How to Use the Command `llvm-config` (with Examples)

llvm-config is a utility provided by the LLVM project, vital for software developers working with LLVM, a collection of modular and reusable compiler and toolchain technologies. This command-line tool offers various configuration details necessary to compile programs that utilize LLVM components. Its typical application is within build systems, such as Makefiles or configure scripts, providing flags and options that simplify and streamline the build process of LLVM-based applications. For more information, you can visit the official LLVM documentation at LLVM Command Guide .

Use Case 1: Compile and Link an LLVM Based Program

Code:

clang++ $(llvm-config --cxxflags --ldflags --libs) --output path/to/output_executable path/to/source.cc

Motivation: Compiling and linking a program that uses LLVM can be complex, given the need to link against various libraries and include multiple headers. This command simplifies the process by automatically providing the necessary compiler flags (--cxxflags), linker flags (--ldflags), and LLVM libraries (--libs). This allows developers to focus on their source code development without worrying about manually specifying the multitude of details typically needed in a complex software build environment that incorporates LLVM.

Explanation:

• clang++: This is the C++ compiler from the LLVM Clang project. It is used to compile and link C++ programs.
• $(llvm-config --cxxflags --ldflags --libs): This is a shell command substitution that dynamically adds include paths, library paths, and libraries required to compile and link against LLVM. Each of the flags have specific roles:
  • --cxxflags: Provides the necessary C++ compiler flags, such as include directories for the LLVM headers.
  • --ldflags: Supplies the linker flags required to find the LLVM libraries.
  • --libs: Specifies the actual libraries needed for linking the application against LLVM.
• --output path/to/output_executable: Defines the output file for the final executable. You need to replace path/to/output_executable with the actual path and name of the executable file you want to create.
• path/to/source.cc: This is the path to your source code file written in C++ that you wish to compile and link against LLVM libraries.

Example Output: Assuming a successful compilation, there will be no specific output printed to the terminal. However, the specified executable will be created at the path you provided.

Use Case 2: Print the PREFIX of Your LLVM Installation

Code:

llvm-config --prefix

Motivation: Identifying the installation prefix of LLVM on your system is crucial for configuring, compiling, and installing other software packages that depend on LLVM. Knowing this location allows you to verify where the LLVM binaries, libraries, and include files are stored, which can be vital for troubleshooting or customizing the build and installation of LLVM components.

Explanation:

• llvm-config: This is the base command-line utility provided by LLVM.
• --prefix: An option that fetches and displays the prefix directory of your LLVM installation. The prefix typically indicates the root directory where LLVM is installed, enabling you to locate all relevant LLVM resources for development and configuration processes.

Example Output:

/usr/local/llvm

The output provides the root directory where LLVM is installed on your system.

Use Case 3: Print all Targets Supported by Your LLVM Build

Code:

llvm-config --targets-built

Motivation: When working on projects that involve cross-compilation or target-specific optimizations, knowing which targets your LLVM build supports is invaluable. This information helps in configuring projects to enable or disable features based on the availability of specific backends, ensuring that your code generation and optimizations align with the intended hardware architectures.

Explanation:

• llvm-config: Again, this is the primary utility that interfaces with LLVM configurations.
• --targets-built: This option queries and lists all targets (or architectures) supported by your current LLVM build configuration. These could include a variety of CPU architectures like x86, ARM, or MIPS, among others.

Example Output:

X86 ARM AArch64

The output indicates that the LLVM build on the system supports code generation for the X86, ARM, and AArch64 architectures.

Conclusion:

The llvm-config utility equips developers with essential configuration information needed to effectively compile and link programs against the LLVM framework. Each use case demonstrates a different aspect of its utility — from simplifying the build process of LLVM-based applications and retrieving installation details, to understanding target architecture capabilities. These features enable developers to build efficient and customized applications with ease and precision, fully taking advantage of the powerful LLVM infrastructure.