Embedded Swift support for ESP32-S3, ESP32-S2 and original ESP32 (Xtensa arch)

Platform Embedded
1

There have been several community requests to add support for older Espressif MCUs based on the Xtensa architecture—specifically, the ESP32-S3, ESP32-S2, and the original ESP32. While newer MCUs are based on RISC-V and are supported by the upstream compiler, those using Xtensa require a custom compiler build.

I’ve prepared a repository containing a build script that should generate a compatible version of the compiler: GitHub - georgik/swift-xtensa: ESP32 - Xtensa support build of Swift

The build process requires two additional forked repositories (the script downloads them automatically):

This work is derived from efforts made for the Rust compiler. LLVM patches from Espressif’s LLVM version have been applied to the Swift fork of LLVM. Additionally, Xtensa-related targets have been registered in the Swift fork. In theory, this approach should work.

However, the test build is currently failing with the following error:

Error details:
<unknown>:0: error: embedded swift cannot be enabled in a compiler built without Swift sources

Installed Swift libraries:
swift-xtensa/install/lib/swift/shims/EmbeddedShims.h
swift-xtensa/install/lib/swift/macosx/libswiftCore.dylib
swift-xtensa/install/lib/swift/macosx/libswiftCompatibilitySpan.dylib
swift-xtensa/install/lib/swift/macosx/libswiftSwiftOnoneSupport.dylib

It seems that stdlib is not properly built.
I would appreciate any insights into what might be missing.

3 Likes
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What's the build-script invocation that you're using to build this?

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Part specific to custom build of Swift is here: swift-xtensa/swift-xtensa-build.sh at main · georgik/swift-xtensa · GitHub

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To be able to build this more easily and upstream these changes if that's possible at all, I'd suggest migrating to build-script instead of invoking cmake directly on the top-level swift directory. There are a lot of assumptions in CMake files that expect options passed in certain combinations from build-script and corresponding presets, especially on macOS.

1 Like
5

Thank you for the suggestion, I've added new branch feature/xtensa-2025-08, changing the build method to build-script.
The script is here: swift-xtensa/swift-xtensa-build-script.sh at feature/build-script · georgik/swift-xtensa · GitHub
The build seems to be failing on several places. It will require further investigation.

1 Like
6

I'd surmise you have to clean up --llvm-cmake-options and --swift-cmake-options, as most of these are inferred by build-script and some things overridden manually could conflict with inferred options.

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Wondering is there any progress on this?

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Hi. Unfortunatelly there is no progress on Xtensa part. There are many issues. I would recommend using RISC-V MCUs from Espressif portfolio which works with Swift without problem.

9

Thanks for the answer!