diff --git a/clang/lib/Basic/Targets.cpp b/clang/lib/Basic/Targets.cpp
index ba91d0439..42265d547 100644
--- a/clang/lib/Basic/Targets.cpp
+++ b/clang/lib/Basic/Targets.cpp
@@ -149,6 +149,8 @@ TargetInfo *AllocateTarget(const llvm::Triple &Triple,
       return new NetBSDTargetInfo<AArch64leTargetInfo>(Triple, Opts);
     case llvm::Triple::OpenBSD:
       return new OpenBSDTargetInfo<AArch64leTargetInfo>(Triple, Opts);
+    case llvm::Triple::Serenity:
+      return new SerenityTargetInfo<AArch64leTargetInfo>(Triple, Opts);
     case llvm::Triple::Win32:
       switch (Triple.getEnvironment()) {
       case llvm::Triple::GNU:
@@ -538,6 +540,8 @@ TargetInfo *AllocateTarget(const llvm::Triple &Triple,
       return new MCUX86_32TargetInfo(Triple, Opts);
     case llvm::Triple::Hurd:
       return new HurdTargetInfo<X86_32TargetInfo>(Triple, Opts);
+    case llvm::Triple::Serenity:
+      return new SerenityTargetInfo<X86_32TargetInfo>(Triple, Opts);
     default:
       return new X86_32TargetInfo(Triple, Opts);
     }
@@ -590,6 +594,8 @@ TargetInfo *AllocateTarget(const llvm::Triple &Triple,
       return new NaClTargetInfo<X86_64TargetInfo>(Triple, Opts);
     case llvm::Triple::PS4:
       return new PS4OSTargetInfo<X86_64TargetInfo>(Triple, Opts);
+    case llvm::Triple::Serenity:
+      return new SerenityTargetInfo<X86_64TargetInfo>(Triple, Opts);
     default:
       return new X86_64TargetInfo(Triple, Opts);
     }
diff --git a/clang/lib/Basic/Targets/OSTargets.h b/clang/lib/Basic/Targets/OSTargets.h
index 3fe39ed64..51e7a6cca 100644
--- a/clang/lib/Basic/Targets/OSTargets.h
+++ b/clang/lib/Basic/Targets/OSTargets.h
@@ -966,6 +966,22 @@ public:
   }
 };
 
+// SerenityOS target
+template <typename Target>
+class LLVM_LIBRARY_VISIBILITY SerenityTargetInfo : public OSTargetInfo<Target> {
+protected:
+  void getOSDefines(const LangOptions &Opts, const llvm::Triple &Triple,
+                    MacroBuilder &Builder) const override {
+    Builder.defineMacro("__serenity__");
+    DefineStd(Builder, "unix", Opts);
+    Builder.defineMacro("__ELF__");
+  }
+
+public:
+  SerenityTargetInfo(const llvm::Triple &Triple, const TargetOptions &Opts)
+      : OSTargetInfo<Target>(Triple, Opts) {}
+};
+
 } // namespace targets
 } // namespace clang
 #endif // LLVM_CLANG_LIB_BASIC_TARGETS_OSTARGETS_H
diff --git a/clang/lib/Driver/CMakeLists.txt b/clang/lib/Driver/CMakeLists.txt
index 08be9f011..69038ff00 100644
--- a/clang/lib/Driver/CMakeLists.txt
+++ b/clang/lib/Driver/CMakeLists.txt
@@ -67,6 +67,7 @@ add_clang_library(clangDriver
   ToolChains/OpenBSD.cpp
   ToolChains/PS4CPU.cpp
   ToolChains/RISCVToolchain.cpp
+  ToolChains/Serenity.cpp
   ToolChains/Solaris.cpp
   ToolChains/TCE.cpp
   ToolChains/VEToolchain.cpp
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index 94a7553e2..c6b3210f6 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -41,6 +41,7 @@
 #include "ToolChains/PPCLinux.h"
 #include "ToolChains/PS4CPU.h"
 #include "ToolChains/RISCVToolchain.h"
+#include "ToolChains/Serenity.h"
 #include "ToolChains/Solaris.h"
 #include "ToolChains/TCE.h"
 #include "ToolChains/VEToolchain.h"
@@ -5299,6 +5300,9 @@ const ToolChain &Driver::getToolChain(const ArgList &Args,
     case llvm::Triple::Fuchsia:
       TC = std::make_unique<toolchains::Fuchsia>(*this, Target, Args);
       break;
+    case llvm::Triple::Serenity:
+      TC = std::make_unique<toolchains::Serenity>(*this, Target, Args);
+      break;
     case llvm::Triple::Solaris:
       TC = std::make_unique<toolchains::Solaris>(*this, Target, Args);
       break;
diff --git a/clang/lib/Driver/ToolChain.cpp b/clang/lib/Driver/ToolChain.cpp
index 6c1b88141..c77ff78be 100644
--- a/clang/lib/Driver/ToolChain.cpp
+++ b/clang/lib/Driver/ToolChain.cpp
@@ -404,6 +404,8 @@ StringRef ToolChain::getOSLibName() const {
     return "sunos";
   case llvm::Triple::AIX:
     return "aix";
+  case llvm::Triple::Serenity:
+    return "serenity";
   default:
     return getOS();
   }
diff --git a/clang/lib/Driver/ToolChains/Arch/X86.cpp b/clang/lib/Driver/ToolChains/Arch/X86.cpp
index 12749c7ec..91f744a26 100644
--- a/clang/lib/Driver/ToolChains/Arch/X86.cpp
+++ b/clang/lib/Driver/ToolChains/Arch/X86.cpp
@@ -100,6 +100,7 @@ std::string x86::getX86TargetCPU(const ArgList &Args,
   case llvm::Triple::OpenBSD:
     return "i586";
   case llvm::Triple::FreeBSD:
+  case llvm::Triple::Serenity:
     return "i686";
   default:
     // Fallback to p4.
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 58ae08a38..8e9a3fee6 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -5695,7 +5695,13 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
                            options::OPT_fno_visibility_inlines_hidden_static_local_var);
   Args.AddLastArg(CmdArgs, options::OPT_fvisibility_global_new_delete_hidden);
 
