diff --git a/clang/lib/Driver/ToolChains/Fuchsia.cpp b/clang/lib/Driver/ToolChains/Fuchsia.cpp
--- a/clang/lib/Driver/ToolChains/Fuchsia.cpp
+++ b/clang/lib/Driver/ToolChains/Fuchsia.cpp
@@ -358,17 +358,36 @@
 
 void Fuchsia::AddClangCXXStdlibIncludeArgs(const ArgList &DriverArgs,
                                            ArgStringList &CC1Args) const {
+  const Driver &D = getDriver();
+
   if (DriverArgs.hasArg(options::OPT_nostdlibinc) ||
       DriverArgs.hasArg(options::OPT_nostdincxx))
     return;
 
+  auto AddIncludePath = [&](StringRef Path) {
+    std::string Version = detectLibcxxVersion(Path);
+    if (Version.empty())
+      return;
+
+    SmallString<128> P;
+
+    // First add the per-target include path.
+    P.assign(Path);
+    llvm::sys::path::append(P, getTriple().str(), "c++", Version);
+    if (getVFS().exists(P))
+      addSystemInclude(DriverArgs, CC1Args, P);
+
+    // Second add the generic one.
+    P.assign(Path);
+    llvm::sys::path::append(P, "c++", Version);
+    addSystemInclude(DriverArgs, CC1Args, P);
+  };
+
   switch (GetCXXStdlibType(DriverArgs)) {
   case ToolChain::CST_Libcxx: {
-    SmallString<128> P(getDriver().Dir);
+    SmallString<128> P(D.Dir);
     llvm::sys::path::append(P, "..", "include");
-    std::string Version = detectLibcxxVersion(P);
-    llvm::sys::path::append(P, "c++", Version);
-    addSystemInclude(DriverArgs, CC1Args, P.str());
+    AddIncludePath(P);
     break;
   }
 
