CMake: Check LLVM_ENABLE_LIBXML2 in clang

[rinon]

Summary

If LLVM_ENABLE_LIBXML2=OFF, we should not attempt to link clang against
libxml2.

[rnk]

The only think that links against libxml2 is c-index-test. Surely nobody cares about installing c-index-test on some other machine that lacks the local version of libxml2. What's the use case for this to justify the complexity of interacting with LLVM's configuration options?

[rinon]

I'd be fine with not building c-index-test when building clang, but I didn't see any easy way to disable it.

My use-case here is just to build LLVM+clang without having to link against my system libxml2 at all. It's a rather weird build situation, but Android builds a toolchain against a fixed, prebuilt set of headers. My system libxml2 expects unicode support that these prebuilt headers don't have, so it's much simpler to just disable libxml2 support. This led me to the realization that you can't actually disable libxml2 support for clang, which struck me as odd since it's so unimportant.

[rnk]

lgtm

[shafik]

Is this change still relevant of can we close this?

[rinon]

Looks like this was effectively implemented in c4d75361, so this change is no longer necessary.

CMakeLists.txt

if( CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR )
   else()
     set(LLVM_SHLIB_OUTPUT_INTDIR ${LLVM_LIBRARY_OUTPUT_INTDIR})
   endif()
 
   option(LLVM_ENABLE_WARNINGS "Enable compiler warnings." ON)
   option(LLVM_INSTALL_TOOLCHAIN_ONLY
     "Only include toolchain files in the 'install' target." OFF)
 
+  # Ensure that LLVM_ENABLE_LIBXML2 is set for standalone builds
+  if(NOT DEFINED LLVM_ENABLE_LIBXML2)
+    set(LLVM_ENABLE_LIBXML2 "ON" CACHE STRING "Use libxml2 if available. Can be ON, OFF, or FORCE_ON")
+  endif()
+
   option(LLVM_FORCE_USE_OLD_HOST_TOOLCHAIN
     "Set to ON to force using an old, unsupported host toolchain." OFF)
   option(CLANG_ENABLE_BOOTSTRAP "Generate the clang bootstrap target" OFF)
 
   include(AddLLVM)
   include(TableGen)
   include(HandleLLVMOptions)
   include(VersionFromVCS)

 endif()
 
 # Make sure that our source directory is on the current cmake module path so that
 # we can include cmake files from this directory.
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")
 
 # Don't look for libxml if we're using MSan, since uninstrumented third party
 # code may call MSan interceptors like strlen, leading to false positives.
-if(NOT LLVM_USE_SANITIZER MATCHES "Memory.*")
+if((LLVM_ENABLE_LIBXML2) AND (NOT LLVM_USE_SANITIZER MATCHES "Memory.*"))
   set (LIBXML2_FOUND 0)
   find_package(LibXml2 2.5.3 QUIET)
   if (LIBXML2_FOUND)
     set(CLANG_HAVE_LIBXML 1)
   endif()
 endif()
 
+if (LLVM_ENABLE_LIBXML2 STREQUAL "FORCE_ON" AND NOT CLANG_HAVE_LIBXML)
+  message(FATAL_ERROR "Failed to configure libxml2")
+endif()
+
 include(CheckIncludeFile)
 check_include_file(sys/resource.h CLANG_HAVE_RLIMITS)
 
 set(CLANG_RESOURCE_DIR "" CACHE STRING
   "Relative directory from the Clang binary to its resource files.")
 
 set(C_INCLUDE_DIRS "" CACHE STRING
   "Colon separated list of directories clang will search for headers.")