-root"
-      mkdir "$rsrc"
-      ln -s "${cc.lib}/lib/clang/${metadata.release_version}/include" "$rsrc"
-      echo "-resource-dir=$rsrc" >> $out/nix-support/cc-cflags
-    '';
-    mkExtraBuildCommandsBasicRt = cc: mkExtraBuildCommands0 cc + ''
-      ln -s "${targetLlvmLibraries.compiler-rt-no-libc.out}/lib" "$rsrc/lib"
-      ln -s "${targetLlvmLibraries.compiler-rt-no-libc.out}/share" "$rsrc/share"
-    '';
-    mkExtraBuildCommands = cc: mkExtraBuildCommands0 cc + ''
-      ln -s "${targetLlvmLibraries.compiler-rt.out}/lib" "$rsrc/lib"
-      ln -s "${targetLlvmLibraries.compiler-rt.out}/share" "$rsrc/share"
-    '';
-
-  bintoolsNoLibc' =
-    if bootBintoolsNoLibc == null
-    then tools.bintoolsNoLibc
-    else bootBintoolsNoLibc;
-  bintools' =
-    if bootBintools == null
-    then tools.bintools
-    else bootBintools;
-
-  in {
-
-    libllvm = callPackage ../common/llvm {
-      src = fetch "llvm" "1pzx9zrmd7r3481sbhwvkms68fwhffpp4mmz45dgrkjpyl2q96kx";
-      polly_src = fetch "polly" "1yfm9ixda4a2sx7ak5vswijx4ydk5lv1c1xh39xmd2kh299y4m12";
-      patches = [
-        # When cross-compiling we configure llvm-config-native with an approximation
-        # of the flags used for the normal LLVM build. To avoid the need for building
-        # a native libLLVM.so (which would fail) we force llvm-config to be linked
-        # statically against the necessary LLVM components always.
-        ../common/llvm/llvm-config-link-static.patch
-        # Fix llvm being miscompiled by some gccs. See llvm/llvm-project#49955
-        # Fix llvm being miscompiled by some gccs. See https://github.com/llvm/llvm-project/issues/49955
-        ./llvm/fix-llvm-issue-49955.patch
-
-        ./llvm/gnu-install-dirs.patch
-        # On older CPUs (e.g. Hydra/wendy) we'd be getting an error in this test.
-        (fetchpatch {
-          name = "uops-CMOV16rm-noreg.diff";
-          url = "https://github.com/llvm/llvm-project/commit/9e9f991ac033.diff";
-          sha256 = "sha256:12s8vr6ibri8b48h2z38f3afhwam10arfiqfy4yg37bmc054p5hi";
-          stripLen = 1;
-        })
-
-        # Fix musl build.
-        (fetchpatch {
-          url = "https://github.com/llvm/llvm-project/commit/5cd554303ead0f8891eee3cd6d25cb07f5a7bf67.patch";
-          relative = "llvm";
-          hash = "sha256-XPbvNJ45SzjMGlNUgt/IgEvM2dHQpDOe6woUJY+nUYA=";
-        })
-
-        # Backport gcc-13 fixes with missing includes.
-        (fetchpatch {
-          name = "signals-gcc-13.patch";
-          url = "https://github.com/llvm/llvm-project/commit/ff1681ddb303223973653f7f5f3f3435b48a1983.patch";
-          hash = "sha256-CXwYxQezTq5vdmc8Yn88BUAEly6YZ5VEIA6X3y5NNOs=";
-          stripLen = 1;
-        })
-        (fetchpatch {
-          name = "base64-gcc-13.patch";
-          url = "https://github.com/llvm/llvm-project/commit/5e9be93566f39ee6cecd579401e453eccfbe81e5.patch";
-          hash = "sha256-PAwrVrvffPd7tphpwCkYiz+67szPRzRB2TXBvKfzQ7U=";
-          stripLen = 1;
-        })
-      ];
-      pollyPatches = [
-        ./llvm/gnu-install-dirs-polly.patch
-      ];
-    };
-
-    # `llvm` historically had the binaries.  When choosing an output explicitly,
-    # we need to reintroduce `outputSpecified` to get the expected behavior e.g. of lib.get*
-    llvm = tools.libllvm;
-
-    libclang = callPackage ../common/clang {
-      src = fetch "clang" "0px4gl27az6cdz6adds89qzdwb1cqpjsfvrldbz9qvpmphrj34bf";
-      patches = [
-        ./clang/purity.patch
-        # https://reviews.llvm.org/D51899
-        ./clang/gnu-install-dirs.patch
-        (substituteAll {
-          src = ../common/clang/clang-11-15-LLVMgold-path.patch;
-          libllvmLibdir = "${tools.libllvm.lib}/lib";
-        })
-      ];
-    };
-
-    clang-unwrapped = tools.libclang;
-
-    # disabled until recommonmark supports sphinx 3
-    #Llvm-manpages = lowPrio (tools.libllvm.override {
-    #  enableManpages = true;
-    #  python3 = pkgs.python3;  # don't use python-boot
