[libc++] Use a proper CMake target to represent libc++ headers

[ldionne]

Summary

Instead of having complex logic around how to include the libc++ headers
and config_site, handle that by defining cxx-headers as an INTERFACE
library and linking against it. After this patch, linking against cxx-headers
is sufficient to get the right config_site include and include paths
for libc++.

[ldionne]

@phosek I know this change breaks the runtimes build, but I can't figure out why. I spent some time trying to understand why, but I'd welcome some help.

[ldionne]

Ping @phosek .

[ldionne]

Ping @phosek

[phosek]

I spent some time debugging this, and to be honest, I don't know what's going on. The problem is that in the multilib variant case, the header includes are missing. So normally, the generated Ninja entries would look like this:

build libcxx/src/CMakeFiles/cxx_static.dir/filesystem/operations.cpp.obj: CXX_COMPILER__cxx_static /src/clang-llvm/llvm-project/libcxx/src/filesystem/operations.cpp || cmake_object_order_depends_target_cxx_static
  DEFINES = -DNDEBUG -D_LIBCPP_BUILDING_LIBRARY -D_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS
  DEP_FILE = libcxx/src/CMakeFiles/cxx_static.dir/filesystem/operations.cpp.obj.d
  FLAGS = --target=x86_64-unknown-fuchsia -I/usr/local/google/home/phosek/clang-llvm/sdk/pkg/fdio/include -fPIC -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wstring-conversion -ffunction-sections -fdata-sections -ffile-prefix-map=/src/clang-llvm/llvm-build/fuchsia/runtimes/runtimes-x86_64-unknown-fuchsia-bins=../llvm-build/fuchsia/runtimes/runtimes-x86_64-unknown-fuchsia-bins -ffile-prefix-map=/src/clang-llvm/llvm-project/= -no-canonical-prefixes -O2 -g -DNDEBUG   -DLIBCXX_BUILDING_LIBCXXABI -nostdinc++ -fvisibility-inlines-hidden -fvisibility=hidden -Wall -Wextra -W -Wwrite-strings -Wno-unused-parameter -Wno-long-long -Werror=return-type -Wextra-semi -Wno-user-defined-literals -Wno-covered-switch-default -Wno-ignored-attributes -Wno-error -fvisibility-global-new-delete-hidden -include /src/clang-llvm/llvm-build/fuchsia/runtimes/runtimes-x86_64-unknown-fuchsia-bins/libcxx/__config_site -std=c++14
  INCLUDES = -Ilibcxx/include/c++build -I/src/clang-llvm/llvm-build/fuchsia/include/c++/v1
  OBJECT_DIR = libcxx/src/CMakeFiles/cxx_static.dir
  OBJECT_FILE_DIR = libcxx/src/CMakeFiles/cxx_static.dir/filesystem

but with this patch, we would end up with this:

