If you distribute binaries for command line tools built with Swift Package Manager, you might have previously built your distribution binary with:

% swift build --configuration release

If you inspect the binary, you can see it was built for the current machine's architecture by default:

% file .build/release/package
.build/release/package: Mach-O 64-bit executable x86_64

Previously, this was sufficient since macOS only supported one architecture. Now, in order to fully utilize the native performance of Apple Silicon chips, we need to produce a fat binary that contains a slice for both x86_64 and arm64.

Swift Package Manager has a few different ways to achieve this. The easiest way, as far as I can tell, is to pass the hidden --arch flag once for each architecture:

% swift build --configuration release --arch arm64 --arch x86_64

This goes through a different code path in Swift Package Manager, and utilizes Xcode's underlying XCBuild tool. This results in the built binary being in a different path than usual. Inspecting the new artifact, we can see we have a binary containing both requested architectures:

% file .build/apple/Products/Release/package
.build/apple/Products/Release/package: Mach-O universal binary with 2 architectures: [x86_64:Mach-O 64-bit executable x86_64] [arm64:Mach-O 64-bit executable arm64]
.build/apple/Products/Release/package (for architecture x86_64):        Mach-O 64-bit executable x86_64
.build/apple/Products/Release/package (for architecture arm64): Mach-O 64-bit executable arm64

Another option is to build once for each architecture, and then combine the binaries using lipo. Unlike the --arch option, this approach also works on Linux. Here's an example:

% swift build --configuration release --triple arm64-apple-macosx
% swift build --configuration release --triple x86_64-apple-macosx
% lipo -create -output package .build/arm64-apple-macosx/release/package .build/x86_64-apple-macosx/release/package

Inspecting our final binary we can see it correctly has both architectures:

% file package
package: Mach-O universal binary with 2 architectures: [x86_64:Mach-O 64-bit executable x86_64] [arm64:Mach-O 64-bit executable arm64]
package (for architecture x86_64):      Mach-O 64-bit executable x86_64
package (for architecture arm64):       Mach-O 64-bit executable arm64