diff --git a/clang/lib/Driver/ToolChains/Gnu.cpp b/clang/lib/Driver/ToolChains/Gnu.cpp
--- a/clang/lib/Driver/ToolChains/Gnu.cpp
+++ b/clang/lib/Driver/ToolChains/Gnu.cpp
@@ -2895,14 +2895,26 @@
 void
 Generic_GCC::addLibCxxIncludePaths(const llvm::opt::ArgList &DriverArgs,
                                    llvm::opt::ArgStringList &CC1Args) const {
+  const Driver &D = getDriver();
+  std::string SysRoot = computeSysRoot();
+  std::string MultiarchTriple = getMultiarchTriple(D, getTriple(), SysRoot);
+
   auto AddIncludePath = [&](std::string Path) {
     std::string Version = detectLibcxxVersion(Path);
-    std::string IncludePath = Path + "/c++/" + Version;
-    if (Version.empty() || !getVFS().exists(IncludePath))
+    if (Version.empty())
       return false;
-    addSystemInclude(DriverArgs, CC1Args, IncludePath);
+
+    // First add the multiarch include path if it exists.
+    std::string MultiarchDir = Path + "/" + MultiarchTriple + "/c++/" + Version;
+    llvm::outs() << MultiarchDir << "\n";
+    if (!MultiarchTriple.empty() && D.getVFS().exists(MultiarchDir))
+      addSystemInclude(DriverArgs, CC1Args, MultiarchDir);
+
+    // Second add the generic one.
+    addSystemInclude(DriverArgs, CC1Args, Path + "/c++/" + Version);
     return true;
   };
+
   // Android never uses the libc++ headers installed alongside the toolchain,
   // which are generally incompatible with the NDK libraries anyway.
   if (!getTriple().isAndroid())
@@ -2911,7 +2923,6 @@
   // If this is a development, non-installed, clang, libcxx will
   // not be found at ../include/c++ but it likely to be found at
   // one of the following two locations:
-  std::string SysRoot = computeSysRoot();
   if (AddIncludePath(SysRoot + "/usr/local/include"))
     return;
   if (AddIncludePath(SysRoot + "/usr/include"))
diff --git a/clang/test/Driver/Inputs/basic_fuchsia_tree/include/aarch64-unknown-fuchsia/c++/v1/.keep b/clang/test/Driver/Inputs/basic_fuchsia_tree/include/aarch64-unknown-fuchsia/c++/v1/.keep
new file mode 100644
diff --git a/clang/test/Driver/Inputs/basic_fuchsia_tree/include/riscv64-unknown-fuchsia/c++/v1/.keep b/clang/test/Driver/Inputs/basic_fuchsia_tree/include/riscv64-unknown-fuchsia/c++/v1/.keep
new file mode 100644
diff --git a/clang/test/Driver/Inputs/basic_fuchsia_tree/include/x86_64-unknown-fuchsia/c++/v1/.keep b/clang/test/Driver/Inputs/basic_fuchsia_tree/include/x86_64-unknown-fuchsia/c++/v1/.keep
new file mode 100644
diff --git a/clang/test/Driver/Inputs/basic_linux_libcxx_tree/usr/include/x86_64-linux-gnu/c++/v1/.keep b/clang/test/Driver/Inputs/basic_linux_libcxx_tree/usr/include/x86_64-linux-gnu/c++/v1/.keep
new file mode 100644
diff --git a/clang/test/Driver/Inputs/basic_linux_libcxxv2_tree/usr/include/x86_64-linux-gnu/c++/v2/.keep b/clang/test/Driver/Inputs/basic_linux_libcxxv2_tree/usr/include/x86_64-linux-gnu/c++/v2/.keep
new file mode 100644
diff --git a/clang/test/Driver/Inputs/basic_linux_libstdcxx_libcxxv2_tree/usr/include/x86_64-linux-gnu/c++/v2/.keep b/clang/test/Driver/Inputs/basic_linux_libstdcxx_libcxxv2_tree/usr/include/x86_64-linux-gnu/c++/v2/.keep
new file mode 100644
diff --git a/clang/test/Driver/fuchsia.cpp b/clang/test/Driver/fuchsia.cpp
--- a/clang/test/Driver/fuchsia.cpp
+++ b/clang/test/Driver/fuchsia.cpp
@@ -20,6 +20,9 @@
 // CHECK-NOT: "-fno-use-init-array"
 // CHECK: "-resource-dir" "[[RESOURCE_DIR:[^"]+]]"
 // CHECK: "-isysroot" "[[SYSROOT:[^"]+]]"
+// CHECK-X86_64: "-internal-isystem" "{{.*[/\\]}}include{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}c++{{/|\\\\}}v1"
+// CHECK-AARCH64: "-internal-isystem" "{{.*[/\\]}}include{{/|\\\\}}aarch64-unknown-fuchsia{{/|\\\\}}c++{{/|\\\\}}v1"
+// CHECK-RISCV64: "-internal-isystem" "{{.*[/\\]}}include{{/|\\\\}}riscv64-unknown-fuchsia{{/|\\\\}}c++{{/|\\\\}}v1"
 // CHECK: "-internal-isystem" "{{.*[/\\]}}include{{/|\\\\}}c++{{/|\\\\}}v1"
 // CHECK: "-internal-externc-isystem" "[[SYSROOT]]{{/|\\\\}}include"
 // CHECK: {{.*}}ld.lld{{.*}}" "-z" "now" "-z" "rodynamic" "-z" "separate-loadable-segments"
diff --git a/clang/test/Driver/linux-header-search.cpp b/clang/test/Driver/linux-header-search.cpp
--- a/clang/test/Driver/linux-header-search.cpp
+++ b/clang/test/Driver/linux-header-search.cpp
@@ -13,6 +13,7 @@
 // RUN:   | FileCheck --check-prefix=CHECK-BASIC-LIBCXX-SYSROOT %s
 // CHECK-BASIC-LIBCXX-SYSROOT: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 // CHECK-BASIC-LIBCXX-SYSROOT: "-isysroot" "[[SYSROOT:[^"]+]]"
+// CHECK-BASIC-LIBCXX-SYSROOT: "-internal-isystem" "[[SYSROOT]]/usr/include/x86_64-linux-gnu/c++/v1"
 // CHECK-BASIC-LIBCXX-SYSROOT: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v1"
 // CHECK-BASIC-LIBCXX-SYSROOT: "-internal-isystem" "[[SYSROOT]]/usr/local/include"
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
@@ -25,6 +26,7 @@
 // RUN:   | FileCheck --check-prefix=CHECK-BASIC-LIBCXX-INSTALL %s
 // CHECK-BASIC-LIBCXX-INSTALL: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 // CHECK-BASIC-LIBCXX-INSTALL: "-isysroot" "[[SYSROOT:[^"]+]]"
+// CHECK-BASIC-LIBCXX-INSTALL: "-internal-isystem" "[[SYSROOT]]/usr/bin/../include/x86_64-linux-gnu/c++/v1"
 // CHECK-BASIC-LIBCXX-INSTALL: "-internal-isystem" "[[SYSROOT]]/usr/bin/../include/c++/v1"
 // CHECK-BASIC-LIBCXX-INSTALL: "-internal-isystem" "[[SYSROOT]]/usr/local/include"
 //
@@ -38,6 +40,7 @@
 // RUN:   | FileCheck --check-prefix=CHECK-BASIC-LIBCXXV2-SYSROOT %s
 // CHECK-BASIC-LIBCXXV2-SYSROOT: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 // CHECK-BASIC-LIBCXXV2-SYSROOT: "-isysroot" "[[SYSROOT:[^"]+]]"
+// CHECK-BASIC-LIBCXXV2-SYSROOT: "-internal-isystem" "[[SYSROOT]]/usr/include/x86_64-linux-gnu/c++/v2"
 // CHECK-BASIC-LIBCXXV2-SYSROOT: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v2"
 // CHECK-BASIC-LIBCXXV2-SYSROOT: "-internal-isystem" "[[SYSROOT]]/usr/local/include"
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
@@ -50,6 +53,7 @@
 // RUN:   | FileCheck --check-prefix=CHECK-BASIC-LIBCXXV2-INSTALL %s
 // CHECK-BASIC-LIBCXXV2-INSTALL: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 // CHECK-BASIC-LIBCXXV2-INSTALL: "-isysroot" "[[SYSROOT:[^"]+]]"
+// CHECK-BASIC-LIBCXXV2-INSTALL: "-internal-isystem" "[[SYSROOT]]/usr/bin/../include/x86_64-linux-gnu/c++/v2"
 // CHECK-BASIC-LIBCXXV2-INSTALL: "-internal-isystem" "[[SYSROOT]]/usr/bin/../include/c++/v2"
 // CHECK-BASIC-LIBCXXV2-INSTALL: "-internal-isystem" "[[SYSROOT]]/usr/local/include"
 //
@@ -64,6 +68,7 @@
 // RUN:   | FileCheck --check-prefix=CHECK-BASIC-LIBSTDCXX-LIBCXXV2-SYSROOT %s
 // CHECK-BASIC-LIBSTDCXX-LIBCXXV2-SYSROOT: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 // CHECK-BASIC-LIBSTDCXX-LIBCXXV2-SYSROOT: "-isysroot" "[[SYSROOT:[^"]+]]"
+// CHECK-BASIC-LIBSTDCXX-LIBCXXV2-SYSROOT: "-internal-isystem" "[[SYSROOT]]/usr/include/x86_64-linux-gnu/c++/v2"
 // CHECK-BASIC-LIBSTDCXX-LIBCXXV2-SYSROOT: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v2"
 // CHECK-BASIC-LIBSTDCXX-LIBCXXV2-SYSROOT: "-internal-isystem" "[[SYSROOT]]/usr/local/include"
 
diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -408,7 +408,10 @@
 if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND NOT APPLE)
   set(LIBCXX_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR}/${LLVM_DEFAULT_TARGET_TRIPLE}/c++)
   set(LIBCXX_GENERATED_INCLUDE_DIR "${LLVM_BINARY_DIR}/include/c++/v1")
+  set(LIBCXX_GENERATED_INCLUDE_TARGET_DIR "${LLVM_BINARY_DIR}/include/${LLVM_DEFAULT_TARGET_TRIPLE}/c++/v1")
   set(LIBCXX_INSTALL_LIBRARY_DIR lib${LLVM_LIBDIR_SUFFIX}/${LLVM_DEFAULT_TARGET_TRIPLE}/c++)
+  set(LIBCXX_INSTALL_INCLUDE_DIR "include/c++/v1")
+  set(LIBCXX_INSTALL_INCLUDE_TARGET_DIR "include/${LLVM_DEFAULT_TARGET_TRIPLE}/c++/v1")
   if(LIBCXX_LIBDIR_SUBDIR)
     string(APPEND LIBCXX_LIBRARY_DIR /${LIBCXX_LIBDIR_SUBDIR})
     string(APPEND LIBCXX_INSTALL_LIBRARY_DIR /${LIBCXX_LIBDIR_SUBDIR})
@@ -416,11 +419,17 @@
 elseif(LLVM_LIBRARY_OUTPUT_INTDIR)
   set(LIBCXX_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR})
   set(LIBCXX_GENERATED_INCLUDE_DIR "${LLVM_BINARY_DIR}/include/c++/v1")
+  set(LIBCXX_GENERATED_INCLUDE_TARGET_DIR "${LIBCXX_GENERATED_INCLUDE_DIR}")
   set(LIBCXX_INSTALL_LIBRARY_DIR lib${LIBCXX_LIBDIR_SUFFIX})
+  set(LIBCXX_INSTALL_INCLUDE_DIR "include/c++/v1")
+  set(LIBCXX_INSTALL_INCLUDE_TARGET_DIR "${LIBCXX_INSTALL_INCLUDE_DIR}")
 else()
   set(LIBCXX_LIBRARY_DIR ${CMAKE_BINARY_DIR}/lib${LIBCXX_LIBDIR_SUFFIX})
   set(LIBCXX_GENERATED_INCLUDE_DIR "${CMAKE_BINARY_DIR}/include/c++/v1")
+  set(LIBCXX_GENERATED_INCLUDE_TARGET_DIR "${LIBCXX_GENERATED_INCLUDE_DIR}")
   set(LIBCXX_INSTALL_LIBRARY_DIR lib${LIBCXX_LIBDIR_SUFFIX})
+  set(LIBCXX_INSTALL_INCLUDE_DIR "include/c++/v1")
+  set(LIBCXX_INSTALL_INCLUDE_TARGET_DIR "${LIBCXX_INSTALL_INCLUDE_DIR}")
 endif()
 
