Dependencies

A short reference for the third-party software the project depends on, both at build time and at runtime, and what the policies around adding new dependencies look like.

Vendored third-party libraries

These live under third-party/:

Library	Path	Purpose
Google Test / Google Mock	third-party/unittest/	Unit-test framework used by every subproject's unittests/
Google Benchmark	third-party/benchmark/	Microbenchmark framework
Boost Math	third-party/boost-math/	Reference high-precision math used in libc and libc++ tests
SipHash	third-party/siphash/	Fast keyed-hash; pulled in for the rare uses

Update scripts ship alongside each:

• third-party/update_benchmark.sh
• third-party/update_boost_math.sh

The Integer Set Library (ISL) used by Polly is not under third-party/; it lives under polly/lib/External/ with its own update flow.

Build-time tools

Tool	Required?	Notes
CMake	Yes	≥ 3.20
Python	Yes	≥ 3.8
Ninja	Recommended	The default for any non-toy build
ccache / sccache	Optional	-DLLVM_CCACHE_BUILD=ON
C++17 host compiler	Yes	GCC ≥ 7.4, Clang ≥ 5, MSVC ≥ recent
lit	Bundled	Built/installed from llvm/utils/lit/ as needed
FileCheck	Bundled	Built from llvm/utils/FileCheck/

For documentation builds, Sphinx plus a couple of extensions is needed; the relevant requirements are pinned in each subproject's docs/ directory.

Runtime dependencies

The runtime story varies by subproject:

Subproject	Runtime dependency
clang	The C++ runtime (libc++ or libstdc++) of the host build at runtime; compiler-rt builtins for codegen
lld	Just the host C++ runtime
lldb	Python (for the script bridge), platform debug-server libraries, ptrace/equivalent on the host
flang	flang-rt, MLIR static libs, LLVM static libs
compiler-rt sanitizers	Host kernel features (ptrace, /proc, mmap protections), often glibc-specific entry points
libcxx	libc++abi, libunwind
libc++abi	libunwind
libunwind	None beyond the OS
openmp	A pthreads implementation, the host C library
offload	Vendor SDKs at runtime — CUDA driver / ROCm runtime / HSA, etc.
bolt	None at apply time; needs perf to collect input profile

Adding a new dependency

The project policy is conservative: new third-party dependencies require a public discussion thread on Discourse and clear justification (no smaller in-tree alternative; license compatible with Apache 2.0 with LLVM Exceptions; a maintainer willing to keep it updated). Most contributions rewrite or vendor a small piece rather than introduce a system dependency.

For dependencies that already live in third-party/, updates are made through the corresponding update_*.sh script and reviewed alongside the actual import.

Reference

• LLVM Developer Policy — third-party software
• third-party/README.md