[wasi] .NET 10: WasmEnableSIMD with AOT fails because WasiApp.targets has -msimd128 commented out

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
78/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
csharp, wasm
Domain
build-system

Research direction

Start with the .NET 10 WasiApp.targets file in Microsoft.NET.Runtime.WebAssembly.Wasi.Sdk and compare its SIMD flag with the .NET 11 version. Run the reported dotnet publish -c Release reproduction; done means the AOT publish succeeds and produces an app that prints 42.

Written by the indexing model from the issue text.

Description

area-Build-mono untriaged
Description

With the .NET 10 SDK 10.0.401 and the wasi-experimental workload 10.0.12, a wasi-wasm app with WasmEnableSIMD and RunAOTCompilation fails to publish. WasiApp.targets in Microsoft.NET.Runtime.WebAssembly.Wasi.Sdk 10.0.12 has the SIMD flag commented out:

<!--<_WasiClangCommonFlags Include="-msimd128"                         Condition="'$(WasmEnableSIMD)' == 'true'" />-->

so the AOT bitcode, which uses SIMD, is compiled by wasi-sdk's clang for a target without SIMD, and the LLVM backend aborts. The same line is active in the .NET 11 workload (11.0.0-rc.3.26522.148), so this looks like a fix that .NET 10 servicing is missing.

Found while building a SIMD-enabled Mono AOT baseline for the benchmarks of NetWasm, an ahead-of-time C# to WebAssembly compiler.

Reproduction Steps

macOS arm64, wasi-sdk 25.0 (the version the 10.0 targets expect), WASI_SDK_PATH set.

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net10.0</TargetFramework>
    <RuntimeIdentifier>wasi-wasm</RuntimeIdentifier>
    <RunAOTCompilation>true</RunAOTCompilation>
    <WasmSingleFileBundle>true</WasmSingleFileBundle>
    <WasmEnableSIMD>true</WasmEnableSIMD>
  </PropertyGroup>
</Project>

System.Console.WriteLine(42);

dotnet publish -c Release
Expected behavior

An AOT-compiled AppBundle/Hello.wasm that prints 42, as without WasmEnableSIMD.

Actual behavior
WasmApp.Common.targets(816,5): error : Failed to compile .../obj/Release/net10.0/wasi-wasm/wasm/for-publish/aot-instances.dll.bc -> .../aot-instances.dll.o
error : clang version 19.1.5-wasi-sdk (https://github.com/llvm/llvm-project ab4b5a2db582958af1ee308a790cfdb42bd24720)
error : Target: wasm32-unknown-wasip2
fatal error: error in backend: Do not know how to split this operator's operand!
Regression?

Not checked against earlier 10.0 servicing releases. The .NET 11 RC3 targets have the line enabled.

Known Workarounds

Add <WasiClangFlags>-msimd128</WasiClangFlags> to the project. With it the app publishes and prints 42 under Wasmtime.

Configuration
  • .NET SDK 10.0.401, wasi-experimental workload 10.0.12 (manifest 10.0.112)
  • wasi-sdk 25.0 (clang 19.1.5-wasi-sdk), macOS arm64
Other information

Uncommenting the -msimd128 line in the 10.0 WasiApp.targets, as in 11.0, should fix it.

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