 file(MAKE_DIRECTORY "${LIBCXX_BINARY_INCLUDE_DIR}")
diff --git a/libcxx/benchmarks/CMakeLists.txt b/libcxx/benchmarks/CMakeLists.txt
--- a/libcxx/benchmarks/CMakeLists.txt
+++ b/libcxx/benchmarks/CMakeLists.txt
@@ -15,6 +15,10 @@
     -Wl,-rpath,${LIBCXX_LIBRARY_DIR}
     ${SANITIZER_FLAGS}
     )
+if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND NOT APPLE)
+  list(APPEND BENCHMARK_LIBCXX_COMPILE_FLAGS
+    -isystem "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}")
+endif()
 if (DEFINED LIBCXX_CXX_ABI_LIBRARY_PATH)
   list(APPEND BENCHMARK_LIBCXX_COMPILE_FLAGS
           -L${LIBCXX_CXX_ABI_LIBRARY_PATH}
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -191,9 +191,9 @@
   wctype.h
   )
 
-configure_file("__config_site.in" "${LIBCXX_GENERATED_INCLUDE_DIR}/__config_site" @ONLY)
+configure_file("__config_site.in" "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}/__config_site" @ONLY)
 
-set(_all_includes "${LIBCXX_GENERATED_INCLUDE_DIR}/__config_site")
+set(_all_includes "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}/__config_site")
 foreach(f ${files})
   set(src "${CMAKE_CURRENT_SOURCE_DIR}/${f}")
   set(dst "${LIBCXX_GENERATED_INCLUDE_DIR}/${f}")
@@ -210,24 +210,26 @@
 add_dependencies(cxx-headers generate-cxx-headers ${LIBCXX_CXX_ABI_HEADER_TARGET})
 # TODO: Use target_include_directories once we figure out why that breaks the runtimes build
 if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC" OR "${CMAKE_CXX_SIMULATE_ID}" STREQUAL "MSVC")
-  target_compile_options(cxx-headers INTERFACE /I "${LIBCXX_GENERATED_INCLUDE_DIR}")
+  target_compile_options(cxx-headers INTERFACE /I "${LIBCXX_GENERATED_INCLUDE_DIR}"
+                                     INTERFACE /I "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}")
 else()
-  target_compile_options(cxx-headers INTERFACE -I "${LIBCXX_GENERATED_INCLUDE_DIR}")
+  target_compile_options(cxx-headers INTERFACE -I${LIBCXX_GENERATED_INCLUDE_DIR}
+                                     INTERFACE -I${LIBCXX_GENERATED_INCLUDE_TARGET_DIR})
 endif()
 
 if (LIBCXX_INSTALL_HEADERS)
   foreach(file ${files})
     get_filename_component(dir ${file} DIRECTORY)
     install(FILES ${file}
-      DESTINATION ${LIBCXX_INSTALL_HEADER_PREFIX}include/c++/v1/${dir}
+      DESTINATION ${LIBCXX_INSTALL_HEADER_PREFIX}${LIBCXX_INSTALL_HEADER_DIR}/${dir}
       COMPONENT cxx-headers
       PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
     )
   endforeach()
 
   # Install the generated __config_site.
-  install(FILES ${LIBCXX_GENERATED_INCLUDE_DIR}/__config_site
-    DESTINATION ${LIBCXX_INSTALL_HEADER_PREFIX}include/c++/v1
+  install(FILES ${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}/__config_site
+    DESTINATION ${LIBCXX_INSTALL_HEADER_PREFIX}${LIBCXX_INSTALL_INCLUDE_TARGET_DIR}
     PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
     COMPONENT cxx-headers)