build libcxx/src/CMakeFiles/cxx_static.dir/filesystem/operations.cpp.obj: CXX_COMPILER__cxx_static /src/clang-llvm/llvm-project/libcxx/src/filesystem/operations.cpp || cmake_object_order_depends_target_cxx_static
  DEFINES = -DNDEBUG -D_LIBCPP_BUILDING_LIBRARY -D_LIBCPP_DISABLE_NEW_DELETE_DEFINITIONS -D_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS
  DEP_FILE = libcxx/src/CMakeFiles/cxx_static.dir/filesystem/operations.cpp.obj.d
  FLAGS = --target=x86_64-unknown-fuchsia -I/usr/local/google/home/phosek/clang-llvm/sdk/pkg/fdio/include -fPIC -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wstring-conversion -fno-omit-frame-pointer -fsanitize=address -fsanitize-address-use-after-scope -ffunction-sections -fdata-sections -ffile-prefix-map=/src/clang-llvm/llvm-build/fuchsia/runtimes/runtimes-x86_64-unknown-fuchsia+asan+noexcept-bins=../llvm-build/fuchsia/runtimes/runtimes-x86_64-unknown-fuchsia+asan+noexcept-bins -ffile-prefix-map=/src/clang-llvm/llvm-project/= -no-canonical-prefixes -O2 -g -DNDEBUG   -DLIBCXX_BUILDING_LIBCXXABI -fno-omit-frame-pointer -gline-tables-only -fsanitize=address -fsanitize=address -nostdinc++ -fvisibility-inlines-hidden -fvisibility=hidden -Wall -Wextra -W -Wwrite-strings -Wno-unused-parameter -Wno-long-long -Werror=return-type -Wextra-semi -Wno-user-defined-literals -Wno-covered-switch-default -Wno-ignored-attributes -Wno-error -fno-exceptions -include /src/clang-llvm/llvm-build/fuchsia/runtimes/runtimes-x86_64-unknown-fuchsia+asan+noexcept-bins/libcxx/__config_site -std=c++14
  INCLUDES = -Ilibcxx/include/c++build
  OBJECT_DIR = libcxx/src/CMakeFiles/cxx_static.dir
  OBJECT_FILE_DIR = libcxx/src/CMakeFiles/cxx_static.dir/filesystem

See the INCLUDES variable, in the latter case, -I/src/clang-llvm/llvm-build/fuchsia/include/c++/v1 is missing so compiler fails to find libc++ headers.

I have no idea why that happens. I inserted debugging prints into various places and I haven't seen any obvious issue. The only difference between the two runtime builds is (aside from ASan instrumentation) that in the latter case, compiler-rt isn't being built as part of runtimes, but I still don't see why that would should matter here.

[phosek]

In D82702#2146195, @phosek wrote:

I spent some time debugging this, and to be honest, I don't know what's going on. The problem is that in the multilib variant case, the header includes are missing. So normally, the generated Ninja entries would look like this:

build libcxx/src/CMakeFiles/cxx_static.dir/filesystem/operations.cpp.obj: CXX_COMPILER__cxx_static /src/clang-llvm/llvm-project/libcxx/src/filesystem/operations.cpp || cmake_object_order_depends_target_cxx_static
  DEFINES = -DNDEBUG -D_LIBCPP_BUILDING_LIBRARY -D_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS
  DEP_FILE = libcxx/src/CMakeFiles/cxx_static.dir/filesystem/operations.cpp.obj.d
  FLAGS = --target=x86_64-unknown-fuchsia -I/usr/local/google/home/phosek/clang-llvm/sdk/pkg/fdio/include -fPIC -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wstring-conversion -ffunction-sections -fdata-sections -ffile-prefix-map=/src/clang-llvm/llvm-build/fuchsia/runtimes/runtimes-x86_64-unknown-fuchsia-bins=../llvm-build/fuchsia/runtimes/runtimes-x86_64-unknown-fuchsia-bins -ffile-prefix-map=/src/clang-llvm/llvm-project/= -no-canonical-prefixes -O2 -g -DNDEBUG   -DLIBCXX_BUILDING_LIBCXXABI -nostdinc++ -fvisibility-inlines-hidden -fvisibility=hidden -Wall -Wextra -W -Wwrite-strings -Wno-unused-parameter -Wno-long-long -Werror=return-type -Wextra-semi -Wno-user-defined-literals -Wno-covered-switch-default -Wno-ignored-attributes -Wno-error -fvisibility-global-new-delete-hidden -include /src/clang-llvm/llvm-build/fuchsia/runtimes/runtimes-x86_64-unknown-fuchsia-bins/libcxx/__config_site -std=c++14
  INCLUDES = -Ilibcxx/include/c++build -I/src/clang-llvm/llvm-build/fuchsia/include/c++/v1
  OBJECT_DIR = libcxx/src/CMakeFiles/cxx_static.dir
  OBJECT_FILE_DIR = libcxx/src/CMakeFiles/cxx_static.dir/filesystem