-  Args.AddLastArg(CmdArgs, options::OPT_ftlsmodel_EQ);
+  if (Triple.isOSSerenity()) {
+    StringRef tls_model =
+        Args.getLastArgValue(options::OPT_ftlsmodel_EQ, "initial-exec");
+    CmdArgs.push_back(Args.MakeArgString("-ftls-model=" + tls_model));
+  } else {
+    Args.AddLastArg(CmdArgs, options::OPT_ftlsmodel_EQ);
+  }
 
   if (Args.hasFlag(options::OPT_fno_operator_names,
                    options::OPT_foperator_names, false))
diff --git a/clang/lib/Driver/ToolChains/Serenity.cpp b/clang/lib/Driver/ToolChains/Serenity.cpp
new file mode 100644
index 000000000..8f3e125dd
--- /dev/null
+++ b/clang/lib/Driver/ToolChains/Serenity.cpp
@@ -0,0 +1,275 @@
+//===---- Serenity.cpp - SerenityOS ToolChain Implementation ----*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Serenity.h"
+#include "CommonArgs.h"
+#include "clang/Driver/Compilation.h"
+#include "clang/Driver/Driver.h"
+#include "clang/Driver/DriverDiagnostic.h"
+#include "clang/Driver/Options.h"
+#include "clang/Driver/SanitizerArgs.h"
+#include "llvm/Option/ArgList.h"
+#include "llvm/Support/VirtualFileSystem.h"
+
+using namespace clang::driver;
+using namespace clang::driver::toolchains;
+using namespace clang;
+using namespace llvm::opt;
+
+static bool getPIE(const ArgList &Args, const ToolChain &TC) {
+  if (Args.hasArg(options::OPT_static, options::OPT_shared,
+                  options::OPT_static_pie))
+    return false;
+  Arg *Last = Args.getLastArg(options::OPT_pie, options::OPT_no_pie,
+                              options::OPT_nopie);
+  return Last ? Last->getOption().matches(options::OPT_pie) : TC.isPIEDefault();
+}
+
+void tools::serenity::Linker::ConstructJob(Compilation &C, const JobAction &JA,
+                                           const InputInfo &Output,
+                                           const InputInfoList &Inputs,
+                                           const ArgList &Args,
+                                           const char *LinkingOutput) const {
+  const auto &TC = getToolChain();
+  const auto &D = TC.getDriver();
+  const bool IsShared = Args.hasArg(options::OPT_shared);
+  const bool IsStatic =
+      Args.hasArg(options::OPT_static) && !Args.hasArg(options::OPT_static_pie);
+  const bool IsRdynamic = Args.hasArg(options::OPT_rdynamic);
+  const bool IsStaticPIE = Args.hasArg(options::OPT_static_pie);
+  const bool IsPIE = getPIE(Args, TC);
+  ArgStringList CmdArgs;
+
+  if (!D.SysRoot.empty())
+    CmdArgs.push_back(Args.MakeArgString("--sysroot=" + D.SysRoot));
+
+  if (IsPIE || IsStaticPIE)
+    CmdArgs.push_back("-pie");
+
+  if (IsShared)
+    CmdArgs.push_back("-shared");
+
+  if (IsStatic || IsStaticPIE)
+    CmdArgs.push_back("-static");
+
+  if (IsStaticPIE) {
+    CmdArgs.push_back("--no-dynamic-linker");
+    CmdArgs.push_back("-z");
+    CmdArgs.push_back("text");
+  }
+
+  if (!IsStatic && !IsStaticPIE) {
+    if (IsRdynamic)
+      CmdArgs.push_back("-export-dynamic");
+    CmdArgs.push_back("-dynamic-linker");
+    CmdArgs.push_back("/usr/lib/Loader.so");
+  }
+
+  if (!IsStatic || IsStaticPIE)
+    CmdArgs.push_back("--eh-frame-hdr");
+
+  if (Output.isFilename()) {
+    CmdArgs.push_back("-o");
+    CmdArgs.push_back(Output.getFilename());
+  }
+
+  if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles)) {
+    CmdArgs.push_back(Args.MakeArgString(
+        TC.GetFilePath((IsShared) ? "crt0_shared.o" : "crt0.o")));
+    CmdArgs.push_back(Args.MakeArgString(TC.GetFilePath("crti.o")));
+
+    std::string crtbegin_path;
+    if (TC.GetRuntimeLibType(Args) == ToolChain::RLT_CompilerRT) {
+      std::string crtbegin =
+          TC.getCompilerRT(Args, "crtbegin", ToolChain::FT_Object);
+      if (TC.getVFS().exists(crtbegin))
+        crtbegin_path = crtbegin;
+    }
+    if (crtbegin_path.empty()) {
+      const char *crtbegin = (IsShared || IsPIE) ? "crtbeginS.o" : "crtbegin.o";
+      crtbegin_path = TC.GetFilePath(crtbegin);
+    }
+    CmdArgs.push_back(Args.MakeArgString(crtbegin_path));
+  }
+
+  Args.AddAllArgs(CmdArgs, options::OPT_L);
+  Args.AddAllArgs(CmdArgs, options::OPT_u);
+
+  TC.AddFilePathLibArgs(Args, CmdArgs);
+
+  if (D.isUsingLTO()) {
+    assert(!Inputs.empty() && "Must have at least one input.");
+    addLTOOptions(TC, Args, CmdArgs, Output, Inputs[0],
+                  D.getLTOMode() == LTOK_Thin);
+  }
+
+  Args.AddAllArgs(CmdArgs, options::OPT_T_Group);
+  Args.AddAllArgs(CmdArgs, options::OPT_e);
+  Args.AddAllArgs(CmdArgs, options::OPT_s);
+  Args.AddAllArgs(CmdArgs, options::OPT_t);
+  Args.AddAllArgs(CmdArgs, options::OPT_Z_Flag);
+  Args.AddAllArgs(CmdArgs, options::OPT_r);
+
+  addLinkerCompressDebugSectionsOption(TC, Args, CmdArgs);
+
