Rust bindings to LLVM's C API.

Usage

[dependencies]
llvm-sys = "180"

There must be a compatible version of LLVM available. By default llvm-sys will look for llvm-config on PATH to find a system-wide copy of LLVM and use that if it is a compatible version. Custom options for finding LLVM on your system can be specified via environment variables. See LLVM compatibility for more information.

Documentation

See the examples directory in this repository for API examples. There also exist some other projects using these bindings which may be informative or useful:

• Safe, "Rusty" APIs for using LLVM:
  • Inkwell
  • llvm-ir
• Tari's merthc
• Wilfred's BF compiler
• Jay Phelps wrote about building a minimal compiler targeting WebAssembly
  • Lyle extended this for arm64 using clang
• Lyle wrote a small PoC compiler Calculon using LLVM with LALRPOP

Most of the interfaces are not documented in these bindings. Refer to the LLVM documentation for more information, particularly the generated API documentation.

If you have your own project using these bindings that you think is worth mentioning here, by all means let me know.

Cargo features

This crate provides a few feature flags that can be important to particular uses. These are mostly useful only at build-time; If you encounter problems building something that depends on llvm-sys, enabling one or more option may be useful; for example:

cargo build --features=llvm-sys/prefer-dynamic

Most crates that depend on llvm-sys should not automatically enable any of these features, because the appropriateness of any given option usually depends on the configuration of the system on which the library is being compiled.

• Dynamic/static linking preferences prefer-dynamic, force-dynamic, prefer-static, force-static. Only one may be specified, indicating how LLVM should be linked. If none are enabled, prefer-static is assumed. (Prior to crate version 191, the default linking behavior is force-static.)

  With dynamic linking, the crate will link against a LLVM shared library (a .dll on Windows, .so on Linux and so forth) which must be available at runtime; static linking instead uses a static library archive (.lib on Windows or .a on most Unix-like systems) that includes all of the library code in the final binary (increasing file size but eliminating any dependency on additional files to run the program).

  If a prefer- option is enabled, the crate will attempt to use the specified kind of linking (static or dynamic) but fall back to the other if the first kind is unavailable. Setting a force- option will attempt only to do the specified kind of linking and fail otherwise.

• strict-versioning: causes the build to fail if a version of LLVM exactly matching the current crate version is not found. If enabled, linking crate version 150.1.0 against LLVM 16.0 (for example) would fail whereas it is normally permitted.

  This may be useful if there is known to be an incompatibility between some LLVM versions, causing the build to fail rather than possibly causing errors at runtime.

• disable-alltargets-init: disable building the functions that initialize LLVM components for all supported targets. These functions need to be compiled from C, so if they are unneeded then turning them off can allow the build to proceed if a working C compiler is unavailable.

• no-llvm-linking: prevents llvm-sys from instructing the compiler to link against any LLVM libraries. This can be useful if another crate links LLVM in some different way (preventing conflicts) but means the other crate must ensure llvm-sys' library requirements are satisfied.

Build requirements

llvm-sys requires a copy of llvm-config corresponding to the desired version of LLVM to build: llvm-config allows it to probe what libraries need to be linked and what compiler options are required.

Binary distributions of LLVM (including the official release packages) generally do not include a copy of llvm-config, making them unsuited to use for building programs with llvm-sys. Known exceptions (that do include a copy of llvm-config) include:

• Official Debian/Ubuntu packages from apt.llvm.org
• Fedora/Red Hat's llvm-devel package
• Arch Linux's llvm package

If a suitable binary package is not available for your platform, compiling from source is usually the best option. See Compiling LLVM in this document for details.

It may be difficult or even impossible to provide a compatible LLVM version system-wide for a given project (consider a program using two libraries that internally use different versions of LLVM!) so environment variables can be set to help the build scripts find your copy of the libraries. This is also helpful if you are unable to provide a system-wide version of LLVM but can still compile it yourself.

LLVM_SYS_<version>_PREFIX specifies the install prefix for a compiled and installed copy of the libraries, where <version> is the major version of llvm-sys (for example, LLVM_SYS_37_PREFIX). The llvm-sys build scripts will look for a llvm-config binary in the directory <PREFIX>/bin/ in addition to searching for a copy on $PATH, and verify that the LLVM version reported by llvm-config is compatible with the current crate version.

