build: avoid hardcoding the libxml2 library name

[compnerd]

Summary

FindLibXml2 will set the LIBXML2_LIBRARIES variable to the libraries that we must link against. This will be an empty string if libxml2 is not found. Avoid hardcoding the library name as xml2 in the configuration. Simplify the usage in the WindowsManifest library.

[beanz]

LGTM

[compnerd]

GIT 340e7c0b77a

[srj]

This has injected a bug into llvm-config --system-libs (at least, on OSX and Linux) for me; previously, the output would be something like

-lz -lrt -ldl -lpthread -lm -lxml2

but now, it's

-lz -lrt -ldl -lpthread -lm -l/usr/lib/x86_64-linux-gnu/libxml2.so

which is not a legal set of args to the linker. (This is breaking Halide's downstream use of LLVM, as it relies on llvm-config --system-libs.)

[srj]

I haven't found any reasonable workarounds for this breakage; CMake is apparently splitting -l/usr/lib/x86_64-linux-gnu/libxml2.so into -l/usr/lib/x86_64 and -linux-gnu/libxml2.so, neither of which are valid dependencies, so our builds are totally broken. If a fix isn't forthcoming, can we please roll this change back at your earliest convenience?

[compnerd]

There isn't a workaround for this, its just a bug in our CMake. I've fixed the issue with our CMake logic in GIT 372ad32734ecb455f9fb4d0601229ca2dfc78b66.

cmake/config-ix.cmake

if((LLVM_ENABLE_LIBXML2) AND ((CMAKE_SYSTEM_NAME MATCHES "Linux") AND (ICONV_LIBRARY_PATH) OR APPLE))
       find_package(LibXml2)
       if (LIBXML2_FOUND)
         set(LLVM_LIBXML2_ENABLED 1)
         if ((CMAKE_OSX_SYSROOT) AND (EXISTS ${CMAKE_OSX_SYSROOT}/${LIBXML2_INCLUDE_DIR}))
           include_directories(${CMAKE_OSX_SYSROOT}/${LIBXML2_INCLUDE_DIR})
         else()
           include_directories(${LIBXML2_INCLUDE_DIR})
         endif()
-        set(LIBXML2_LIBS "xml2")
       endif()
     endif()
   endif()
 endif()
 
 if (LLVM_ENABLE_LIBXML2 STREQUAL "FORCE_ON" AND NOT LLVM_LIBXML2_ENABLED)
   message(FATAL_ERROR "Failed to congifure libxml2")
 endif()

lib/WindowsManifest/CMakeLists.txt

-set(system_libs)
-if( CMAKE_HOST_UNIX )
-  if( LLVM_LIBXML2_ENABLED )
-    set(system_libs ${system_libs} ${LIBXML2_LIBS})
-  endif()
-endif()
-
 add_llvm_library(LLVMWindowsManifest
   WindowsManifestMerger.cpp
 
   ADDITIONAL_HEADER_DIRS
   ${LLVM_MAIN_INCLUDE_DIR}/llvm/WindowsManifest
   ${Backtrace_INCLUDE_DIRS}
 
-  LINK_LIBS ${system_libs}
+  LINK_LIBS ${LIBXML2_LIBRARIES}
   )
-
-set_property(TARGET LLVMWindowsManifest PROPERTY LLVM_SYSTEM_LIBS "${system_libs}")
+set_target_properties(LLVMWindowsManifest PROPERTIES
+  LLVM_SYSTEM_LIBS ${LIBXML2_LIBRARIES})