but with this patch, we would end up with this:

build libcxx/src/CMakeFiles/cxx_static.dir/filesystem/operations.cpp.obj: CXX_COMPILER__cxx_static /src/clang-llvm/llvm-project/libcxx/src/filesystem/operations.cpp || cmake_object_order_depends_target_cxx_static
  DEFINES = -DNDEBUG -D_LIBCPP_BUILDING_LIBRARY -D_LIBCPP_DISABLE_NEW_DELETE_DEFINITIONS -D_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS
  DEP_FILE = libcxx/src/CMakeFiles/cxx_static.dir/filesystem/operations.cpp.obj.d
  FLAGS = --target=x86_64-unknown-fuchsia -I/usr/local/google/home/phosek/clang-llvm/sdk/pkg/fdio/include -fPIC -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wstring-conversion -fno-omit-frame-pointer -fsanitize=address -fsanitize-address-use-after-scope -ffunction-sections -fdata-sections -ffile-prefix-map=/src/clang-llvm/llvm-build/fuchsia/runtimes/runtimes-x86_64-unknown-fuchsia+asan+noexcept-bins=../llvm-build/fuchsia/runtimes/runtimes-x86_64-unknown-fuchsia+asan+noexcept-bins -ffile-prefix-map=/src/clang-llvm/llvm-project/= -no-canonical-prefixes -O2 -g -DNDEBUG   -DLIBCXX_BUILDING_LIBCXXABI -fno-omit-frame-pointer -gline-tables-only -fsanitize=address -fsanitize=address -nostdinc++ -fvisibility-inlines-hidden -fvisibility=hidden -Wall -Wextra -W -Wwrite-strings -Wno-unused-parameter -Wno-long-long -Werror=return-type -Wextra-semi -Wno-user-defined-literals -Wno-covered-switch-default -Wno-ignored-attributes -Wno-error -fno-exceptions -include /src/clang-llvm/llvm-build/fuchsia/runtimes/runtimes-x86_64-unknown-fuchsia+asan+noexcept-bins/libcxx/__config_site -std=c++14
  INCLUDES = -Ilibcxx/include/c++build
  OBJECT_DIR = libcxx/src/CMakeFiles/cxx_static.dir
  OBJECT_FILE_DIR = libcxx/src/CMakeFiles/cxx_static.dir/filesystem

See the INCLUDES variable, in the latter case, -I/src/clang-llvm/llvm-build/fuchsia/include/c++/v1 is missing so compiler fails to find libc++ headers.

I have no idea why that happens. I inserted debugging prints into various places and I haven't seen any obvious issue. The only difference between the two runtime builds is (aside from ASan instrumentation) that in the latter case, compiler-rt isn't being built as part of runtimes, but I still don't see why that would should matter here.

One more thing I've noticed is that both of those invocations include __config_site which is added by the cxx-headers interface library, so that library must be used, it's just as if target_include_directories(${CXX_HEADER_TARGET} INTERFACE ${output_dir}) was ignored when building the instrumented version.

[phosek]

In D82702#2146536, @phosek wrote:

One more thing I've noticed is that both of those invocations include __config_site which is added by the cxx-headers interface library, so that library must be used, it's just as if target_include_directories(${CXX_HEADER_TARGET} INTERFACE ${output_dir}) was ignored when building the instrumented version.

Another thing I just found out, this is only affecting paths that end with include/c++/v1, if I add any other path like include/c++/v2, it's works fine. So it seems likes something is filtering out paths that end with include/c++/v1 from include directories. I'm starting to suspect a bug in CMake (I'm using version 3.16.3 for reference).

[phosek]

libcxx/include/CMakeLists.txt

239

When I replace this line with:

if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC" OR "${CMAKE_CXX_SIMULATE_ID}" STREQUAL "MSVC")
  target_compile_options(${CXX_HEADER_TARGET} INTERFACE /I "${output_dir}")
else()
  target_compile_options(${CXX_HEADER_TARGET} INTERFACE -I "${output_dir}")
endif()

everything seems to be working. So whatever is filtering out the include path seems to be only affecting target_include_directories.

