llvm-ir: LLVM IR in natural Rust data structures

llvm-ir seeks to provide a Rust-y representation of LLVM IR. It's based on the idea that an LLVM Instruction shouldn't be an opaque datatype, but rather an enum with variants like Add, Call, and Store. Likewise, types like BasicBlock, Function, and Module should be Rust structs containing as much information as possible.

Unlike other safe LLVM bindings such as inkwell, llvm-ir does not rely on continuous FFI to the LLVM API. It uses the LLVM API only for its initial parsing step, to pull in all the data it needs to construct its rich representation of LLVM IR. Once llvm-ir creates a Module data structure by parsing an LLVM file (using the excellent llvm-sys low-level LLVM bindings), it drops the LLVM FFI objects and makes no further FFI calls. This allows you to work with the resulting LLVM IR in pure safe Rust.

llvm-ir is intended for consumption of LLVM IR, and not necessarily production of LLVM IR (yet). That is, it is aimed at program analysis and related applications which want to read and analyze LLVM IR. In the future, perhaps llvm-ir could be able to output its Modules back into LLVM files, or even send them directly to the LLVM library for compiling. If this interests you, contributions are welcome! (Or in the meantime, check out inkwell for a different safe interface for producing LLVM IR.) But if you're looking for a nice read-oriented representation of LLVM IR for working in pure Rust, that's exactly what llvm-ir can provide today.

Getting started

This crate is on crates.io, so you can simply add it as a dependency in your Cargo.toml, selecting the feature corresponding to the LLVM version you want:

[dependencies]
llvm-ir = { version = "0.11.1", features = ["llvm-18"] }

Currently, the supported LLVM versions are llvm-9, llvm-10, llvm-11, llvm-12, llvm-13, llvm-14, llvm-15, llvm-16, llvm-17, and llvm-18.

Then, the easiest way to get started is to parse some existing LLVM IR into this crate's data structures. To do this, you need LLVM bitcode (*.bc) or text-format IR (*.ll) files. If you currently have C/C++ sources (say, source.c), you can generate *.bc files with clang's -c and -emit-llvm flags:

clang -c -emit-llvm source.c -o source.bc

Alternately, to compile Rust sources to LLVM bitcode, you can use rustc's --emit=llvm-bc flag.

In either case, once you have a bitcode file, then you can use llvm-ir's Module::from_bc_path function:

use llvm_ir::Module;
let module = Module::from_bc_path("path/to/my/file.bc")?;

or if you have a text-format IR file, you can use Module::from_ir_path().

You may also be interested in the llvm-ir-analysis crate, which computes control-flow graphs, dominator trees, etc for llvm-ir functions.

Documentation

Documentation for llvm-ir can be found on docs.rs, or of course you can generate local documentation with cargo doc --open. The documentation includes links to relevant parts of the LLVM documentation when appropriate.

Note that some data structures differ slightly depending on your choice of LLVM version. The docs.rs documentation is generated with the llvm-10 feature; for other LLVM versions, you can get appropriate documentation with cargo doc --features=llvm-<x> --open where <x> is the LLVM version you're using.

Compatibility

Starting with llvm-ir 0.7.0, LLVM versions are selected by a Cargo feature flag. This means that a single crate version can be used for any supported LLVM version. Currently, llvm-ir supports LLVM versions 9 through 18, selected by feature flags llvm-9 through llvm-18.

You should select the LLVM version corresponding to the version of the LLVM library you are linking against (i.e., that is available on your system.) Newer LLVMs should be able to read bitcode produced by older LLVMs, so you should be able to use this crate to parse bitcode older than the LLVM version you select via crate feature, even bitcode produced by LLVMs older than LLVM 9. However, this is not extensively tested by us.

llvm-ir works on stable Rust. As of this writing, it requires Rust 1.65+.

Development/Debugging

For development or debugging, you may want LLVM text-format (*.ll) files in addition to *.bc files.

For C/C++ sources, you can generate these by passing -S -emit-llvm to clang, instead of -c -emit-llvm. E.g.,

clang -S -emit-llvm source.c -o source.ll

For Rust sources, you can use rustc's --emit=llvm-ir flag.

