μSwift[Core]

The Swift language is implemented as a small layer of sugar over LLVM IR. This includes the core types such as Bool. In order to support even basic constructs, the language requires the standard library to be available. The reference standard library is a large code base and has additional dependencies such as libicu. This makes the library inconvenient for certain environments.

μSwift is a minimal standard library that provides a few of the core interfaces required for using basic constructs in Swift. The long term vision for this library is to provide conditional control over the features that the implementation vends. This enables the use of Swift in embedded systems which may not be amenable to large libraries and do not need the complete core functionality from Swift (e.g. Unicode support).

Components

1. swiftCore: the standard library.
2. swiftOnoneSupport: the support library for building with -Onone
3. swiftRuntime: the language runtime support (merged into swiftCore)

Build Requirements

• clang compiler (11.0+)
• Swift compiler (5.4+)
• CMake (3.18+)
• Ninja (1.8+)

Building

Building a dynamically linked version of the libraries is controlled by the BUILD_SHARED_LIBS standard parameter.

Building with CMake requires the Ninja build tool.

NOTE: There is some support which is required in the Swift compiler itself in order to build the Swift stanard library. This includes support for the architecture and the OS spelling. Without this, the target may not be recognised properly and the Standard Library may fail to compile.

The following builds a release (optimized) configuration of the statically linked variant of the standard library for a freestanding ELF environment:

cmake -B out -D BUILD_SHARED_LIBS=NO -D CMAKE_BUILD_TYPE=Release -D CMAKE_Swift_COMPILER_TARGET=aarch64-unknown-none-elf -D CMAKE_Swift_COMPILER_WORKS=YES -G Ninja -S .
ninja -C out

NOTE: This support requires patches to the Swift compiler which have not yet been merged. The changes are available at apple/swift#35970.

Parameters

1. BUILD_SHARED_LIBS: Boolean
   Indicates if the libraries should be shared (dynamically linked) or not.

2. CMAKE_Swift_COMPILER_TARGET: String
   Identifiers the target triple of the platform that the standard library should be built. Defaults to the build.

3. CMAKE_Swift_COMPILER_WORKS: Boolean
   Required parameter. Must always specify a true boolean value. This is required to skip the checks that the compiler can build code for the target. The Swift compiler cannot build code without the standard library available. Because we are building the standard library, we must skip the checks and assume that the compiler functions properly.

Artifacts

swiftCore.dll/libswiftCore.so/libswiftCore.dylib

The runtime component of the standard library.

swiftCore.lib/libswiftCore.so/libswiftCore.dylib

The build/SDK component of the standard library for linking.

Swift.swiftmodule

The build/SDK component of the standard library for building.