+  AddLinkerInputs(TC, Inputs, Args, CmdArgs, JA);
+
+  if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
+    AddRunTimeLibs(TC, D, CmdArgs, Args);
+
+    // We supply our own sanitizer runtimes
+    // FIXME: What if we want to use Clang-supplied ones as well?
+    const SanitizerArgs &Sanitize = TC.getSanitizerArgs();
+    if (Sanitize.needsUbsanRt())
+      CmdArgs.push_back("-lubsan");
+  }
+
+  if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
+    if (D.CCCIsCXX()) {
+      if (TC.ShouldLinkCXXStdlib(Args))
+        TC.AddCXXStdlibLibArgs(Args, CmdArgs);
+      CmdArgs.push_back("-lm");
+    }
+  }
+
+  if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
+    if (Args.hasArg(options::OPT_pthread, options::OPT_pthreads))
+      CmdArgs.push_back("-lpthread");
+
+    if (!Args.hasArg(options::OPT_nolibc))
+      CmdArgs.push_back("-lc");
+  }
+
+  if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles)) {
+    std::string crtend_path;
+    if (TC.GetRuntimeLibType(Args) == ToolChain::RLT_CompilerRT) {
+      std::string crtend =
+          TC.getCompilerRT(Args, "crtend", ToolChain::FT_Object);
+      if (TC.getVFS().exists(crtend))
+        crtend_path = crtend;
+    }
+    if (crtend_path.empty()) {
+      const char *crtend = (IsShared || IsPIE) ? "crtendS.o" : "crtend.o";
+      crtend_path = TC.GetFilePath(crtend);
+    }
+    CmdArgs.push_back(Args.MakeArgString(crtend_path));
+
+    CmdArgs.push_back(Args.MakeArgString(TC.GetFilePath("crtn.o")));
+  }
+
+  Args.AddAllArgs(CmdArgs, options::OPT_T);
+
+  const char *Exec = Args.MakeArgString(TC.GetLinkerPath());
+  C.addCommand(std::make_unique<Command>(JA, *this,
+                                         ResponseFileSupport::AtFileCurCP(),
+                                         Exec, CmdArgs, Inputs, Output));
+}
+
+Serenity::Serenity(const Driver &D, const llvm::Triple &Triple,
+                   const ArgList &Args)
+    : ToolChain(D, Triple, Args) {
+  getProgramPaths().push_back(getDriver().getInstalledDir());
+  if (getDriver().getInstalledDir() != getDriver().Dir)
+    getProgramPaths().push_back(getDriver().Dir);
+  getFilePaths().push_back(getDriver().SysRoot + "/usr/lib");
+}
+
+Tool *Serenity::buildLinker() const {
+  return new tools::serenity::Linker(*this);
+}
+
+void Serenity::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
+                                         ArgStringList &CC1Args) const {
+  const Driver &D = getDriver();
+
+  if (DriverArgs.hasArg(options::OPT_nostdinc))
+    return;
+
+  if (!DriverArgs.hasArg(options::OPT_nobuiltininc)) {
+    addSystemInclude(DriverArgs, CC1Args, D.ResourceDir + "/include");
+  }
+
+  if (DriverArgs.hasArg(options::OPT_nostdlibinc))
+    return;
+
+  if (!D.SysRoot.empty()) {
+    addSystemInclude(DriverArgs, CC1Args, D.SysRoot + "/usr/local/include");
+    addExternCSystemInclude(DriverArgs, CC1Args, D.SysRoot + "/usr/include");
+  }
+}
+
+void Serenity::AddClangCXXStdlibIncludeArgs(const ArgList &DriverArgs,
+                                            ArgStringList &CC1Args) const {
+  if (DriverArgs.hasArg(options::OPT_nostdinc, options::OPT_nostdincxx,
+                        +options::OPT_nostdlibinc))
+    return;
+
+  if (GetCXXStdlibType(DriverArgs) != ToolChain::CST_Libcxx)
+    llvm_unreachable("Only libc++ is supported on the Serenity target");
+
+  const Driver &D = getDriver();
+  std::string SysRoot = computeSysRoot();
+  std::string Target = getTripleString();
+
+  auto AddIncludePath = [&](std::string Path) {
+    std::string Version = detectLibcxxVersion(Path);
+    if (Version.empty())
+      return false;
+
+    std::string TargetDir = Path + "/" + Target + "/c++/" + Version;
+    if (D.getVFS().exists(TargetDir))
+      addSystemInclude(DriverArgs, CC1Args, TargetDir);
+
+    addSystemInclude(DriverArgs, CC1Args, Path + "/c++/" + Version);
+    return true;
+  };
+
+  if (AddIncludePath(getDriver().Dir + "/../include"))
+    return;
+  if (AddIncludePath(SysRoot + "/usr/local/include"))
+    return;
+  if (AddIncludePath(SysRoot + "/usr/include"))
+    return;
+}
+
+void Serenity::addClangTargetOptions(
+    const llvm::opt::ArgList &DriverArgs, llvm::opt::ArgStringList &CC1Args,
+    Action::OffloadKind DeviceOffloadKind) const {
+  if (!DriverArgs.hasFlag(options::OPT_fuse_init_array,
+                          options::OPT_fno_use_init_array, true))
+    CC1Args.push_back("-fno-use-init-array");
+}
+
+ToolChain::UnwindLibType
+Serenity::GetUnwindLibType(const llvm::opt::ArgList &Args) const {
+
+  const Arg *A = Args.getLastArg(options::OPT_unwindlib_EQ);
+  StringRef LibName = A ? A->getValue() : "";
+
+  if (LibName == "none") {
+    return ToolChain::UNW_None;