[EricWF]

IDK if I know what's causing the issue, but this change LGTM.

[ldionne]

Update with Petr's workaround

[aeubanks]

Hi, I think this is causing our Mac bots to fail when building libcxx, with

/opt/s/w/ir/cache/xcode_ios_11a1027.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ -DNDEBUG -D_LIBCPP_BUILDING_LIBRARY -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -DSTDC_CONSTANT_MACROS -DSTDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -Iprojects/libcxx/src -I/opt/s/w/ir/cache/builder/src/third_party/llvm/libcxx/src -I/opt/s/w/ir/cache/xcode_ios_11a1027.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/usr/include/libxml2 -Iinclude -I/opt/s/w/ir/cache/builder/src/third_party/llvm/llvm/include -Iprojects/libcxx/include/c++build -DLLVM_FORCE_HEAD_REVISION -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wno-long-long -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wstring-conversion -fdiagnostics-color -O3 -isysroot /opt/s/w/ir/cache/xcode_ios_11a1027.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -mmacosx-version-min=10.14 -DLIBCXX_BUILDING_LIBCXXABI -UNDEBUG -nostdinc++ -fvisibility-inlines-hidden -fvisibility=hidden -Wall -Wextra -W -Wwrite-strings -Wno-unused-parameter -Wno-long-long -Werror=return-type -Wextra-semi -Wno-user-defined-literals -Wno-covered-switch-default -Wno-ignored-attributes -Wno-error -std=c++14 -MD -MT projects/libcxx/src/CMakeFiles/cxx_experimental.dir/experimental/memory_resource.cpp.o -MF projects/libcxx/src/CMakeFiles/cxx_experimental.dir/experimental/memory_resource.cpp.o.d -o projects/libcxx/src/CMakeFiles/cxx_experimental.dir/experimental/memory_resource.cpp.o -c /opt/s/w/ir/cache/builder/src/third_party/llvm/libcxx/src/experimental/memory_resource.cpp
/opt/s/w/ir/cache/builder/src/third_party/llvm/libcxx/src/experimental/memory_resource.cpp:9:10: fatal error: 'experimental/memory_resource' file not found
#include "experimental/memory_resource"

^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1 error generated.

https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8874769072373415488/+/steps/gclient_runhooks/0/stdout

Is this intended?

[phosek]

In D82702#2151557, @aeubanks wrote:

Hi, I think this is causing our Mac bots to fail when building libcxx, with

/opt/s/w/ir/cache/xcode_ios_11a1027.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ -DNDEBUG -D_LIBCPP_BUILDING_LIBRARY -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -DSTDC_CONSTANT_MACROS -DSTDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -Iprojects/libcxx/src -I/opt/s/w/ir/cache/builder/src/third_party/llvm/libcxx/src -I/opt/s/w/ir/cache/xcode_ios_11a1027.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/usr/include/libxml2 -Iinclude -I/opt/s/w/ir/cache/builder/src/third_party/llvm/llvm/include -Iprojects/libcxx/include/c++build -DLLVM_FORCE_HEAD_REVISION -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wno-long-long -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wstring-conversion -fdiagnostics-color -O3 -isysroot /opt/s/w/ir/cache/xcode_ios_11a1027.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -mmacosx-version-min=10.14 -DLIBCXX_BUILDING_LIBCXXABI -UNDEBUG -nostdinc++ -fvisibility-inlines-hidden -fvisibility=hidden -Wall -Wextra -W -Wwrite-strings -Wno-unused-parameter -Wno-long-long -Werror=return-type -Wextra-semi -Wno-user-defined-literals -Wno-covered-switch-default -Wno-ignored-attributes -Wno-error -std=c++14 -MD -MT projects/libcxx/src/CMakeFiles/cxx_experimental.dir/experimental/memory_resource.cpp.o -MF projects/libcxx/src/CMakeFiles/cxx_experimental.dir/experimental/memory_resource.cpp.o.d -o projects/libcxx/src/CMakeFiles/cxx_experimental.dir/experimental/memory_resource.cpp.o -c /opt/s/w/ir/cache/builder/src/third_party/llvm/libcxx/src/experimental/memory_resource.cpp
/opt/s/w/ir/cache/builder/src/third_party/llvm/libcxx/src/experimental/memory_resource.cpp:9:10: fatal error: 'experimental/memory_resource' file not found
#include "experimental/memory_resource"

