Subprojects

This section is a per-subproject deep dive. Each entry describes what the subproject is for, where its source lives, the most important pieces inside it, and where to start if you want to modify it.

The repository's own vocabulary calls these directories "subprojects" — the term is used in llvm/CMakeLists.txt (LLVM_ENABLE_PROJECTS), in release notes, and in the maintainers files. We use the same term here.

Subproject map

Subproject	Page	Role
llvm	llvm	Core IR, optimizer, codegen, targets, support libraries
clang	clang	C / C++ / Objective-C front end
clang-tools-extra	clang-tools-extra	clangd, clang-tidy, clang-format, clang-doc, etc.
lld	lld	Cross-platform linker
lldb	lldb	Debugger
mlir	mlir	Multi-Level IR framework
flang	flang	Fortran front end
flang-rt	flang-rt	Fortran runtime
libc	libc	LLVM's C standard library
libcxx	libcxx	libc++ — the C++ standard library
libcxxabi	libcxxabi	C++ ABI runtime
libunwind	libunwind	Itanium-ABI stack-unwinding library
polly	polly	Polyhedral loop optimizer
openmp	openmp	OpenMP runtimes
compiler-rt	compiler-rt	Compiler builtins, sanitizers, profile runtimes
bolt	bolt	Post-link binary optimizer
offload	offload	Accelerator-offload runtime
libsycl	libsycl	SYCL 2020 runtime
libclc	libclc	OpenCL C library
orc-rt	orc-rt	ORC JIT runtime support
llvm-libgcc	llvm-libgcc	LLVM-built libgcc-compatible shim
cross-project-tests	cross-project-tests	Cross-subproject integration tests

Naming pattern

The standard top-level directory layout for a subproject is:

<subproject>/
├── CMakeLists.txt
├── README.md or README.txt
├── Maintainers.md (if applicable)
├── docs/                # User-facing documentation
├── include/             # Public headers
├── lib/                 # Implementation
├── test/                # lit regression tests
├── unittests/           # Google-Test unit tests (where applicable)
├── tools/               # Driver executables
├── utils/               # Helper tools (TableGen backends, scripts)
└── examples/            # Standalone examples (where applicable)

Most pages in this section assume that layout and only call it out when the subproject deviates.

Cross-cutting build/test conventions

Every subproject:

• Builds via CMake when its name appears in LLVM_ENABLE_PROJECTS (compilers/tools) or LLVM_ENABLE_RUNTIMES (runtime libraries).
• Provides a check-<name> target that runs its lit suite and unit tests.
• Lives by the same coding standards (llvm/docs/CodingStandards.rst).
• Is reviewed under the same developer policy (llvm/docs/DeveloperPolicy.rst).