[libcxxabi][cmake] Allow building without libcxx again

[Ericson2314]

Summary

This allows -DLLVM_ENABLE_RUNTIMES=libcxxabi to work.

This functionality was removed in D125561 (among other such removals) as
dead code, because it was only available as part of the standalone build
before.

The functionality as added back as it was, except target_include_directories is used instead of the cruder target_compile_options (with an MSVC vs not MSVC conditional split). That is just wholly better.

[Ericson2314]

Ah I would have to check both lists to support the legacy case. Ew.

D132324 removes support for the deprecated legacy case, in which case this will pass CI.

[phosek]

libcxxabi/CMakeLists.txt

143–147

Can yo use target_include_directories instead?

[Ericson2314]

libcxxabi/CMakeLists.txt

143–147

I had gotten simply by adding back what was removed, but yes target_include_directories would be much better.

[Ericson2314]

Use target_include_directories

[phosek]

LGTM

[Ericson2314]

The legacy build is no longer CI'd so the one failure here is no longer an issue.

[kadircet]

this is breaking builds with:
cmake -G Ninja ../llvm "-DLLVM_ENABLE_PROJECTS=all" "-DCMAKE_BUILD_TYPE=Release" "-DCMAKE_C_COMPILER=clang" "-DCMAKE_CXX_COMPILER=clang++"

Looking at https://libcxx.llvm.org/BuildingLibcxx.html, I assume these have been deprecated for a while but there were no warnings from CMake. Moreover LIBCXXABI_LIBCXX_INCLUDES was not being used or required previously by default at all (and LLVM_ENABLE_RUNTIMES).

After this change it enforces either having LLVM_ENABLE_RUNTIMES or LIBCXXABI_LIBCXX_INCLUDES set in the build configuration to build libcxx. Since I am not familiar with libcxx I can't really say if that's the desired behavior. Can you explicitly verify that's the case (and if so tell me how the builds with the config mentioned above was working above? they weren't able to build libcxx at all?)

libcxxabi/CMakeLists.txt

136

all is also a possible value for LLVM_ENABLE_RUNTIMES (here and below, in test/CMakeLists.txt) can you get that fixed? see

[kadircet]

Looks like the error messages didn't show up because expansion of all happened after the check. sent out D132454 to fix it.

since libcxx(abi) can't be build through llvm_enable_projects, should we exclude it from the list in https://github.com/llvm/llvm-project/blob/main/llvm/CMakeLists.txt#L112 ?

libcxxabi/CMakeLists.txt

136

okay this was happening because LLVM_ENABLE_PROJECTS=all would also include this directory, without going through runtimes, which performs the expansion.

[Ericson2314]

Thanks for that sleuthing, @kadircet.

Also see D132411 which should help fix some things too.

libcxxabi/CMakeLists.txt

 option(LIBCXXABI_SILENT_TERMINATE "Set this to make the terminate handler default to a silent alternative" OFF)
 option(LIBCXXABI_NON_DEMANGLING_TERMINATE "Set this to make the terminate handler
 avoid demangling" OFF)
 
 if (NOT LIBCXXABI_ENABLE_SHARED AND NOT LIBCXXABI_ENABLE_STATIC)
   message(FATAL_ERROR "libc++abi must be built as either a shared or static library.")
 endif()
 
-# TODO: Remove this, which shouldn't be necessary since we know we're being built
-#       side-by-side with libc++.
 set(LIBCXXABI_LIBCXX_INCLUDES "" CACHE PATH
     "Specify path to libc++ includes.")
+if (NOT libcxx IN_LIST LLVM_ENABLE_RUNTIMES)
+  if (NOT IS_DIRECTORY ${LIBCXXABI_LIBCXX_INCLUDES})
+    message(FATAL_ERROR
+      "LIBCXXABI_LIBCXX_INCLUDES=${LIBCXXABI_LIBCXX_INCLUDES} is not a valid directory. "
+      "Please provide the path to where the libc++ headers have been installed.")
+  endif()
+  add_library(cxx-headers INTERFACE)
+  target_include_directories(cxx-headers INTERFACE "${LIBCXXABI_LIBCXX_INCLUDES}")
+endif()
 
 set(LIBCXXABI_HERMETIC_STATIC_LIBRARY_DEFAULT OFF)
 if (WIN32)
   set(LIBCXXABI_HERMETIC_STATIC_LIBRARY_DEFAULT ON)
 endif()
 option(LIBCXXABI_HERMETIC_STATIC_LIBRARY
   "Do not export any symbols from the static library." ${LIBCXXABI_HERMETIC_STATIC_LIBRARY_DEFAULT})
 
 if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
   set(LIBCXXABI_DEFAULT_TEST_CONFIG "llvm-libc++abi-shared-gcc.cfg.in")
 elseif(MINGW)

libcxxabi/test/CMakeLists.txt

 else()
   set(LIBCXXABI_TEST_DEPS cxxabi_static)
 endif()
 
 if (LIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY)
   list(APPEND LIBCXXABI_TEST_DEPS cxx_external_threads)
 endif()
 
+if(libcxx IN_LIST LLVM_ENABLE_RUNTIMES)
   list(APPEND LIBCXXABI_TEST_DEPS cxx)
+endif()
+if(libunwind IN_LIST LLVM_ENABLE_RUNTIMES)
   if (LIBCXXABI_USE_LLVM_UNWINDER AND TARGET unwind)
     list(APPEND LIBCXXABI_TEST_DEPS unwind)
   endif()
+endif()
 
 set(AUTO_GEN_COMMENT "## Autogenerated by libcxxabi configuration.\n# Do not edit!")
 set(SERIALIZED_LIT_PARAMS "# Lit parameters serialized here for llvm-lit to pick them up\n")
 
 macro(serialize_lit_param param value)
   string(APPEND SERIALIZED_LIT_PARAMS "config.${param} = ${value}\n")
 endmacro()