^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1 error generated.

https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8874769072373415488/+/steps/gclient_runhooks/0/stdout

Is this intended?

This looks the issue I've been investigating, but it seems like the workaround I suggested doesn't help here.

[aeubanks]

Is it ok to revert this then?

[ldionne]

In D82702#2151903, @aeubanks wrote:

Is it ok to revert this then?

No.

The issue is that your builder builds with -DLIBCXX_ENABLE_SHARED=OFF -DLIBCXX_ENABLE_STATIC=OFF, but LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY is implicitly set to ON. In other words, you're trying to build the libc++ experimental library without building the static or the shared library. If you're trying to disable libc++ from the build, why include it in LLVM_ENABLE_PROJECTS?

[aeubanks]

In D82702#2153130, @ldionne wrote:

In D82702#2151903, @aeubanks wrote:

Is it ok to revert this then?

No.

The issue is that your builder builds with -DLIBCXX_ENABLE_SHARED=OFF -DLIBCXX_ENABLE_STATIC=OFF, but LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY is implicitly set to ON. In other words, you're trying to build the libc++ experimental library without building the static or the shared library. If you're trying to disable libc++ from the build, why include it in LLVM_ENABLE_PROJECTS?

Ah, thanks for the explanation.
Does it make sense for LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY to only be implicitly set to ON if either of LIBCXX_ENABLE_SHARED or LIBCXX_ENABLE_STATIC are on?

[thakis]

In D82702#2153130, @ldionne wrote:

In D82702#2151903, @aeubanks wrote:

Is it ok to revert this then?

No.

The issue is that your builder builds with -DLIBCXX_ENABLE_SHARED=OFF -DLIBCXX_ENABLE_STATIC=OFF, but LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY is implicitly set to ON. In other words, you're trying to build the libc++ experimental library without building the static or the shared library. If you're trying to disable libc++ from the build, why include it in LLVM_ENABLE_PROJECTS?

See https://bugs.chromium.org/p/chromium/issues/detail?id=1067216#c7 for background :)

[ldionne]

In D82702#2153620, @aeubanks wrote:

Ah, thanks for the explanation.
Does it make sense for LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY to only be implicitly set to ON if either of LIBCXX_ENABLE_SHARED or LIBCXX_ENABLE_STATIC are on?

Either that, or we could have cxx_experimental link against cxx-headers, which would make the build work (even thought that's not what you want). Or we could issue an error to make it clearer.

I think the most consistent thing to do is to link cxx_experimental against cxx-headers, since it does use the libc++ headers after all.

In D82702#2153626, @thakis wrote:

See https://bugs.chromium.org/p/chromium/issues/detail?id=1067216#c7 for background :)

Fair enough.

[bcain]

I'm troubleshooting a build failure that happens when this commit lands on a downstream builder. The symptom looks similar to the one reported by @aeubanks where the includes aren't present but I've ruled out the EXPERIMENTAL_LIBRARY (explicitly set OFF in the failing build).

Can someone share a link to a reference bot that is still working well after this change? Maybe I have the wrong recipe.

