cmake: Install gtest lib&headers, to use in stand-alone builds

[mgorny]

Summary

Install gtest & gtest_main libraries and the gtest headers in dedicated subdirectories to support stand-alone builds of external LLVM projects. Use dedicated subdirectories in order to clearly indicate that the files are not parts of the LLVM API and avoid collisions with other packages (and system-wide installs of gtest).

The libraries are installed to 'llvm' subdirectory of library directory, while the headers are installed to 'llvm-gtest' subdirectory of header directory. The library names are unchanged so that all existing rules in LLVM and external projects continue to work. Additional target checks are added to unittests subdirectory to prevent target collisions in
external projects adding the utils/unittest directory to their CMake build.

Full compatibility for existing builds is preserved, both for combined builds of LLVM and subprojects, and for stand-alone builds done using LLVM source directory. In the latter case, the installed gtest library will be preferred over building another one if available. Additionally, it is now possible to do unittest builds in external projects against
installed LLVM after adding the necessary include directory.

Example use in clang: https://reviews.llvm.org/D23959

[mgorny]

Turns out it's not that simple. For example, compiler-rt uses LLVM-provided gtest sources to build gtest with different -D flags. I don't know if we would be able to come up with a common gtest build that would satisfy all the projects.

cmake/modules/AddLLVM.cmake

if ("${name}" STREQUAL gtest OR "${name}" STREQUAL gtest_main)
     set(_is_gtest TRUE)
   else()
     set(_is_gtest FALSE)
     set_property( GLOBAL APPEND PROPERTY LLVM_LIBS ${name} )
   endif()
 
   if( EXCLUDE_FROM_ALL )
     set_target_properties( ${name} PROPERTIES EXCLUDE_FROM_ALL ON)
-  elseif(NOT _is_gtest)
+  else()
     if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY OR ${name} STREQUAL "LTO")
       set(install_dir lib${LLVM_LIBDIR_SUFFIX})
       if(ARG_SHARED OR BUILD_SHARED_LIBS)
         if(WIN32 OR CYGWIN OR MINGW)
           set(install_type RUNTIME)
           set(install_dir bin)
         else()
           set(install_type LIBRARY)
         endif()
       else()
         set(install_type ARCHIVE)
       endif()
 
+      if( _is_gtest )
+        set(install_dir "${install_dir}/llvm")
+      endif()
+
       install(TARGETS ${name}
             EXPORT LLVMExports
             ${install_type} DESTINATION ${install_dir}
             COMPONENT ${name})
 
       if (NOT CMAKE_CONFIGURATION_TYPES)
         add_custom_target(install-${name}
                           DEPENDS ${name}
                           COMMAND "${CMAKE_COMMAND}"
                                   -DCMAKE_INSTALL_COMPONENT=${name}
                                   -P "${CMAKE_BINARY_DIR}/cmake_install.cmake")
       endif()
     endif()
     set_property(GLOBAL APPEND PROPERTY LLVM_EXPORTS ${name})
   endif()
+
+  if( NOT _is_gtest )
     set_target_properties(${name} PROPERTIES FOLDER "Libraries")
+  else()
+    set_target_properties(${name} PROPERTIES FOLDER "Internal use libraries")
+  endif()
 endmacro(add_llvm_library name)
 
 macro(add_llvm_loadable_module name)
   llvm_add_library(${name} MODULE ${ARGN})
   if(NOT TARGET ${name})
     # Add empty "phony" target
     add_custom_target(${name})
   else()

utils/unittest/CMakeLists.txt

 # Project-wide settings
 
 # Where gtest's .h files can be found.
 include_directories(
   googletest/include
   googletest
   )
 
+# Define conditionally to avoid breaking revdeps that explicitly include
+# this directory to obtain gtest target (exported now)
+if( NOT TARGET gtest )
   if(WIN32)
     add_definitions(-DGTEST_OS_WINDOWS=1)
   endif()
 
   if(SUPPORTS_NO_VARIADIC_MACROS_FLAG)
     add_definitions("-Wno-variadic-macros")
   endif()
 
   set(LLVM_REQUIRES_RTTI 1)
   add_definitions( -DGTEST_HAS_RTTI=0 )
 
   if (NOT LLVM_ENABLE_THREADS)
     add_definitions( -DGTEST_HAS_PTHREAD=0 )
   endif()
 
   find_library(PTHREAD_LIBRARY_PATH pthread)
   if (PTHREAD_LIBRARY_PATH)
     list(APPEND LIBS pthread)
   endif()
 
   add_llvm_library(gtest
     googletest/src/gtest-all.cc
 
     LINK_LIBS
     ${LIBS}
 
     LINK_COMPONENTS
     Support # Depends on llvm::raw_ostream
   )
 
+  if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
+    install(DIRECTORY googletest/include/
+      DESTINATION include/llvm-gtest
+      COMPONENT llvm-gtest-headers
+      FILES_MATCHING
+      PATTERN "*.h"
+      )
+
+    if (NOT CMAKE_CONFIGURATION_TYPES)
+      add_custom_target(installgtesthdrs
+                        DEPENDS ${name}
+                        COMMAND "${CMAKE_COMMAND}"
+                                -DCMAKE_INSTALL_COMPONENT=llvm-gtest-headers
+                                -P "${CMAKE_BINARY_DIR}/cmake_install.cmake")
+    endif()
+  endif()
+endif()
+
 add_subdirectory(UnitTestMain)

utils/unittest/UnitTestMain/CMakeLists.txt

+# Define conditionally to avoid breaking revdeps that explicitly include
+# this directory to obtain gtest target (exported now)
+if( NOT TARGET gtest_main )
   add_llvm_library(gtest_main
     TestMain.cpp
 
     LINK_LIBS
     gtest
 
     LINK_COMPONENTS
     Support # Depends on llvm::cl
     )
+endif()