-    #});
-
-    clang-manpages = lowPrio (tools.libclang.override {
-      enableManpages = true;
-      python3 = pkgs.python3;  # don't use python-boot
-    });
-
-    # disabled until recommonmark supports sphinx 3
-    # lldb-manpages = lowPrio (tools.lldb.override {
-    #   enableManpages = true;
-    #   python3 = pkgs.python3;  # don't use python-boot
-    # });
-
-    # Wrapper for standalone command line utilities
-    clang-tools = callPackage ../common/clang-tools { };
-
-    # pick clang appropriate for package set we are targeting
-    clang =
-      /**/ if stdenv.targetPlatform.libc == null then tools.clangNoLibc
-      else if stdenv.targetPlatform.useLLVM or false then tools.clangUseLLVM
-      else if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU then tools.libstdcxxClang
-      else tools.libcxxClang;
-
-    libstdcxxClang = wrapCCWith rec {
-      cc = tools.clang-unwrapped;
-      # libstdcxx is taken from gcc in an ad-hoc way in cc-wrapper.
-      libcxx = null;
-      extraPackages = [
-        targetLlvmLibraries.compiler-rt
-      ];
-      extraBuildCommands = mkExtraBuildCommands cc;
-    };
-
-    libcxxClang = wrapCCWith rec {
-      cc = tools.clang-unwrapped;
-      libcxx = targetLlvmLibraries.libcxx;
-      extraPackages = [
-        targetLlvmLibraries.compiler-rt
-      ];
-      extraBuildCommands = mkExtraBuildCommands cc;
-    };
-
-    lld = callPackage ../common/lld {
-      src = fetch "lld" "0qg3fgc7wj34hdkqn21y03zcmsdd01szhhm1hfki63iifrm3y2v9";
-      patches = [
-        ./lld/gnu-install-dirs.patch
-      ];
-      inherit (libraries) libunwind;
-    };
-
-    lldb = callPackage ../common/lldb.nix {
-      src = fetch "lldb" "0g3pj1m3chafavpr35r9fynm85y2hdyla6klj0h28khxs2613i78";
-      patches =
-        let
-          resourceDirPatch = callPackage
-            ({ substituteAll, libclang }: substituteAll
-              {
-                src = ./lldb/resource-dir.patch;
-                clangLibDir = "${lib.getLib libclang}/lib";
-              })
-            { };
-        in
-        [
-          ./lldb/procfs.patch
-          resourceDirPatch
-          ./lldb/gnu-install-dirs.patch
-        ];
-    };
-
-    # Below, is the LLVM bootstrapping logic. It handles building a
-    # fully LLVM toolchain from scratch. No GCC toolchain should be
-    # pulled in. As a consequence, it is very quick to build different
-    # targets provided by LLVM and we can also build for what GCC
-    # doesn’t support like LLVM. Probably we should move to some other
-    # file.
-
-    bintools-unwrapped = callPackage ../common/bintools.nix { };
-
-    bintoolsNoLibc = wrapBintoolsWith {
-      bintools = tools.bintools-unwrapped;
-      libc = preLibcCrossHeaders;
-    };
-
-    bintools = wrapBintoolsWith {
-      bintools = tools.bintools-unwrapped;
-    };
-
-    clangUseLLVM = wrapCCWith rec {
-      cc = tools.clang-unwrapped;
-      libcxx = targetLlvmLibraries.libcxx;
-      bintools = bintools';
-      extraPackages = [
-        targetLlvmLibraries.compiler-rt
-      ] ++ lib.optionals (!stdenv.targetPlatform.isWasm) [
-        targetLlvmLibraries.libunwind
-      ];
-      extraBuildCommands = ''
-        echo "-rtlib=compiler-rt -Wno-unused-command-line-argument" >> $out/nix-support/cc-cflags
-        echo "-B${targetLlvmLibraries.compiler-rt}/lib" >> $out/nix-support/cc-cflags
-      '' + lib.optionalString (!stdenv.targetPlatform.isWasm) ''
-        echo "--unwindlib=libunwind" >> $out/nix-support/cc-cflags
-        echo "-L${targetLlvmLibraries.libunwind}/lib" >> $out/nix-support/cc-ldflags
-      '' + lib.optionalString (!stdenv.targetPlatform.isWasm && stdenv.targetPlatform.useLLVM or false) ''
-        echo "-lunwind" >> $out/nix-support/cc-ldflags
-      '' + lib.optionalString stdenv.targetPlatform.isWasm ''
-        echo "-fno-exceptions" >> $out/nix-support/cc-cflags
-      '' + mkExtraBuildCommands cc;
-    };
-