I was hoping that I could use the premerge check associated with this review as a reference builder but that one seems to fail as well (but a different symptom: attempt to explicitly link to "cxxabi_shared" fails -- old cmake? or related?)

https://buildkite.com/llvm-project/premerge-checks/builds/3740#35262bb7-c20c-4117-9f03-1811a95be68b

libcxx/CMakeLists.txt

if(LIBCXX_LIBDIR_SUBDIR)
     string(APPEND LIBCXX_INSTALL_LIBRARY_DIR /${LIBCXX_LIBDIR_SUBDIR})
   endif()
 elseif(LLVM_LIBRARY_OUTPUT_INTDIR)
   set(LIBCXX_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR})
   set(LIBCXX_HEADER_DIR  ${LLVM_BINARY_DIR})
   set(LIBCXX_INSTALL_LIBRARY_DIR lib${LIBCXX_LIBDIR_SUFFIX})
 else()
   set(LIBCXX_LIBRARY_DIR ${CMAKE_BINARY_DIR}/lib${LIBCXX_LIBDIR_SUFFIX})
+  set(LIBCXX_HEADER_DIR  ${CMAKE_BINARY_DIR})
   set(LIBCXX_INSTALL_LIBRARY_DIR lib${LIBCXX_LIBDIR_SUFFIX})
 endif()
 
 file(MAKE_DIRECTORY "${LIBCXX_BINARY_INCLUDE_DIR}")
 
 set(LIBCXX_INSTALL_PREFIX "" CACHE STRING "Define libc++ destination prefix.")
 set(LIBCXX_INSTALL_HEADER_PREFIX "" CACHE STRING "Define libc++ header destination prefix.")
 

 # By default libc++ on Windows expects to use a shared library, which requires
 # the headers to use DLL import/export semantics. However when building a
 # static library only we modify the headers to disable DLL import/export.
 if (DEFINED WIN32 AND LIBCXX_ENABLE_STATIC AND NOT LIBCXX_ENABLE_SHARED)
   message(STATUS "Generating custom __config for non-DLL Windows build")
   config_define(ON _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
 endif()
 
-# We generate a __config_site header (see libcxx/include/CMakeLists.txt) and
-# we make sure to include it when building the library.
-function(cxx_add_config_site target)
-  if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC" OR "${CMAKE_CXX_SIMULATE_ID}" STREQUAL "MSVC")
-    target_compile_options(${target} PUBLIC /FI "${LIBCXX_BINARY_DIR}/__config_site")
-  else()
-    target_compile_options(${target} PUBLIC -include "${LIBCXX_BINARY_DIR}/__config_site")
-  endif()
-endfunction()
-
 # Setup all common build flags =================================================
 function(cxx_add_common_build_flags target)
   cxx_add_basic_build_flags(${target})
   cxx_add_warning_flags(${target})
   cxx_add_windows_flags(${target})
-  cxx_add_config_site(${target})
   cxx_add_exception_flags(${target})
   cxx_add_rtti_flags(${target})
   cxx_add_module_flags(${target})
   cxx_link_system_libraries(${target})
 endfunction()
 
 #===============================================================================
 # Setup Source Code And Tests
 #===============================================================================
-include_directories(include)
 add_subdirectory(include)
 add_subdirectory(src)
 
 set(LIBCXX_TEST_DEPS "")
 
 if (LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
   list(APPEND LIBCXX_TEST_DEPS cxx_experimental)
 endif()

libcxx/cmake/Modules/DefineLinkerScript.cmake

function(define_linker_script target)
   set(symlink "$<TARGET_LINKER_FILE:${target}>")
   set(soname "$<TARGET_SONAME_FILE_NAME:${target}>")
 
   get_target_property(interface_libs "${target}" INTERFACE_LINK_LIBRARIES)
 