+  } else if (LibName == "platform" || LibName == "") {
+    ToolChain::RuntimeLibType RtLibType = GetRuntimeLibType(Args);
+    if (RtLibType == ToolChain::RLT_CompilerRT) {
+      return ToolChain::UNW_CompilerRT;
+    } else if (RtLibType == ToolChain::RLT_Libgcc)
+      return ToolChain::UNW_Libgcc;
+  } else if (LibName == "libunwind") {
+    if (GetRuntimeLibType(Args) == RLT_Libgcc)
+      getDriver().Diag(diag::err_drv_incompatible_unwindlib);
+    return ToolChain::UNW_CompilerRT;
+  } else if (LibName == "libgcc") {
+    return ToolChain::UNW_Libgcc;
+  }
+
+  if (A)
+    getDriver().Diag(diag::err_drv_invalid_unwindlib_name)
+        << A->getAsString(Args);
+
+  return ToolChain::UNW_None;
+}
\ No newline at end of file
diff --git a/clang/lib/Driver/ToolChains/Serenity.h b/clang/lib/Driver/ToolChains/Serenity.h
new file mode 100644
index 000000000..50b0dbe62
--- /dev/null
+++ b/clang/lib/Driver/ToolChains/Serenity.h
@@ -0,0 +1,99 @@
+//===---- Serenity.h - SerenityOS ToolChain Implementation ------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_LIB_DRIVER_TOOLCHAINS_SERENITY_H
+#define LLVM_CLANG_LIB_DRIVER_TOOLCHAINS_SERENITY_H
+
+#include "clang/Basic/LangOptions.h"
+#include "clang/Driver/Tool.h"
+#include "clang/Driver/ToolChain.h"
+
+namespace clang {
+namespace driver {
+namespace tools {
+namespace serenity {
+
+class LLVM_LIBRARY_VISIBILITY Linker : public Tool {
+public:
+  Linker(const ToolChain &TC) : Tool("serenity::Linker", "linker", TC) {}
+
+  bool hasIntegratedCPP() const override { return false; }
+  bool isLinkJob() const override { return true; }
+
+  void ConstructJob(Compilation &C, const JobAction &JA,
+                    const InputInfo &Output, const InputInfoList &Inputs,
+                    const llvm::opt::ArgList &TCArgs,
+                    const char *LinkingOutput) const override;
+};
+} // end namespace serenity
+} // end namespace tools
+
+namespace toolchains {
+
+class LLVM_LIBRARY_VISIBILITY Serenity : public ToolChain {
+public:
+  Serenity(const Driver &D, const llvm::Triple &Triple,
+           const llvm::opt::ArgList &Args);
+
+  void
+  AddClangSystemIncludeArgs(const llvm::opt::ArgList &DriverArgs,
+                            llvm::opt::ArgStringList &CC1Args) const override;
+
+  void AddClangCXXStdlibIncludeArgs(
+      const llvm::opt::ArgList &DriverArgs,
+      llvm::opt::ArgStringList &CC1Args) const override;
+
+  RuntimeLibType GetDefaultRuntimeLibType() const override {
+    return ToolChain::RLT_CompilerRT;
+  }
+
+  CXXStdlibType GetDefaultCXXStdlibType() const override {
+    return ToolChain::CST_Libcxx;
+  }
+
+  UnwindLibType GetUnwindLibType(const llvm::opt::ArgList &Args) const override;
+
+  void
+  addClangTargetOptions(const llvm::opt::ArgList &DriverArgs,
+                        llvm::opt::ArgStringList &CC1Args,
+                        Action::OffloadKind DeviceOffloadKind) const override;
+
+  const char *getDefaultLinker() const override { return "ld.lld"; }
+
+  bool HasNativeLLVMSupport() const override { return true; }
+
+  bool IsIntegratedAssemblerDefault() const override { return true; }
+
+  bool isPICDefault() const override { return true; }
+  bool isPIEDefault() const override { return true; }
+  bool isPICDefaultForced() const override { return false; }
+
+  bool IsMathErrnoDefault() const override { return false; }
+
+  bool IsUnwindTablesDefault(const llvm::opt::ArgList &Args) const override {
+    return true;
+  }
+
+  bool useRelaxRelocations() const override { return true; }
+
+  LangOptions::StackProtectorMode
+  GetDefaultStackProtectorLevel(bool KernelOrKext) const override {
+    return LangOptions::SSPStrong;
+  }
+
+  unsigned GetDefaultDwarfVersion() const override { return 5; }
+
+protected:
+  Tool *buildLinker() const override;
+};
+
+} // end namespace toolchains
+} // end namespace driver
+} // end namespace clang
+
+#endif // LLVM_CLANG_LIB_DRIVER_TOOLCHAINS_SERENITY_H
diff --git a/clang/lib/Frontend/InitHeaderSearch.cpp b/clang/lib/Frontend/InitHeaderSearch.cpp
index ba9f96384..4aecfeee2 100644
--- a/clang/lib/Frontend/InitHeaderSearch.cpp
+++ b/clang/lib/Frontend/InitHeaderSearch.cpp
@@ -233,6 +233,7 @@ void InitHeaderSearch::AddDefaultCIncludePaths(const llvm::Triple &triple,
     case llvm::Triple::PS4:
     case llvm::Triple::ELFIAMCU:
     case llvm::Triple::Fuchsia:
+    case llvm::Triple::Serenity:
       break;
     case llvm::Triple::Win32:
       if (triple.getEnvironment() != llvm::Triple::Cygnus)
@@ -432,6 +433,7 @@ void InitHeaderSearch::AddDefaultIncludePaths(const LangOptions &Lang,
   case llvm::Triple::Solaris:
   case llvm::Triple::WASI:
   case llvm::Triple::AIX:
+  case llvm::Triple::Serenity:
     return;
 