-    clangWithLibcAndBasicRtAndLibcxx = wrapCCWith rec {
-      cc = tools.clang-unwrapped;
-      libcxx = targetLlvmLibraries.libcxx;
-      bintools = bintools';
-      extraPackages = [
-        targetLlvmLibraries.compiler-rt-no-libc
-      ] ++ lib.optionals (!stdenv.targetPlatform.isWasm) [
-        targetLlvmLibraries.libunwind
-      ];
-      extraBuildCommands = ''
-        echo "-rtlib=compiler-rt" >> $out/nix-support/cc-cflags
-        echo "-Wno-unused-command-line-argument" >> $out/nix-support/cc-cflags
-        echo "-B${targetLlvmLibraries.compiler-rt-no-libc}/lib" >> $out/nix-support/cc-cflags
-      '' + mkExtraBuildCommandsBasicRt cc;
-    };
-
-    clangWithLibcAndBasicRt = wrapCCWith rec {
-      cc = tools.clang-unwrapped;
-      libcxx = null;
-      bintools = bintools';
-      extraPackages = [
-        targetLlvmLibraries.compiler-rt-no-libc
-      ];
-      extraBuildCommands = ''
-        echo "-rtlib=compiler-rt" >> $out/nix-support/cc-cflags
-        echo "-B${targetLlvmLibraries.compiler-rt-no-libc}/lib" >> $out/nix-support/cc-cflags
-        echo "-nostdlib++" >> $out/nix-support/cc-cflags
-      '' + mkExtraBuildCommandsBasicRt cc;
-    };
-
-    clangNoLibcWithBasicRt = wrapCCWith rec {
-      cc = tools.clang-unwrapped;
-      libcxx = null;
-      bintools = bintoolsNoLibc';
-      extraPackages = [
-        targetLlvmLibraries.compiler-rt-no-libc
-      ];
-      extraBuildCommands = ''
-        echo "-rtlib=compiler-rt" >> $out/nix-support/cc-cflags
-        echo "-B${targetLlvmLibraries.compiler-rt-no-libc}/lib" >> $out/nix-support/cc-cflags
-      '' + mkExtraBuildCommandsBasicRt cc;
-    };
-
-    clangNoLibcNoRt = wrapCCWith rec {
-      cc = tools.clang-unwrapped;
-      libcxx = null;
-      bintools = bintoolsNoLibc';
-      extraPackages = [ ];
-      extraBuildCommands = ''
-        echo "-nostartfiles" >> $out/nix-support/cc-cflags
-      '' + mkExtraBuildCommands0 cc;
-    };
-
-    # This is an "oddly ordered" bootstrap just for Darwin. Probably
-    # don't want it otherwise.
-    clangNoCompilerRtWithLibc = wrapCCWith rec {
-      cc = tools.clang-unwrapped;
-      libcxx = null;
-      bintools = bintools';
-      extraPackages = [ ];
-      extraBuildCommands = mkExtraBuildCommands0 cc;
-    };
-
-    # Aliases
-    clangNoCompilerRt = tools.clangNoLibcNoRt;
-    clangNoLibc = tools.clangNoLibcWithBasicRt;
-    clangNoLibcxx = tools.clangWithLibcAndBasicRt;
-  });
-
-  libraries = lib.makeExtensible (libraries: let
-    callPackage = newScope (libraries // buildLlvmTools // args // metadata);
-  in {
-
-    compiler-rt-libc = callPackage ../common/compiler-rt (let
-      stdenv =
-        if args.stdenv.hostPlatform.useLLVM or false then
-          overrideCC args.stdenv buildLlvmTools.clangWithLibcAndBasicRtAndLibcxx
-        else
-          args.stdenv;
-    in {
-      src = fetch "compiler-rt" "1950rg294izdwkaasi7yjrmadc9mzdd5paf0q63jjcq2m3rdbj5l";
-      patches = [
-        ../common/compiler-rt/7-12-codesign.patch # Revert compiler-rt commit that makes codesign mandatory
-        ./compiler-rt/X86-support-extension.patch # Add support for i486 i586 i686 by reusing i386 config
-        ./compiler-rt/gnu-install-dirs.patch
-        # ld-wrapper dislikes `-rpath-link //nix/store`, so we normalize away the
-        # extra `/`.
-        ./compiler-rt/normalize-var.patch
-        ../common/compiler-rt/darwin-plistbuddy-workaround.patch
-        ./compiler-rt/armv7l.patch
-        # Fix build on armv6l
-        ../common/compiler-rt/armv6-mcr-dmb.patch
-        ../common/compiler-rt/armv6-sync-ops-no-thumb.patch
-        ../common/compiler-rt/armv6-no-ldrexd-strexd.patch
-      ];
-      inherit stdenv;
-    } // lib.optionalAttrs (stdenv.hostPlatform.useLLVM or false) {
-      libxcrypt = (libxcrypt.override { inherit stdenv; }).overrideAttrs (old: {
-        configureFlags = old.configureFlags ++ [ "--disable-symvers" ];
-      });
-    });
-