LLVM compatibility

Because the LLVM C API stability guarantees are relatively weak, this crate enforces that the LLVM release in use match the one it was made for. The crate version is constructed by treating the LLVM version as a real number and multiplying by 10, ignoring any fractional part. Thus llvm-sys version 37 is compatible with LLVM 3.7.x, and llvm-sys 41 would be compatible with LLVM 4.1.x.

The build scripts will not enforce this compatibility matrix strictly, permitting compilation against any version of LLVM that is at least as new as the crate target version. This is safe in most cases because the LLVM C API is meant to maintain binary compatibility across releases with the exception of when functions are deprecated and later removed. An incompatible LLVM version will generally fail to compile with a link-time error, rather than cause runtime errors. Where versions are known to break binary compatibility, the build script will prevent compilation.

Depending on your use of the C API, your program may require that only a version of LLVM exactly matching your crate version be allowed. This can be set with the cargo feature flag strict-versioning or by setting the environment variable LLVM_SYS_<version>_STRICT_VERSIONING (where <version> is the target crate version) to any value.

llvm-sys blocklists some versions of LLVM that are known to be binary-incompatible. If you're feeling lucky, setting LLVM_SYS_<version>_IGNORE_BLOCKLIST to "YES" will permit the use of blocklisted library versions (which may cause vexing bugs).

This crate declares that it links to llvm-<MAJOR VERSION>, not just llvm. This makes it possible to declare a crate that depends on multiple versions of llvm-sys (corresponding to different versions of LLVM) as long as only one of them is actually used:

llvm-sys-90 = { package = "llvm-sys", version = "90", optional = true }
llvm-sys-100 = { package = "llvm-sys", version = "100", optional = true }

This requires that the target LLVM version (llvm-10 for instance) be declared as the linking target rather than just llvm because Cargo requires that all linked libraries be unique regardless of what is actually enabled. Note that although Cargo will not prevent you from enabling multiple versions of LLVM at once as a result, doing so will likely cause errors at link time.

Compiling LLVM

If you need to compile LLVM or manage multiple versions, llvmenv may simplify the process. Consider using it if you don't have special requirements or previous experience with LLVM!

While the getting started guide is the official guide to compiling LLVM, this section will attempt to provide minimum guidance in creating usable libraries. If you encounter problems, refer to the official documentation.

Download sources

Download and unpack a copy of the source for the required version.

wget https://llvm.org/releases/3.9.0/llvm-3.9.0.src.tar.xz
tar xJf llvm-3.9.0.src.tar.xz

Note that you do not need to compile Clang or the test suite.

Configure

Configure the build using CMake (you will need a copy of CMake installed).

mkdir -p llvm-3.9.0.src/build
cd llvm-3.9.0.src/build
cmake .. -DCMAKE_INSTALL_PREFIX=$HOME/llvm-3.9.0

Some of the useful options that can be specified at configuration-time (via -D passed to CMake):

• CMAKE_INSTALL_PREFIX sets the location to install everything in the install step (later). In the above example this is under your home directory.
• CMAKE_BUILD_TYPE specifies the build mode, one of Debug, Release, MinSizeRel or RelWithDebInfo. Unless you plan to debug LLVM itself, Release or MinSizeRel is probably a good choice.
• LLVM_ENABLE_ASSERTIONS enables internal sanity checks, highly recommended when writing and debugging your own program that uses LLVM because it can detect many usage errors that could otherwise result in hard-to-debug crashes.

Passing -G <generator> to CMake will make it use a different build system, but by default it will select one appropriate to your system. If you have ninja available, it is recommended due to its speed (-G Ninja).

Compile and install

cmake --build . --target install

This will automatically invoke the build system and copy binaries into the prefix specified at configuration-time when done. Some build tools (like Visual Studio on Windows) support all configurations concurrently so you also need to specify the build type (which defaults to Debug on Windows), adding an option like --config MinSizeRel to this invocation of cmake.

After building and installing via CMake, you can compile llvm-sys against it.

cd your/crate/path
LLVM_SYS_39_PREFIX=$HOME/llvm-3.9.0 cargo build

Windows

You must use a version of Rust that uses the same compiler as you build LLVM with, either MSVC or MinGW. Fortunately, a mismatch like this will cause errors at compile-time when llvm-config provides options which are supported by only one of them, so if you're using the other it will cause the build to fail.

Cross-compilation

Will theoretically work, but hasn't been tested. Let us know if you try.