MLIR Bindings for Swift
This project intends to make MLIR APIs accessible from Swift via the MLIR C Bindings.
Usage
The best reference for how to use this package is currently the tests (Module Tests is probably the most interesting).
If you are looking for an example of using MLIRSwift to bind MLIR dialects, check out Edith which uses MLIRSwift to bind dialects from CIRCT.
MLIR
Installation
MLIR is build using the LLVM build infrastructure which uses cmake. This is incompatible with Swift Package Manager, so for now developers will need to install MLIR separately in order for this project to work. Once the MLIR C API settles and Swift Package Manager get better support for binary targets on Linux, we will likely make this dependency available as a precompiled binary.
In the meantime, you can manually install MLIR using the Tools/build-dependencies script. This script requires recent versions of cmake and ninja to be present on your macine. These can be installed on macOS using Homebrew (brew install cmake ninja) or on Ubuntu via apt-get (apt-get install cmake ninja-build). Linux systems might also require the libncurses5-dev and python3 packages.
Troubleshooting
Insufficient permissions to create pkg-config file
One error you may run into is not having sufficient permissions to create LLVM-for-Swift.pc in /usr/local/lib/pkgconfig. This can be solved by making sure that folder exists and granting the correct user read/write permissions to that folder (an aggressive way to accomplish this is sudo chmod -R 777 /usr/local/lib/pkgconfig, which grants all users very open permissions to that folder). The reason this is necessary is that the only cross-platform way to include a precompiled binary in a Swift project is to have that binary installed on the system and accessible via a pkg-config (.pc) file. Tools/build-dependencies works by creating this file for you, and pointing it to the version of CIRCT it built. Unfortunately, Xcode seems to only check certain well known locations for .pc files, the main one being /usr/local/lib/pkgconfig. If we are using SwiftPM from the command line (via a command like swift test) we can modify this behavior using PKG_CONFIG_PATH (the Linux CI uses this approach).
Stale caches when updating pkg-config file
Sometimes, if you change the contents of the pkg-config file (creating it for the first time, or switching between install directories) Xcode will not pick up this change, even after quitting and re-opening. The solution to this is to delete Xcode’s “DerivedData” directory, which can be located by going to “Preferences” > “Locations” (an arrow by the path will open that location in Finder). An advanced approach is to store DerivedData “Relative” to the project (I use “.build/DerivedData” as the path). This way, it is easier to delete from the command line.
Updating
We do not include MLIR (llvm) as a submodule, because this would cause Swift Package Manager to pull in all of LLVM in any project depending on MLIRSwift. Instead, we store the hash we care about in Tools/llvm-commit file, update this file to a new commit to update MLIR. Note that this file must be a hash and not a branch like main, since the contents of this file is used to cache the LLVM build on GitHub Actions.
Using an external MLIR checkout
You can point use your own local version of MLIR in a number of ways, the most flexible is simply to install a custom “LLVM-for-Swift.pc” file that points to your locally built version (consult Tools/build-dependencies for an example). A simpler option may be to run Tools/build-dependencies with the environment variable LLVM_REPO set to "local", and LLVM_REPO_PATH set to the path to the repo you want to use. There are a number of other knobs you can turn in Tools/build-dependencies to customize this approach.