-    compiler-rt-no-libc = callPackage ../common/compiler-rt {
-      src = fetch "compiler-rt" "1950rg294izdwkaasi7yjrmadc9mzdd5paf0q63jjcq2m3rdbj5l";
-      patches = [
-        ../common/compiler-rt/7-12-codesign.patch # Revert compiler-rt commit that makes codesign mandatory
-        ./compiler-rt/X86-support-extension.patch # Add support for i486 i586 i686 by reusing i386 config
-        ./compiler-rt/gnu-install-dirs.patch
-        # ld-wrapper dislikes `-rpath-link //nix/store`, so we normalize away the
-        # extra `/`.
-        ./compiler-rt/normalize-var.patch
-        ../common/compiler-rt/darwin-plistbuddy-workaround.patch
-        ./compiler-rt/armv7l.patch
-        # Fix build on armv6l
-        ../common/compiler-rt/armv6-mcr-dmb.patch
-        ../common/compiler-rt/armv6-sync-ops-no-thumb.patch
-        ../common/compiler-rt/armv6-no-ldrexd-strexd.patch
-      ];
-      stdenv =
-        if stdenv.hostPlatform.isDarwin && stdenv.hostPlatform == stdenv.buildPlatform then
-          stdenv
-        else
-          # TODO: make this branch unconditional next rebuild
-          overrideCC stdenv buildLlvmTools.clangNoLibcNoRt;
-    };
-
-    compiler-rt =
-      # Building the with-libc compiler-rt and WASM doesn't yet work,
-      # because wasilibc doesn't provide some expected things. See
-      # compiler-rt's file for further details.
-      if stdenv.hostPlatform.libc == null || stdenv.hostPlatform.isWasm then
-        libraries.compiler-rt-no-libc
-      else
-        libraries.compiler-rt-libc;
-
-    stdenv = overrideCC stdenv buildLlvmTools.clang;
-
-    libcxxStdenv = overrideCC stdenv buildLlvmTools.libcxxClang;
-
-    libcxx = callPackage ../common/libcxx {
-      src = fetchFromGitHub {
-        owner = "llvm";
-        repo = "llvm-project";
-        rev = "refs/tags/llvmorg-${metadata.version}";
-        sparseCheckout = [
-          "libcxx"
-          "libcxxabi"
-          "llvm/cmake"
-          "llvm/utils"
-          "runtimes"
-        ];
-        hash = "sha256-etxgXIdWxMTmbZ83Hsc0w6Jt5OSQSUEPVEWqLkHsNBY=";
-      };
-      patches = [
-        (substitute {
-          src = ../common/libcxxabi/wasm.patch;
-          substitutions = [
-            "--replace-fail" "/cmake/" "/llvm/cmake/"
-          ];
-        })
-      ] ++ lib.optionals stdenv.hostPlatform.isMusl [
-        (substitute {
-          src = ../common/libcxx/libcxx-0001-musl-hacks.patch;
-          substitutions = [
-            "--replace-fail" "/include/" "/libcxx/include/"
-          ];
-        })
-      ];
-      stdenv = overrideCC stdenv buildLlvmTools.clangWithLibcAndBasicRt;
-    };
-
-    libunwind = callPackage ../common/libunwind {
-      src = fetch "libunwind" "192ww6n81lj2mb9pj4043z79jp3cf58a9c2qrxjwm5c3a64n1shb";
-      patches = [
-        ./libunwind/gnu-install-dirs.patch
-      ];
-      stdenv = overrideCC stdenv buildLlvmTools.clangWithLibcAndBasicRt;
-    };
-
-    openmp = callPackage ../common/openmp {
-      src = fetch "openmp" "14dh0r6h2xh747ffgnsl4z08h0ri04azi9vf79cbz7ma1r27kzk0";
-      patches = [
-        # Fix cross.
-        (fetchpatch {
-          url = "https://github.com/llvm/llvm-project/commit/5e2358c781b85a18d1463fd924d2741d4ae5e42e.patch";
-          hash = "sha256-UxIlAifXnexF/MaraPW0Ut6q+sf3e7y1fMdEv1q103A=";
-        })
-      ];
-    };
-  });
-  noExtend = extensible: lib.attrsets.removeAttrs extensible [ "extend" ];
-
-in { inherit tools libraries; inherit (metadata) release_version; } // (noExtend libraries) // (noExtend tools)
diff --git a/pkgs/development/compilers/llvm/common/clang/default.nix b/pkgs/development/compilers/llvm/common/clang/default.nix
index 88f5dfd84d6a..f842e11530e2 100644
--- a/pkgs/development/compilers/llvm/common/clang/default.nix
+++ b/pkgs/development/compilers/llvm/common/clang/default.nix
@@ -16,7 +16,6 @@
 , buildLlvmTools
 , fixDarwinDylibNames
 , enableManpages ? false
-, clang-tools-extra_src ? null
 , devExtraCmakeFlags ? []
 }:
 