   case llvm::Triple::Win32:
diff --git a/libcxx/cmake/Modules/HandleOutOfTreeLLVM.cmake b/libcxx/cmake/Modules/HandleOutOfTreeLLVM.cmake
index ad2820b32..deaa2c380 100644
--- a/libcxx/cmake/Modules/HandleOutOfTreeLLVM.cmake
+++ b/libcxx/cmake/Modules/HandleOutOfTreeLLVM.cmake
@@ -14,6 +14,8 @@ set(LLVM_INCLUDE_DIR ${LLVM_PATH}/include CACHE PATH "Path to llvm/include")
 set(LLVM_PATH ${LLVM_PATH} CACHE PATH "Path to LLVM source tree")
 set(LLVM_MAIN_SRC_DIR ${LLVM_PATH})
 set(LLVM_CMAKE_PATH "${LLVM_PATH}/cmake/modules")
+set(LLVM_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
+set(LLVM_LIBRARY_OUTPUT_INTDIR "${CMAKE_CURRENT_BINARY_DIR}/lib")
 
 if (EXISTS "${LLVM_CMAKE_PATH}")
   list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_PATH}")
diff --git a/libcxx/include/__config b/libcxx/include/__config
index 97e33f315..bfb221230 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -958,6 +958,8 @@ typedef unsigned int   char32_t;
 #  define _LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION
 #elif defined(__MVS__)
 #  define _LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION
+#elif defined(__serenity__) && !defined(KERNEL)
+#  define _LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION
 #endif
 