   set(link_libraries)
   if (interface_libs)
     foreach(lib IN LISTS interface_libs)
+      if ("${lib}" STREQUAL "cxx-headers")
+        continue()
+      endif()
       if (TARGET "${lib}" OR
           (${lib} MATCHES "cxxabi(_static|_shared)?" AND HAVE_LIBCXXABI) OR
           (${lib} MATCHES "unwind(_static|_shared)?" AND HAVE_LIBUNWIND))
         list(APPEND link_libraries "${CMAKE_LINK_LIBRARY_FLAG}$<TARGET_PROPERTY:${lib},OUTPUT_NAME>")
       else()
         list(APPEND link_libraries "${CMAKE_LINK_LIBRARY_FLAG}${lib}")
       endif()
     endforeach()

libcxx/include/CMakeLists.txt

COMMAND ${Python3_EXECUTABLE} ${LIBCXX_SOURCE_DIR}/utils/cat_files.py
     ${LIBCXX_SOURCE_DIR}/include/__config
     -o ${LIBCXX_BINARY_DIR}/__generated_config
   DEPENDS ${LIBCXX_SOURCE_DIR}/include/__config
           ${LIBCXX_BINARY_DIR}/__config_site
 )
 # Add a target that executes the generation commands.
 add_custom_target(cxx-generated-config ALL
   DEPENDS ${LIBCXX_BINARY_DIR}/__generated_config)
-set(generated_config_deps cxx-generated-config)
 
 # In some build configurations (like bootstrapping clang), we need to be able to
 # install the libcxx headers before the CMake configuration for libcxx runs. Making
 # the name of this target configurable allows LLVM/runtimes/CMakeLists.txt to
 # add this subdirectory to the LLVM build to put libcxx's headers in place
 # before libcxx's build configuration is run.
 if (NOT CXX_HEADER_TARGET)
   set(CXX_HEADER_TARGET cxx-headers)
add_custom_command(OUTPUT ${dst}
       COMMENT "Copying CXX header ${f}")
     list(APPEND out_files ${dst})
   endforeach()
 
   # Copy the generated header as __config into build directory.
   set(src ${LIBCXX_BINARY_DIR}/__generated_config)
   set(dst ${output_dir}/__config)
   add_custom_command(OUTPUT ${dst}
-      DEPENDS ${src} ${generated_config_deps}
+      DEPENDS ${src} cxx-generated-config
       COMMAND ${CMAKE_COMMAND} -E copy_if_different ${src} ${dst}
       COMMENT "Copying CXX __config")
   list(APPEND out_files ${dst})
+  add_custom_target(generate-cxx-headers DEPENDS ${out_files})
 
-  add_custom_target(${CXX_HEADER_TARGET} ALL DEPENDS ${out_files} ${LIBCXX_CXX_ABI_HEADER_TARGET})
+  add_library(${CXX_HEADER_TARGET} INTERFACE)
+  add_dependencies(${CXX_HEADER_TARGET} 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_HEADER_TARGET} INTERFACE /I "${output_dir}")
   else()
-  add_custom_target(${CXX_HEADER_TARGET})
+    target_compile_options(${CXX_HEADER_TARGET} INTERFACE -I "${output_dir}")
+  endif()
+
+  # Make sure the generated __config_site header is included when we build the library.
+  if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC" OR "${CMAKE_CXX_SIMULATE_ID}" STREQUAL "MSVC")
+    target_compile_options(${CXX_HEADER_TARGET} INTERFACE /FI "${LIBCXX_BINARY_DIR}/__config_site")
+  else()
+    target_compile_options(${CXX_HEADER_TARGET} INTERFACE -include "${LIBCXX_BINARY_DIR}/__config_site")
+  endif()
+else()
+  add_library(${CXX_HEADER_TARGET} INTERFACE)
 endif()
-set_target_properties(${CXX_HEADER_TARGET} PROPERTIES FOLDER "Misc")
 
 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}
       COMPONENT ${CXX_HEADER_TARGET}
       PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
     )
   endforeach()
 