@@ -38,8 +37,7 @@ let
 
     src = src';
 
-    sourceRoot = if lib.versionOlder release_version "13" then null
-      else "${src.name}/${pname}";
+    sourceRoot = "${src.name}/${pname}";
 
     nativeBuildInputs = [ cmake ]
       ++ (lib.optional (lib.versionAtLeast release_version "15") ninja)
@@ -79,17 +77,22 @@ let
       # Make sure clang passes the correct location of libLTO to ld64
       substituteInPlace lib/Driver/ToolChains/Darwin.cpp \
         --replace-fail 'StringRef P = llvm::sys::path::parent_path(D.Dir);' 'StringRef P = "${lib.getLib libllvm}";'
-    '' + (
+      (cd tools && ln -s ../../clang-tools-extra extra)
+    '' + lib.optionalString (
+      lib.versionOlder release_version "13"
       # See the comment on the `add-nostdlibinc-flag.patch` patch in
       # `../default.nix` for why we skip Darwin here.
-      if lib.versionOlder release_version "13" && (!stdenv.hostPlatform.isDarwin || !stdenv.targetPlatform.isDarwin) then ''
-        sed -i -e 's/DriverArgs.hasArg(options::OPT_nostdlibinc)/true/' \
-               -e 's/Args.hasArg(options::OPT_nostdlibinc)/true/' \
-               lib/Driver/ToolChains/*.cpp
-      '' else ''
-        (cd tools && ln -s ../../clang-tools-extra extra)
-      ''
-    ) + lib.optionalString stdenv.hostPlatform.isMusl ''
+      && (!stdenv.hostPlatform.isDarwin || !stdenv.targetPlatform.isDarwin
+    ) ''
+      sed -i -e 's/DriverArgs.hasArg(options::OPT_nostdlibinc)/true/' \
+             -e 's/Args.hasArg(options::OPT_nostdlibinc)/true/' \
+             lib/Driver/ToolChains/*.cpp
+    ''
+    + lib.optionalString (lib.versionOlder release_version "13") ''
+      substituteInPlace tools/extra/clangd/quality/CompletionModel.cmake \
+        --replace ' ''${CMAKE_SOURCE_DIR}/../clang-tools-extra' ' ''${CMAKE_SOURCE_DIR}/tools/extra'
+    ''
+    + lib.optionalString stdenv.hostPlatform.isMusl ''
       sed -i -e 's/lgcc_s/lgcc_eh/' lib/Driver/ToolChains/*.cpp
     '';
 
@@ -215,18 +218,7 @@ let
     '';
   } else {
     ninjaFlags = [ "docs-clang-man" ];
-  })) // (lib.optionalAttrs (clang-tools-extra_src != null) { inherit clang-tools-extra_src; })
-    // (lib.optionalAttrs (lib.versionOlder release_version "13") {
-      unpackPhase = ''
-        unpackFile $src
-        mv clang-* clang
-        sourceRoot=$PWD/clang
-        unpackFile ${clang-tools-extra_src}
-        mv clang-tools-extra-* $sourceRoot/tools/extra
-        substituteInPlace $sourceRoot/tools/extra/clangd/quality/CompletionModel.cmake \
-          --replace ' ''${CMAKE_SOURCE_DIR}/../clang-tools-extra' ' ''${CMAKE_SOURCE_DIR}/tools/extra'
-      '';
-    })
+  }))
   // (lib.optionalAttrs (lib.versionAtLeast release_version "15") {
     env = lib.optionalAttrs (stdenv.buildPlatform != stdenv.hostPlatform && !stdenv.hostPlatform.useLLVM) {
       # The following warning is triggered with (at least) gcc >=
diff --git a/pkgs/development/compilers/llvm/common/compiler-rt/default.nix b/pkgs/development/compilers/llvm/common/compiler-rt/default.nix
index 27e5254d1728..f859a415b5db 100644
--- a/pkgs/development/compilers/llvm/common/compiler-rt/default.nix
+++ b/pkgs/development/compilers/llvm/common/compiler-rt/default.nix
@@ -73,8 +73,7 @@ stdenv.mkDerivation ({
   inherit pname version patches;
 
   src = src';
-  sourceRoot = if lib.versionOlder release_version "13" then null
-    else "${src'.name}/${baseName}";
+  sourceRoot = "${src'.name}/${baseName}";
 
   nativeBuildInputs = [ cmake ]
     ++ (lib.optional (lib.versionAtLeast release_version "15") ninja)
diff --git a/pkgs/development/compilers/llvm/common/default.nix b/pkgs/development/compilers/llvm/common/default.nix
index 9d74c65eaaf1..5264dd05f012 100644
--- a/pkgs/development/compilers/llvm/common/default.nix
+++ b/pkgs/development/compilers/llvm/common/default.nix
@@ -380,6 +380,17 @@ let
       libllvm = callPackage ./llvm {
         patches =
           lib.optional (lib.versionOlder metadata.release_version "14") ./llvm/llvm-config-link-static.patch
+          ++ lib.optionals (lib.versions.major metadata.release_version == "12") [
+            (metadata.getVersionFile "llvm/fix-llvm-issue-49955.patch")
+
+            # On older CPUs (e.g. Hydra/wendy) we'd be getting an error in this test.
+            (fetchpatch {
+              name = "uops-CMOV16rm-noreg.diff";
+              url = "https://github.com/llvm/llvm-project/commit/9e9f991ac033.diff";
+              sha256 = "sha256:12s8vr6ibri8b48h2z38f3afhwam10arfiqfy4yg37bmc054p5hi";
+              stripLen = 1;
+            })
+          ]
           ++ [ (metadata.getVersionFile "llvm/gnu-install-dirs.patch") ]
           ++ lib.optionals (lib.versionAtLeast metadata.release_version "15") [
             # Running the tests involves invoking binaries (like `opt`) that depend on
@@ -443,7 +454,7 @@ let
                   hash = "sha256-XPbvNJ45SzjMGlNUgt/IgEvM2dHQpDOe6woUJY+nUYA=";
                 }
               )
-          ++ lib.optionals (lib.versions.major metadata.release_version == "13") [
+          ++ lib.optionals (lib.versionOlder metadata.release_version "14") [
             # Backport gcc-13 fixes with missing includes.
             (fetchpatch {
               name = "signals-gcc-13.patch";
@@ -544,7 +555,9 @@ let
           # compilers breaking libclang when we can do Linux‐to‐Darwin
           # cross‐compilation again.
           ++ lib.optional (
-            !args.stdenv.hostPlatform.isDarwin || !args.stdenv.targetPlatform.isDarwin
+            # TODO: This also applies for clang == 12, do we need it?
+            lib.versionAtLeast metadata.release_version "13" &&
+            (!args.stdenv.hostPlatform.isDarwin || !args.stdenv.targetPlatform.isDarwin)
           ) ./clang/add-nostdlibinc-flag.patch
           ++ [
             (substituteAll {
@@ -989,14 +1002,21 @@ let
       );
 
