CMake: Deprecate using llvm-config to detect llvm installation

[tstellar]

Summary

clang currently uses llvm-config to determine the installation paths
for llvm's headers and binaries. clang is also using LLVM's cmake
files to determine other information about the LLVM build, like
LLVM_LIBDIR_SUFFIX, LLVM_VERSION_*, etc. Since the installation
paths are also available via the cmake files, we can simplify the code
by only relying on information from cmake about the LLVM install and
dropping the use of llvm-config altogether.

In addition to simplifying the code, the cmake files have more
accurate information about the llvm installation paths. llvm-config
assumes that the lib, bin, and cmake directories are always located
in the same place relative to the path of the llvm-config executable.
This can be wrong if a user decides to install headers, binaries
or libraries to a non-standard location: e.g. static libraries
installed to /usr/lib/llvm6.0/

This patch takes the first step towards dropping llvm-config by
removing the automatic detection of llvm-config (users can still
manually supply a path to llvm-config by passing
-DLLVM_CONFIG=/usr/bin/llvm-config to cmake) and adding a
deprecation warning when users try to use this option.

[mgorny]

Is LLVM_CONFIG dropped from cache here? I suspect the warning might fire for everyone who has LLVM configured.

[tstellar]

In D51714#1225545, @mgorny wrote:

Is LLVM_CONFIG dropped from cache here? I suspect the warning might fire for everyone who has LLVM configured.

Yes, it is dropped from the cache, is it a problem to having the warning fire for people who have already configured LLVM?

[mgorny]

I don't have a strong opinion here.

[steveire]

Brilliant, I've wanted to do this for years, and I still have many improvements to the LLVM/Clang cmake files on my todo list. Good to know others are similarly motivated.

CMakeLists.txt

13

message(DEPRECATION) is designed for deprecation messages. It announces itself as a "CMake Deprecation Warning at CMakeLists.txt:6 (message):".

The user can silence the warning with -Wno-deprecated or make it an error with Wdeprecated since old versions of CMake: https://cmake.org/pipermail/cmake-developers/2013-July/019634.html

[smeenai]

Is there anything holding this up?

[tstellar]

Use cmake's DEPRECATION message.

[tstellar]

In D51714#1248643, @smeenai wrote:

Is there anything holding this up?

No hold up, I just wanted to address the review comment. I'll commit on Monday if there are no other comments.

CMakeLists.txt

 cmake_minimum_required(VERSION 3.4.3)
 
 # If we are not building as a part of LLVM, build Clang as an
 # standalone project, using LLVM as an external library:
 if( CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR )
   project(Clang)
 
   # Rely on llvm-config.
   set(CONFIG_OUTPUT)
-  find_program(LLVM_CONFIG "llvm-config")
   if(LLVM_CONFIG)
+    set (LLVM_CONFIG_FOUND 1)
     message(STATUS "Found LLVM_CONFIG as ${LLVM_CONFIG}")
+    message(DEPRECATION "Using llvm-config to detect the LLVM installation is \
+          deprecated.  The installed cmake files should be used \
+          instead.  CMake should be able to detect your LLVM install \
+          automatically, but you can also use LLVM_DIR to specify \
+          the path containing LLVMConfig.cmake.")
     set(CONFIG_COMMAND ${LLVM_CONFIG}
       "--assertion-mode"
       "--bindir"
       "--libdir"
       "--includedir"
       "--prefix"
       "--src-root"
       "--cmakedir")
     execute_process(
       COMMAND ${CONFIG_COMMAND}
       RESULT_VARIABLE HAD_ERROR
       OUTPUT_VARIABLE CONFIG_OUTPUT
     )
     if(NOT HAD_ERROR)
       string(REGEX REPLACE
         "[ \t]*[\r\n]+[ \t]*" ";"
         CONFIG_OUTPUT ${CONFIG_OUTPUT})
     else()
       string(REPLACE ";" " " CONFIG_COMMAND_STR "${CONFIG_COMMAND}")
       message(STATUS "${CONFIG_COMMAND_STR}")
       message(FATAL_ERROR "llvm-config failed with status ${HAD_ERROR}")
     endif()
-  else()
-    message(FATAL_ERROR "llvm-config not found -- ${LLVM_CONFIG}")
-  endif()
 
     list(GET CONFIG_OUTPUT 0 ENABLE_ASSERTIONS)
     list(GET CONFIG_OUTPUT 1 TOOLS_BINARY_DIR)
     list(GET CONFIG_OUTPUT 2 LIBRARY_DIR)
     list(GET CONFIG_OUTPUT 3 INCLUDE_DIR)
     list(GET CONFIG_OUTPUT 4 LLVM_OBJ_ROOT)
     list(GET CONFIG_OUTPUT 5 MAIN_SRC_DIR)
     list(GET CONFIG_OUTPUT 6 LLVM_CONFIG_CMAKE_PATH)
 