 #if defined(__APPLE__)
@@ -1148,6 +1150,7 @@ extern "C" _LIBCPP_FUNC_VIS void __sanitizer_annotate_contiguous_container(
       defined(__sun__) || \
       defined(__MVS__) || \
       defined(_AIX) || \
+      defined(__serenity__) || \
       (defined(__MINGW32__) && __has_include(<pthread.h>))
 #    define _LIBCPP_HAS_THREAD_API_PTHREAD
 #  elif defined(__Fuchsia__)
diff --git a/libcxx/include/__locale b/libcxx/include/__locale
index ad742997d..c75dcb458 100644
--- a/libcxx/include/__locale
+++ b/libcxx/include/__locale
@@ -31,7 +31,7 @@
 #elif defined(__sun__)
 # include <xlocale.h>
 # include <__support/solaris/xlocale.h>
-#elif defined(_NEWLIB_VERSION)
+#elif defined(_NEWLIB_VERSION) || defined(__serenity__)
 # include <__support/newlib/xlocale.h>
 #elif defined(__OpenBSD__)
 # include <__support/openbsd/xlocale.h>
@@ -490,7 +490,7 @@ public:
     static const mask xdigit = _ISXDIGIT;
     static const mask blank  = _ISBLANK;
     static const mask __regex_word = 0x80;
-#elif defined(_NEWLIB_VERSION)
+#elif defined(_NEWLIB_VERSION) || defined(__serenity__)
     // Same type as Newlib's _ctype_ array in newlib/libc/include/ctype.h.
     typedef char mask;
     static const mask space  = _S;
diff --git a/libcxx/include/__support/newlib/xlocale.h b/libcxx/include/__support/newlib/xlocale.h
index b75f9263a..f5ffb9003 100644
--- a/libcxx/include/__support/newlib/xlocale.h
+++ b/libcxx/include/__support/newlib/xlocale.h
@@ -9,7 +9,7 @@
 #ifndef _LIBCPP_SUPPORT_NEWLIB_XLOCALE_H
 #define _LIBCPP_SUPPORT_NEWLIB_XLOCALE_H
 