   # Install the generated header as __config.
   install(FILES ${LIBCXX_BINARY_DIR}/__generated_config
     DESTINATION ${LIBCXX_INSTALL_HEADER_PREFIX}include/c++/v1
     PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
     RENAME __config
     COMPONENT ${CXX_HEADER_TARGET})
 
   if (NOT CMAKE_CONFIGURATION_TYPES)
     add_custom_target(install-${CXX_HEADER_TARGET}
-                      DEPENDS ${CXX_HEADER_TARGET} ${generated_config_deps}
+                      DEPENDS ${CXX_HEADER_TARGET} cxx-generated-config
                       COMMAND "${CMAKE_COMMAND}"
                               -DCMAKE_INSTALL_COMPONENT=${CXX_HEADER_TARGET}
                               -P "${CMAKE_BINARY_DIR}/cmake_install.cmake")
     # Stripping is a no-op for headers
     add_custom_target(install-${CXX_HEADER_TARGET}-stripped DEPENDS install-${CXX_HEADER_TARGET})
   endif()
 endif()

libcxx/src/CMakeLists.txt

 split_list(LIBCXX_LINK_FLAGS)
 
 # Build the shared library.
 if (LIBCXX_ENABLE_SHARED)
   add_library(cxx_shared SHARED ${exclude_from_all} ${LIBCXX_SOURCES} ${LIBCXX_HEADERS})
   if(COMMAND llvm_setup_rpath)
     llvm_setup_rpath(cxx_shared)
   endif()
-  target_link_libraries(cxx_shared PRIVATE ${LIBCXX_LIBRARIES})
+  target_link_libraries(cxx_shared PUBLIC cxx-headers
+                                   PRIVATE ${LIBCXX_LIBRARIES})
   set_target_properties(cxx_shared
     PROPERTIES
       COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
       LINK_FLAGS    "${LIBCXX_LINK_FLAGS}"
       OUTPUT_NAME   "c++"
       VERSION       "${LIBCXX_ABI_VERSION}.0"
       SOVERSION     "${LIBCXX_ABI_VERSION}"
       DEFINE_SYMBOL ""
if(WIN32 AND NOT MINGW AND NOT "${CMAKE_HOST_SYSTEM_NAME}" STREQUAL "Windows")
     set_target_properties(cxx_shared PROPERTIES
                           APPEND_STRING PROPERTY LINK_FLAGS " /MANIFEST:NO")
   endif()
 endif()
 
 # Build the static library.
 if (LIBCXX_ENABLE_STATIC)
   add_library(cxx_static STATIC ${exclude_from_all} ${LIBCXX_SOURCES} ${LIBCXX_HEADERS})
-  target_link_libraries(cxx_static PRIVATE ${LIBCXX_LIBRARIES})
+  target_link_libraries(cxx_static PUBLIC cxx-headers
+                                   PRIVATE ${LIBCXX_LIBRARIES})
   set(CMAKE_STATIC_LIBRARY_PREFIX "lib")
   set_target_properties(cxx_static
     PROPERTIES
       COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
       LINK_FLAGS    "${LIBCXX_LINK_FLAGS}"
       OUTPUT_NAME   "c++"
   )
   cxx_add_common_build_flags(cxx_static)
ARGS
       "${MERGE_ARCHIVES_ABI_TARGET}"
       "${MERGE_ARCHIVES_SEARCH_PATHS}"
     WORKING_DIRECTORY ${LIBCXX_BUILD_DIR}
     )
   endif()
 endif()
 
 # Add a meta-target for both libraries.
-add_custom_target(cxx DEPENDS cxx-headers ${LIBCXX_BUILD_TARGETS})
+add_custom_target(cxx DEPENDS ${LIBCXX_BUILD_TARGETS})
 
 if (LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
   set(LIBCXX_EXPERIMENTAL_SOURCES
     experimental/memory_resource.cpp
     )
   add_library(cxx_experimental STATIC ${LIBCXX_EXPERIMENTAL_SOURCES})
   if (LIBCXX_ENABLE_SHARED)
     target_link_libraries(cxx_experimental PRIVATE cxx_shared)