Simplify cross-compilation. Don't use native-compiled llvm-config.

[garious]

Summary

Simplify cross-compilation. Don't use native-compiled llvm-config. Instead use CMake's find_package() feature. This assumes CMAKE_PREFIX_PATH contains a path to the LLVM install directory.

Note: AddLLVM.cmake does not expose the LLVM source directory.

So if you want to run the test suite, you need to either:

1) set LLVM_MAIN_SRC_DIR explicitly (to find lit.py)
2) change AddLLVM.cmake to point to an installed 'lit'.
3) add_subdirectory(compiler-rt/test) from clang instead of compiler-rt.

[samsonov]

Could you please describe the full suggested workflow? Is this a replacement of LLVM_BUILD_EXTERNAL_COMPILER_RT option and the code added for it in tools/clang/runtime/CMakeLists.txt? I.e., if we land this change, what is the sequence of commands one should write to use just-built Clang to build compiler-rt libs?

CMakeLists.txt

81

Will this work for multi-configuration build systems (the ones that can build Debug and Release binaries in a single build tree)?

[garious]

The change is meant to be independent of LLVM_BUILD_EXTERNAL_COMPILER_RT. For that build, in "clang/runtime/CMakeLists.txt", use:

-DCMAKE_PREFIX_PATH=${CMAKE_BINARY_DIR}/${CMAKE_CFG_INTDIR}

instead of:

-DLLVM_CONFIG_PATH=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-config

But doing that only works when the CMAKE_BINARY_DIR is the same for both the clang and llvm builds. That happens to be true for your build, but locally, I build clang against the llvm install directory using the same LLVM_PREFIX_PATH technique.

I don't use LLVM_BUILD_EXTERNAL_COMPILER_RT. Instead I have a custom little package manager that points each standalone build to its dependencies. If you don't have a package manager, you can get by with a Bash script similar to:

mkdir -p llvm/out && cd llvm/out
cmake .. -DCMAKE_INSTALL_PREFIX=ship
ninja check-all install
cd -

mkdir -p compiler-rt/out && cd compiler-rt/out
cmake .. -DCMAKE_PREFIX_PATH=`pwd`/../../llvm/out/ship

-DCMAKE_INSTALL_PREFIX=ship

ninja check-all install
cd -

mkdir -p clang/out && cd clang/out
cmake ..  -DCMAKE_PREFIX_PATH=`pwd`/../../llvm/out/ship:`pwd`/../../compiler-rt/out/ship

-DCMAKE_INSTALL_PREFIX=ship

ninja check-all install
cd -

CMakeLists.txt

81

Yes. There can be a single build tree with a separate install directory for each build variant of each component. Locally, my directory structure contains two top-level directories: 'llvm' and 'compiler-rt'. Within each of those directories are build directories: "out/release" and "out/debug". Within each of those directories is a 'ship' directory, which the 'install' step copies files to. So, for example, in the Release build of compiler-rt, LLVM_INSTALL_PREFIX points to "llvm/out/release/ship".

CMakeLists.txt

   set(COMPILER_RT_TEST_COMPILER ${CMAKE_C_COMPILER})
   set(COMPILER_RT_TEST_COMPILER_ID ${CMAKE_C_COMPILER_ID})
 
-  if (NOT LLVM_CONFIG_PATH)
-    find_program(LLVM_CONFIG_PATH "llvm-config"
-                 DOC "Path to llvm-config binary")
-    if (NOT LLVM_CONFIG_PATH)
-      message(FATAL_ERROR "llvm-config not found: specify LLVM_CONFIG_PATH")
-    endif()
-  endif()
-  execute_process(
-    COMMAND ${LLVM_CONFIG_PATH} "--obj-root" "--bindir" "--libdir" "--src-root"
-    RESULT_VARIABLE HAD_ERROR
-    OUTPUT_VARIABLE CONFIG_OUTPUT)
-  if (HAD_ERROR)
-    message(FATAL_ERROR "llvm-config failed with status ${HAD_ERROR}")
-  endif()
-  string(REGEX REPLACE "[ \t]*[\r\n]+[ \t]*" ";" CONFIG_OUTPUT ${CONFIG_OUTPUT})
-  list(GET CONFIG_OUTPUT 0 LLVM_BINARY_DIR)
-  list(GET CONFIG_OUTPUT 1 LLVM_TOOLS_BINARY_DIR)
-  list(GET CONFIG_OUTPUT 2 LLVM_LIBRARY_DIR)
-  list(GET CONFIG_OUTPUT 3 LLVM_MAIN_SRC_DIR)
-
-  # Make use of LLVM CMake modules.
-  file(TO_CMAKE_PATH ${LLVM_BINARY_DIR} LLVM_BINARY_DIR_CMAKE_STYLE)
-  set(LLVM_CMAKE_PATH "${LLVM_BINARY_DIR_CMAKE_STYLE}/share/llvm/cmake")
-  list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_PATH}")
-  # Get some LLVM variables from LLVMConfig.
-  include("${LLVM_CMAKE_PATH}/LLVMConfig.cmake")
+  find_package(LLVM REQUIRED)
+  list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_DIR}")
+
+  # Variables that AddLLVM.cmake depends on (included by AddCompilerRT)
+  set(LLVM_TOOLS_BINARY_DIR "${LLVM_INSTALL_PREFIX}/bin")
+  set(LLVM_LIBRARY_DIR "${LLVM_INSTALL_PREFIX}/lib")
 
   set(LLVM_LIBRARY_OUTPUT_INTDIR ${CMAKE_BINARY_DIR}/${CMAKE_CFG_INTDIR}/lib)