-#if defined(_NEWLIB_VERSION)
+#if defined(_NEWLIB_VERSION) || defined(__serenity__)
 
 #include <cstdlib>
 #include <clocale>
@@ -22,6 +22,6 @@
 #include <__support/xlocale/__strtonum_fallback.h>
 #endif
 
-#endif // _NEWLIB_VERSION
+#endif // _NEWLIB_VERSION || __serenity__
 
 #endif
diff --git a/libcxx/include/initializer_list b/libcxx/include/initializer_list
index ea1f23467..45631ac2f 100644
--- a/libcxx/include/initializer_list
+++ b/libcxx/include/initializer_list
@@ -43,7 +43,9 @@ template<class E> const E* end(initializer_list<E> il) noexcept; // constexpr in
 */
 
 #include <__config>
+#if !defined(__serenity__) || !defined(KERNEL)
 #include <cstddef>
+#endif
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
diff --git a/libcxx/include/locale b/libcxx/include/locale
index 8e584005d..f29f3453e 100644
--- a/libcxx/include/locale
+++ b/libcxx/include/locale
@@ -206,7 +206,7 @@ template <class charT> class messages_byname;
 
 #if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
 // Most unix variants have catopen.  These are the specific ones that don't.
-#  if !defined(__BIONIC__) && !defined(_NEWLIB_VERSION)
+#  if !defined(__BIONIC__) && !defined(_NEWLIB_VERSION) && !defined(__serenity__)
 #    define _LIBCPP_HAS_CATOPEN 1
 #    include <nl_types.h>
 #  endif
diff --git a/libcxx/src/include/config_elast.h b/libcxx/src/include/config_elast.h
index 7880c733f..87b25cf42 100644
--- a/libcxx/src/include/config_elast.h
+++ b/libcxx/src/include/config_elast.h
@@ -33,6 +33,8 @@
 #define _LIBCPP_ELAST 4095
 #elif defined(__APPLE__)
 // No _LIBCPP_ELAST needed on Apple
+#elif defined(__serenity__)
+// No _LIBCPP_ELAST needed on SerenityOS
 #elif defined(__sun__)
 #define _LIBCPP_ELAST ESTALE
 #elif defined(__MVS__)
diff --git a/libcxx/src/locale.cpp b/libcxx/src/locale.cpp
index d5ab8fb3b..5039c1987 100644
--- a/libcxx/src/locale.cpp
+++ b/libcxx/src/locale.cpp
@@ -1145,6 +1145,8 @@ ctype<char>::classic_table() noexcept
     return __pctype_func();
 #elif defined(__EMSCRIPTEN__)
     return *__ctype_b_loc();
+#elif defined(__serenity__)
+    return _ctype_;
 #elif defined(_NEWLIB_VERSION)
     // Newlib has a 257-entry table in ctype_.c, where (char)0 starts at [1].
     return _ctype_ + 1;
diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
index 0c3419390..75f8b552b 100644
--- a/llvm/cmake/modules/HandleLLVMOptions.cmake
+++ b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -228,11 +228,11 @@ endif()
 
 # Pass -Wl,-z,defs. This makes sure all symbols are defined. Otherwise a DSO
 # build might work on ELF but fail on MachO/COFF.
-if(NOT (CMAKE_SYSTEM_NAME MATCHES "Darwin|FreeBSD|OpenBSD|DragonFly|AIX|SunOS|OS390" OR
-        WIN32 OR CYGWIN) AND
-   NOT LLVM_USE_SANITIZER)
-  set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,-z,defs")
-endif()
+#if(NOT (CMAKE_SYSTEM_NAME MATCHES "Darwin|FreeBSD|OpenBSD|DragonFly|AIX|SunOS|OS390" OR
+#        WIN32 OR CYGWIN) AND
+#   NOT LLVM_USE_SANITIZER)
+#  set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,-z,defs")
+#endif()
 
