Exploring LLVM's Build System Features

• build directives
• in tree/out of tree development
  • in tree: contribute code directly to llvm project.
  • out of tree: creating llvm extensions.

LLVM's CMake directives

Adding New Libraries

• Don't use normal add_library.
• Hard to control shared/static build of libraries using built in add_library.
• Better control over RTTI and exception handling if not using built in directives.
• Use add_llvm_component_library.
• Example:

  add_llvm_component_library(<library name>
  	<source file>)
• Instead of using target_link_libraries and add_dependencies, use LINK_COMPONENTS argument in add_llvm_component_library.
• Example:

  add_llvm_component_library(<library name>
  	<source file>
  	LINK_COMPONENTS
  	<library 1> <library 2>)
• Can use LLVM_LINK_COMPONENTS variable as well.
• Example:

  set(LLVM_LINK_COMPONENTS
  	<library 1> <library 2>)
  add_llvm_component_library(...)
• Component libraries are llvm building blocks. Different than real library names.
• Use llvm_map_components_to_libnames to create mapping from component library name to real one.
• Use LINK_LIBS argument of add_llvm_component_library to link against normal lib.
• Use DEPENDS argument to assign dependencies (instead of add_dependencies).

Add one build target per folder

• Source file detection problem with LLVM Cmake functions.
• LLVM build enforces all C/C++ source files in the same folder are added to the same lib, executable or plugin.
• Use add_subdirectory to add sub-folders and separate out different libs with separate CMakeLists.txt.

Adding executables and tools

• Use add_llvm_executable or add_llvm_tool.
• Targets with add_llvm_tool will be added in installations.
• LLVM_BUILD_TOOLS enables/disables LLVM tool targets.
• Can use DEPENDS.

Adding Pass plugins

• Use add_llvm_pass_plugin.
• LINK_COMPONENTS, LINK_LIBS, and DEPENDS available.

CMake integration for OOT project

• Two ways to create OOT project:
  • llvm-config tool
  • LLVM CMake modules
• Standard CMake package interface.
• Use find_package(LLVM REQUIRED CONFIG) to import configs.
• ${LLVM_INCLUDE_DIRS} and ${LLVM_LIBRARY_DIRS} for include_directories and link_directories, resolves include and library path.
• Use -DLLVM_DIR=<llvm install path>/lib/cmake/llvm to find package.
• Import AddLLVM to get LLVM CMake functions.
• Snippet:

  list(APPEND CMAKE_MODULE_PATH ${LLVM_CMAKE_DIR})
  include(AddLLVM)
• Need to take LLVM definitions into account.
• add_definitions(${LLVM_DEFINITIONS}).
• LLVM not built with RTTI, but non-llvm applications are. Make sure no RTTI mismatch.