       compiler-rtPatches =
-        lib.optional (lib.versionOlder metadata.release_version "15") (
-          metadata.getVersionFile "compiler-rt/codesign.patch"
-        ) # Revert compiler-rt commit that makes codesign mandatory
+        lib.optionals (lib.versions.major metadata.release_version == "12") [
+          # Revert compiler-rt commit that makes codesign mandatory
+          ./compiler-rt/7-12-codesign.patch
+        ]
+        ++ lib.optional (
+          lib.versionAtLeast metadata.release_version "13" && lib.versionOlder metadata.release_version "15"
+        ) (metadata.getVersionFile "compiler-rt/codesign.patch") # Revert compiler-rt commit that makes codesign mandatory
         ++ [
           (metadata.getVersionFile "compiler-rt/X86-support-extension.patch") # Add support for i486 i586 i686 by reusing i386 config
         ]
         ++ lib.optional (
-          lib.versionAtLeast metadata.release_version "14" && lib.versionOlder metadata.release_version "18"
+          lib.versions.major metadata.release_version == "12"
+          || (
+            lib.versionAtLeast metadata.release_version "14" && lib.versionOlder metadata.release_version "18"
+          )
         ) (metadata.getVersionFile "compiler-rt/gnu-install-dirs.patch")
         ++ [
           # ld-wrapper dislikes `-rpath-link //nix/store`, so we normalize away the
@@ -1004,9 +1024,11 @@ let
           (metadata.getVersionFile "compiler-rt/normalize-var.patch")
         ]
         ++
-          lib.optional (lib.versionOlder metadata.release_version "18")
+          lib.optional
+            (lib.versionAtLeast metadata.release_version "13" && lib.versionOlder metadata.release_version "18")
             # Prevent a compilation error on darwin
             (metadata.getVersionFile "compiler-rt/darwin-targetconditionals.patch")
+        # TODO: make unconditional and remove in <15 section below. Causes rebuilds.
         ++ lib.optionals (lib.versionAtLeast metadata.release_version "15") [
           # See: https://github.com/NixOS/nixpkgs/pull/186575
           ./compiler-rt/darwin-plistbuddy-workaround.patch
@@ -1022,13 +1044,18 @@ let
           ./compiler-rt/armv6-mcr-dmb.patch
           ./compiler-rt/armv6-sync-ops-no-thumb.patch
         ]
-        ++ lib.optionals (lib.versionOlder metadata.release_version "18") [
-          # Fix build on armv6l
-          ./compiler-rt/armv6-scudo-no-yield.patch
-        ]
+        ++
+          lib.optionals
+            (lib.versionAtLeast metadata.release_version "13" && lib.versionOlder metadata.release_version "18")
+            [
+              # Fix build on armv6l
+              ./compiler-rt/armv6-scudo-no-yield.patch
+            ]
         ++ [
           # Fix build on armv6l
           ./compiler-rt/armv6-no-ldrexd-strexd.patch
+        ]
+        ++ lib.optionals (lib.versionAtLeast metadata.release_version "13") [
           (metadata.getVersionFile "compiler-rt/armv6-scudo-libatomic.patch")
         ]
         ++ lib.optional (lib.versionAtLeast metadata.release_version "19") (fetchpatch {
diff --git a/pkgs/development/compilers/llvm/common/libcxx/default.nix b/pkgs/development/compilers/llvm/common/libcxx/default.nix
index 453013cdbd27..fe9ae5d0d502 100644
--- a/pkgs/development/compilers/llvm/common/libcxx/default.nix
+++ b/pkgs/development/compilers/llvm/common/libcxx/default.nix
@@ -105,9 +105,13 @@ let
 