 # Pass -Wl,-z,nodelete. This makes sure our shared libraries are not unloaded
 # by dlclose(). We need that since the CLI API relies on cross-references
diff --git a/llvm/include/llvm/ADT/Triple.h b/llvm/include/llvm/ADT/Triple.h
index 76f351405..b81556c55 100644
--- a/llvm/include/llvm/ADT/Triple.h
+++ b/llvm/include/llvm/ADT/Triple.h
@@ -199,7 +199,8 @@ public:
     Hurd,       // GNU/Hurd
     WASI,       // Experimental WebAssembly OS
     Emscripten,
-    LastOSType = Emscripten
+    Serenity,
+    LastOSType = Serenity
   };
   enum EnvironmentType {
     UnknownEnvironment,
@@ -628,6 +629,11 @@ public:
     return getOS() == Triple::AIX;
   }
 
+  /// Tests whether the OS is SerenityOS
+  bool isOSSerenity() const {
+    return getOS() == Triple::Serenity;
+  }
+
   /// Tests whether the OS uses the ELF binary format.
   bool isOSBinFormatELF() const {
     return getObjectFormat() == Triple::ELF;
diff --git a/llvm/lib/Support/Triple.cpp b/llvm/lib/Support/Triple.cpp
index 883115463..22b39d8f5 100644
--- a/llvm/lib/Support/Triple.cpp
+++ b/llvm/lib/Support/Triple.cpp
@@ -218,6 +218,7 @@ StringRef Triple::getOSTypeName(OSType Kind) {
   case PS4: return "ps4";
   case RTEMS: return "rtems";
   case Solaris: return "solaris";
+  case Serenity: return "serenity";
   case TvOS: return "tvos";
   case WASI: return "wasi";
   case WatchOS: return "watchos";
@@ -538,6 +539,7 @@ static Triple::OSType parseOS(StringRef OSName) {
     .StartsWith("hurd", Triple::Hurd)
     .StartsWith("wasi", Triple::WASI)
     .StartsWith("emscripten", Triple::Emscripten)
+    .StartsWith("serenity", Triple::Serenity)
     .Default(Triple::UnknownOS);
 }
 
diff --git a/runtimes/CMakeLists.txt b/runtimes/CMakeLists.txt
index 1ffce323d..552521192 100644
--- a/runtimes/CMakeLists.txt
+++ b/runtimes/CMakeLists.txt
@@ -58,17 +58,28 @@ if(compiler_rt_path)
   endif()
 endif()
 
+# If building standalone by pointing CMake at this runtimes directory,
+# LLVM_BINARY_DIR isn't set, find_package(LLVM) will fail and these
+# intermediate paths are unset.
+if (LLVM_BINARY_DIR STREQUAL "")
+  set(LLVM_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
+endif()
+if (NOT LLVM_FOUND)
+  set(LLVM_TOOLS_BINARY_DIR ${LLVM_BINARY_DIR}/bin)
+  set(LLVM_LIBRARY_DIR ${CMAKE_CURRENT_BINARY_DIR}/lib)
+endif()
+
 # Setting these variables will allow the sub-build to put their outputs into
 # the library and bin directories of the top-level build.
 set(LLVM_LIBRARY_OUTPUT_INTDIR ${LLVM_LIBRARY_DIR})
 set(LLVM_RUNTIME_OUTPUT_INTDIR ${LLVM_TOOLS_BINARY_DIR})
 
 # This variable makes sure that e.g. llvm-lit is found.
-set(LLVM_MAIN_SRC_DIR ${LLVM_BUILD_MAIN_SRC_DIR})
+set(LLVM_MAIN_SRC_DIR ${CMAKE_CURRENT_SOURCE_DIR}/../llvm)
 set(LLVM_CMAKE_PATH ${LLVM_MAIN_SRC_DIR}/cmake/modules)
 
 # This variable is used by individual runtimes to locate LLVM files.
-set(LLVM_PATH ${LLVM_BUILD_MAIN_SRC_DIR})
+set(LLVM_PATH ${CMAKE_CURRENT_SOURCE_DIR}/../llvm)
 
 if(APPLE)
   set(LLVM_ENABLE_LIBCXX ON CACHE BOOL "")