CMake: Turn LLVM_ENABLE_LIBXML2 into a tri-state option

[hans]

Summary

In addition to the current ON and OFF options, this adds the FORCE_ON
option, which causes a configuration error if libxml2 cannot be used.

[hans]

I tested locally that if there's a current LLVM_ENABLE_LIBXML2:BOOL in CMakeCache.txt, that doesn't break things. There is no warning or error, it just seems to ignore the new string option and use the cached bool one.

So I guess this means it won't disrupt current builds, and builds that want to use FORCE_ON need to nuke their cache and reconfigure, which is expected.

[rnk]

lgtm

cmake/config-ix.cmake

163 ↗

(On Diff #122913)

I guess the string value OFF evaluates to false in this context? Scary loose typing, batman!

[hans]

cmake/config-ix.cmake

163 ↗

(On Diff #122913)

Yup, also "0, OFF, NO, FALSE, N, IGNORE, NOTFOUND, the empty string, or ends in the suffix -NOTFOUND" :-)
https://cmake.org/cmake/help/v3.0/command/if.html

llvm/trunk/CMakeLists.txt

 set(FFI_LIBRARY_DIR "" CACHE PATH "Additional directory, where CMake should search for libffi.so")
 set(FFI_INCLUDE_DIR "" CACHE PATH "Additional directory, where CMake should search for ffi.h or ffi/ffi.h")
 
 set(LLVM_TARGET_ARCH "host"
   CACHE STRING "Set target to use for LLVM JIT or use \"host\" for automatic detection.")
 
 option(LLVM_ENABLE_TERMINFO "Use terminfo database if available." ON)
 
-option(LLVM_ENABLE_LIBXML2 "Use libxml2 if available." ON)
+set(LLVM_ENABLE_LIBXML2 "ON" CACHE STRING "Use libxml2 if available. Can be ON, OFF, or FORCE_ON")
 
 option(LLVM_ENABLE_LIBEDIT "Use libedit if available." ON)
 
 option(LLVM_ENABLE_THREADS "Use threads if available." ON)
 
 option(LLVM_ENABLE_ZLIB "Use zlib for compression/decompression if available." ON)
 
 if( LLVM_TARGETS_TO_BUILD STREQUAL "all" )

llvm/trunk/cmake/config-ix.cmake

if((LLVM_ENABLE_LIBXML2) AND ((CMAKE_SYSTEM_NAME MATCHES "Linux") AND (ICONV_LIBRARY_PATH) OR APPLE))
     if (LIBXML2_FOUND)
       set(LLVM_LIBXML2_ENABLED 1)
       include_directories(${LIBXML2_INCLUDE_DIR})
       set(LIBXML2_LIBS "xml2")
     endif()
   endif()
 endif()
 
+if (LLVM_ENABLE_LIBXML2 STREQUAL "FORCE_ON" AND NOT LLVM_LIBXML2_ENABLED)
+  message(FATAL_ERROR "Failed to congifure libxml2")
+endif()
+
 check_library_exists(xar xar_open "" HAVE_LIBXAR)
 if(HAVE_LIBXAR)
   set(XAR_LIB xar)
 endif()
 
 # function checks
 check_symbol_exists(arc4random "stdlib.h" HAVE_DECL_ARC4RANDOM)
 find_package(Backtrace)