   cmakeFlags = [
     "-DLLVM_ENABLE_RUNTIMES=${lib.concatStringsSep ";" runtimes}"
+  ] ++ lib.optionals (
+    stdenv.hostPlatform.isWasm
+    || (lib.versions.major release_version == "12" && stdenv.hostPlatform.isDarwin)
+  ) [
+    "-DCMAKE_CXX_COMPILER_WORKS=ON"
   ] ++ lib.optionals stdenv.hostPlatform.isWasm [
     "-DCMAKE_C_COMPILER_WORKS=ON"
-    "-DCMAKE_CXX_COMPILER_WORKS=ON"
     "-DUNIX=ON" # Required otherwise libc++ fails to detect the correct linker
   ] ++ cxxCMakeFlags
     ++ lib.optionals (cxxabi == null) cxxabiCMakeFlags
diff --git a/pkgs/development/compilers/llvm/common/libunwind/default.nix b/pkgs/development/compilers/llvm/common/libunwind/default.nix
index 63b9737092b8..4f1fa0107c14 100644
--- a/pkgs/development/compilers/llvm/common/libunwind/default.nix
+++ b/pkgs/development/compilers/llvm/common/libunwind/default.nix
@@ -35,11 +35,6 @@ let
 
   hasPatches = builtins.length patches > 0;
 
-  postUnpack = lib.optionalString (lib.versions.major release_version == "12") ''
-    ln -s ${libcxx.src}/libcxx .
-    ln -s ${libcxx.src}/llvm .
-  '';
-
   prePatch = lib.optionalString (lib.versionAtLeast release_version "15" && (hasPatches || lib.versionOlder release_version "18")) ''
     cd ../${pname}
     chmod -R u+w .
@@ -60,8 +55,8 @@ stdenv.mkDerivation (rec {
   src = src';
 
   sourceRoot =
-    if lib.versionOlder release_version "13" then null
-    else if lib.versionAtLeast release_version "15" then "${src.name}/runtimes"
+    if lib.versionAtLeast release_version "15"
+    then "${src.name}/runtimes"
     else "${src.name}/${pname}";
 
   outputs = [ "out" "dev" ];
@@ -72,6 +67,9 @@ stdenv.mkDerivation (rec {
 
   cmakeFlags = lib.optional (lib.versionAtLeast release_version "15") "-DLLVM_ENABLE_RUNTIMES=libunwind"
     ++ lib.optional (!enableShared) "-DLIBUNWIND_ENABLE_SHARED=OFF"
+    ++ lib.optionals (lib.versions.major release_version == "12" && stdenv.hostPlatform.isDarwin) [
+      "-DCMAKE_CXX_COMPILER_WORKS=ON"
+    ]
     ++ devExtraCmakeFlags;
 
   meta = llvm_meta // {
@@ -85,7 +83,6 @@ stdenv.mkDerivation (rec {
       dependency of other runtimes.
     '';
   };
-} // (if postUnpack != "" then { inherit postUnpack; } else {})
-  // (if (lib.versionAtLeast release_version "15") then { inherit postInstall; } else {})
+} // (if (lib.versionAtLeast release_version "15") then { inherit postInstall; } else {})
   // (if prePatch != "" then { inherit prePatch; } else {})
   // (if postPatch != "" then { inherit postPatch; } else {}))
diff --git a/pkgs/development/compilers/llvm/common/lld/default.nix b/pkgs/development/compilers/llvm/common/lld/default.nix
index 32cf5cb46460..4008f2076d73 100644
--- a/pkgs/development/compilers/llvm/common/lld/default.nix
+++ b/pkgs/development/compilers/llvm/common/lld/default.nix
@@ -6,7 +6,6 @@
 , buildLlvmTools
 , monorepoSrc ? null
 , src ? null
-, libunwind ? null
 , runCommand
 , cmake
 , ninja
@@ -30,12 +29,7 @@ let
         mkdir -p "$out/llvm"
       '') else src;
 
-  postPatch = lib.optionalString (lib.versions.major release_version == "12") ''
-    substituteInPlace MachO/CMakeLists.txt --replace \
-      '(''${LLVM_MAIN_SRC_DIR}/' '('
-    mkdir -p libunwind/include
-    tar -xf "${libunwind.src}" --wildcards -C libunwind/include --strip-components=2 "libunwind-*/include/"
-  '' + lib.optionalString (lib.versions.major release_version == "13") ''
+  postPatch = lib.optionalString (lib.versionOlder release_version "14") ''
     substituteInPlace MachO/CMakeLists.txt --replace \
       '(''${LLVM_MAIN_SRC_DIR}/' '(../'
   '';
@@ -45,9 +39,7 @@ stdenv.mkDerivation (rec {
 
   src = src';
 
-  sourceRoot =
-    if lib.versionOlder release_version "13" then null
-    else "${src.name}/${pname}";
+  sourceRoot = "${src.name}/${pname}";
 
   nativeBuildInputs = [ cmake ] ++ lib.optional (lib.versionAtLeast release_version "15") ninja;
   buildInputs = [ libllvm libxml2 ];
diff --git a/pkgs/development/compilers/llvm/common/llvm/default.nix b/pkgs/development/compilers/llvm/common/llvm/default.nix
index bd384108408c..be106b3db295 100644
--- a/pkgs/development/compilers/llvm/common/llvm/default.nix
+++ b/pkgs/development/compilers/llvm/common/llvm/default.nix
@@ -4,7 +4,6 @@
 , pkgsBuildBuild
 , pollyPatches ? []
 , patches ? []
-, polly_src ? null
 , src ? null
 , monorepoSrc ? null
 , runCommand
@@ -107,8 +106,7 @@ stdenv.mkDerivation (finalAttrs: {
   src = src';
   patches = patches';
 