Additionally, you may want to pass the -g flag to clang, clang++, or rustc when generating bitcode. This will generate LLVM bitcode with debuginfo, which will ensure that Instructions, Terminators, GlobalVariables, and Functions have valid DebugLocs attached. (See the HasDebugLoc trait.) Also note that these DebugLocs are only available in LLVM 9 and newer; previous versions of LLVM had a bug in this interface in the C API which would cause segfaults.

Limitations

A few features of LLVM IR are not yet represented in llvm-ir's data structures.

Most notably, llvm-ir recovers debug-location metadata (for mapping back to source locations), but makes no attempt to recover any other debug metadata. LLVM files containing metadata can still be parsed in with no problems, but the resulting Module structures will not contain any of the metadata, except debug locations.

A few other features are missing from llvm-ir's data structures because getters for them are missing from the LLVM C API and the Rust llvm-sys crate, only being present in the LLVM C++ API. These include but are not limited to:

• the "fast-math flags" on various floating-point operations
• contents of inline assembly functions
• information about the clauses in the variadic LandingPad instruction
• information about the operands of a BlockAddress constant expression
• information about TargetExtType types
• the "prefix data" associated with a function
• the values of constant integers which are larger than 64 bits (and don't fit in 64 bits) -- see #5
• the "other labels" reachable from a CallBr terminator (which was introduced in LLVM 9)
• (LLVM 16 and lower -- fixed in LLVM 17 and later) the nsw and nuw flags on Add, Sub, Mul, and Shl, and likewise the exact flag on UDiv, SDiv, LShr, and AShr. The C API has functionality to create new instructions specifying values of these flags, but not to query the values of these flags on existing instructions.
• (LLVM 9 and lower -- fixed in LLVM 10 and later) the opcode for the AtomicRMW instruction, i.e., Xchg, Add, Max, Min, and the like.

More discussion about this is in LLVM bug #42692. Any contributions to filling these gaps in the C API are greatly appreciated!

Acknowledgments

llvm-ir took its original inspiration from the llvm-hs-pure Haskell package. Most of the data structures in the original release of llvm-ir were essentially translations from Haskell to Rust of the data structures in llvm-hs-pure (with some tweaks).

Changelog for 0.7.0

llvm-ir 0.7.0 includes several fairly major changes from previous versions, which are outlined here.

• LLVM versions are now selected via Cargo features. You must select exactly one of the features llvm-8, llvm-9, or llvm-10. Previously, we had the 0.6.x branch for LLVM 10, the 0.5.x branch for LLVM 9, and didn't officially support LLVM 8. Now, a single release supports LLVM 8, 9, and 10.
  • (Note: Versions of this crate beyond 0.7.0 have added support for later LLVM versions as well. For instance, 0.7.3 and later also support LLVM 11; and 0.7.5 and later also support LLVM 12. Crate version 0.11.0 removed support for LLVM 8.)
• FunctionAttribute and ParameterAttribute are now proper enums with descriptive variants such as NoInline, StackProtect, etc. Previously, attributes were opaque numeric codes which were difficult to interpret.
• Several changes to improve runtime performance and especially memory consumption, particularly when parsing large LLVM modules. This involves a number of breaking changes to the public interface:
  • Most users of Type now own a TypeRef rather than a Type directly. This includes Operand::LocalOperand, GlobalVariable, many variants of Instruction, many variants of Constant, and some variants of Type itself, among others. See the documentation on TypeRef.
  • Similarly, most users of Constant now own a ConstantRef rather than a Constant directly. See the documentation on ConstantRef.
  • To get the type of Typed objects, the provided .get_type() method now requires an additional argument; most users will probably prefer module.type_of() (or module.types.type_of()).
  • Type::NamedStructType no longer carries a weak reference to its inner type; instead, you can look up the name using module.types.named_struct_def() to get the definition for any named struct type in the module.
• The required Rust version increased from 1.36+ to 1.39+.
  • (Note: Versions of this crate beyond 0.7.0 have increased this requirement further. For the current required Rust version, see "Compatibility" above.)