swift: how to generate arm7 or arm64 from the command line - swift

Question

Using the Xcode 8.3 beta incarnation of swiftc, how do you generate an Arm7 or Arm64 binary?
I've tried the obvious argument -target-cpu arm64 which gives me a linker message <unknown>:0: warning: argument unused during compilation: '-mcpu=arm64'
ld: library not found for -lobjc and it plows ahead trying to build an x64 target.
Actual command:
swiftc -sdk /Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/ -L /Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/usr/lib -F /Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/System/Library/Frameworks/ -swift-version 3 -target-cpu arm64 somefile.swift

Answer 1

you can use the sdk and target option to do this. Here is an example:
/Projects/Test $ cat main.swift
print("Hello world!");
Compiling for x86_64
/Projects/Test $ swiftc main.swift
/Projects/Test $ lipo -info main
Non-fat file: main is architecture: x86_64
/Projects/Test $ ./main
Hello world!
Compiling for armv7
/Projects/Test $ swiftc main.swift -sdk /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS10.2.sdk -target armv7-apple-ios8.1
/Projects/Test $ lipo -info main
Non-fat file: main is architecture: armv7
Compiling for arm64
/Projects/Test $ swiftc main.swift -sdk /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS10.2.sdk -target arm64-apple-ios8.1
/Projects/Test $ lipo -info main
Non-fat file: main is architecture: arm64
I wrote a script build.sh so that its easier to use:
#!/bin/sh
TARGET_MAC_VERSION=10.11
TARGET_IOS_VERSION=8.1
if [ "$#" -ne 2 ]; then
echo "Usage $0: <armv7/arm64/x86_64> <file>"
exit
fi
if [ "$1" != 'armv7' ] && [ "$1" != 'arm64' ] && [ "$1" != 'x86_64' ]; then
echo "Usage $0: <armv7/arm64/x86_64>"
exit
fi
if [ "$1" == 'x86_64' ]; then
SDK=macosx
TARGET="x86_64-macosx$TARGET_MAC_VERSION"
else
SDK=iphoneos
TARGET="$1-apple-ios$TARGET_IOS_VERSION"
fi
echo "xcrun -sdk $SDK swiftc $2 -target $TARGET"
xcrun -sdk $SDK swiftc $2 -target $TARGET
Output
$ ./build.sh armv7 main.swift
xcrun -sdk iphoneos swiftc main.swift -target armv7-apple-ios8.1
$ lipo -info main
Non-fat file: main is architecture: armv7
$ ./build.sh arm64 main.swift
xcrun -sdk iphoneos swiftc main.swift -target arm64-apple-ios8.1
$ lipo -info main
Non-fat file: main is architecture: arm64
$ ./build.sh x86_64 main.swift
xcrun -sdk macosx swiftc main.swift -target x86_64-macosx10.11
$ lipo -info main
Non-fat file: main is architecture: x86_64
Edit Note: Optimized the script based on the input from #jens. See the comments for more information.