-  sourceRoot = if lib.versionOlder release_version "13" then null
-    else "${finalAttrs.src.name}/${pname}";
+  sourceRoot = "${finalAttrs.src.name}/${pname}";
 
   outputs = [ "out" "lib" "dev" "python" ];
 
@@ -493,18 +491,7 @@ stdenv.mkDerivation (finalAttrs: {
 
   postPatch = null;
   postInstall = null;
-})) // lib.optionalAttrs (lib.versionOlder release_version "13") {
-  inherit polly_src;
-
-  unpackPhase = ''
-    unpackFile $src
-    mv llvm-${release_version}* llvm
-    sourceRoot=$PWD/llvm
-  '' + optionalString enablePolly ''
-    unpackFile $polly_src
-    mv polly-* $sourceRoot/tools/polly
-  '';
-} // lib.optionalAttrs (lib.versionAtLeast release_version "13") {
+})) // lib.optionalAttrs (lib.versionAtLeast release_version "13") {
   nativeCheckInputs = [ which ] ++ lib.optional (stdenv.hostPlatform.isDarwin && lib.versionAtLeast release_version "15") sysctl;
 } // lib.optionalAttrs (lib.versionOlder release_version "15") {
   # hacky fix: created binaries need to be run before installation
diff --git a/pkgs/development/compilers/llvm/common/openmp/default.nix b/pkgs/development/compilers/llvm/common/openmp/default.nix
index 1ad5948edb86..1fe089938f03 100644
--- a/pkgs/development/compilers/llvm/common/openmp/default.nix
+++ b/pkgs/development/compilers/llvm/common/openmp/default.nix
@@ -34,9 +34,7 @@ stdenv.mkDerivation (rec {
 
   src = src';
 
-  sourceRoot =
-    if lib.versionOlder release_version "13" then null
-    else "${src.name}/${pname}";
+  sourceRoot = "${src.name}/${pname}";
 
   outputs = [ "out" ]
     ++ lib.optionals (lib.versionAtLeast release_version "14") [ "dev" ];
diff --git a/pkgs/development/compilers/llvm/default.nix b/pkgs/development/compilers/llvm/default.nix
index 390a1b43eca6..5971dd73b4d3 100644
--- a/pkgs/development/compilers/llvm/default.nix
+++ b/pkgs/development/compilers/llvm/default.nix
@@ -22,6 +22,7 @@
 }@packageSetArgs:
 let
   versions = {
+    "12.0.1".officialRelease.sha256 = "08s5w2db9imb2yaqsvxs6pg21csi1cf6wa35rf8x6q07mam7j8qv";
     "13.0.1".officialRelease.sha256 = "06dv6h5dmvzdxbif2s8njki6h32796v368dyb5945x8gjj72xh7k";
     "14.0.6".officialRelease.sha256 = "sha256-vffu4HilvYwtzwgq+NlS26m65DGbp6OSSne2aje1yJE=";
     "15.0.7".officialRelease.sha256 = "sha256-wjuZQyXQ/jsmvy6y1aksCcEDXGBjuhpgngF3XQJ/T4s=";
diff --git a/pkgs/top-level/all-packages.nix b/pkgs/top-level/all-packages.nix
index e6d18c1a3849..ad8654674b61 100644
--- a/pkgs/top-level/all-packages.nix
+++ b/pkgs/top-level/all-packages.nix
@@ -6685,16 +6685,10 @@ with pkgs;
       stdenv.targetPlatform));
   in pkgs.${"llvmPackages_${minSupported}"};
 
-  llvmPackages_12 = recurseIntoAttrs (callPackage ../development/compilers/llvm/12 {
-    inherit (stdenvAdapters) overrideCC;
-    buildLlvmTools = buildPackages.llvmPackages_12.tools;
-    targetLlvmLibraries = targetPackages.llvmPackages_12.libraries or llvmPackages_12.libraries;
-    targetLlvm = targetPackages.llvmPackages_12.llvm or llvmPackages_12.llvm;
-  });
-
   inherit (rec {
     llvmPackagesSet = recurseIntoAttrs (callPackages ../development/compilers/llvm { });
 
+    llvmPackages_12 = llvmPackagesSet."12";
     llvmPackages_13 = llvmPackagesSet."13";
     llvmPackages_14 = llvmPackagesSet."14";
     llvmPackages_15 = llvmPackagesSet."15";
@@ -6713,7 +6707,8 @@ with pkgs;
     lldb_19 = llvmPackages_19.lldb;
     llvm_19 = llvmPackages_19.llvm;
     bolt_19 = llvmPackages_19.bolt;
-  }) llvmPackages_13
+  }) llvmPackages_12
+    llvmPackages_13
     llvmPackages_14
     llvmPackages_15
     llvmPackages_16