+    # Normalize LLVM_CMAKE_PATH. --cmakedir might contain backslashes.
+    # CMake assumes slashes as PATH.
+    file(TO_CMAKE_PATH ${LLVM_CONFIG_CMAKE_PATH} LLVM_CMAKE_PATH)
+  endif()
+
+
   if(NOT MSVC_IDE)
     set(LLVM_ENABLE_ASSERTIONS ${ENABLE_ASSERTIONS}
       CACHE BOOL "Enable assertions")
     # Assertions should follow llvm-config's.
     mark_as_advanced(LLVM_ENABLE_ASSERTIONS)
   endif()
 
+  find_package(LLVM REQUIRED HINTS "${LLVM_CMAKE_PATH}")
+  list(APPEND CMAKE_MODULE_PATH ${LLVM_DIR})
+
+  # We can't check LLVM_CONFIG here, because find_package(LLVM ...) also sets
+  # LLVM_CONFIG.
+  if (NOT LLVM_CONFIG_FOUND)
+    # Pull values from LLVMConfig.cmake.  We can drop this once the llvm-config
+    # path is removed.
+    set(TOOLS_BINARY_DIR ${LLVM_TOOLS_BINARY_DIR})
+    set(LIBRARY_DIR ${LLVM_LIBRARY_DIR})
+    set(INCLUDE_DIR ${LLVM_INCLUDE_DIR})
+    set(LLVM_OBJ_DIR ${LLVM_BINARY_DIR})
+  endif()
+
   set(LLVM_TOOLS_BINARY_DIR ${TOOLS_BINARY_DIR} CACHE PATH "Path to llvm/bin")
   set(LLVM_LIBRARY_DIR ${LIBRARY_DIR} CACHE PATH "Path to llvm/lib")
   set(LLVM_MAIN_INCLUDE_DIR ${INCLUDE_DIR} CACHE PATH "Path to llvm/include")
   set(LLVM_BINARY_DIR ${LLVM_OBJ_ROOT} CACHE PATH "Path to LLVM build tree")
   set(LLVM_MAIN_SRC_DIR ${MAIN_SRC_DIR} CACHE PATH "Path to LLVM source tree")
 
-  # Normalize LLVM_CMAKE_PATH. --cmakedir might contain backslashes.
-  # CMake assumes slashes as PATH.
-  file(TO_CMAKE_PATH ${LLVM_CONFIG_CMAKE_PATH} LLVM_CMAKE_PATH)
-
   find_program(LLVM_TABLEGEN_EXE "llvm-tblgen" ${LLVM_TOOLS_BINARY_DIR}
     NO_DEFAULT_PATH)
 
-  find_package(LLVM REQUIRED HINTS "${LLVM_CMAKE_PATH}")
-  list(APPEND CMAKE_MODULE_PATH ${LLVM_DIR})
-
   # They are used as destination of target generators.
   set(LLVM_RUNTIME_OUTPUT_INTDIR ${CMAKE_BINARY_DIR}/${CMAKE_CFG_INTDIR}/bin)
   set(LLVM_LIBRARY_OUTPUT_INTDIR ${CMAKE_BINARY_DIR}/${CMAKE_CFG_INTDIR}/lib${LLVM_LIBDIR_SUFFIX})
   if(WIN32 OR CYGWIN)
     # DLL platform -- put DLLs into bin.
     set(LLVM_SHLIB_OUTPUT_INTDIR ${LLVM_RUNTIME_OUTPUT_INTDIR})
   else()
     set(LLVM_SHLIB_OUTPUT_INTDIR ${LLVM_LIBRARY_OUTPUT_INTDIR})