[linker] Convert the EnsureUIThread optimization to a trimmer feature switch. Fixes #26104. by rolfbjarne · Pull Request #26140 · dotnet/macios

rolfbjarne · 2026-07-16T17:16:40Z

Replace the hand-written IL rewrite of [NS|UI]Application.EnsureUIThread with an ObjCRuntime.Runtime.CheckForIllegalCrossThreadCalls trimmer feature switch, driven by a new $(CheckForIllegalCrossThreadCalls) MSBuild property (defaults to the build configuration: on for debug, off for release).

Remove ProcessEnsureUIThread (and its case) from the linker's OptimizeGeneratedCode step, and mark the now-orphaned --optimize=remove-uithread-checks flag as removed (following the existing inline-intptr-size convention).
Emit the ObjCRuntime.Runtime.CheckForIllegalCrossThreadCalls RuntimeHostConfigurationOption from Xamarin.Shared.Sdk.targets and stub the EnsureUIThread method body via ILLink substitutions on all four platforms when the feature is off.
Make [NS|UI]Application.CheckForIllegalCrossThreadCalls trimmable when the checks are off, without an API break: drop the field's = true initializer and instead set it from the application startup path, guarded by an internal feature-switched Runtime.CheckForIllegalCrossThreadCalls getter. When the feature is off the getter substitutes to a constant false, so the assignment is dead-code-eliminated and — with EnsureUIThread also stubbed — the field can be trimmed away entirely.
Document the new $(CheckForIllegalCrossThreadCalls) property and update optimizations.md.

Because the property is emitted as a runtime feature switch, it also takes effect when trimming is disabled (the field reflects the property value at runtime); trimming is only needed to physically remove the check code.

Add EnsureUIThreadChecksTest (with a dedicated EnsureUIThreadApp project that keeps EnsureUIThread reachable) verifying the feature switch value and that, in the app bundle, EnsureUIThread is stubbed and the field is trimmed when the checks are off (iOS + macOS).

Fixes #26104

🤖 Pull request created by Copilot

… switch Replace the hand-written IL rewrite of `[NS|UI]Application.EnsureUIThread` with an `ObjCRuntime.Runtime.CheckForIllegalCrossThreadCalls` trimmer feature switch, driven by a new `$(CheckForIllegalCrossThreadCalls)` MSBuild property (defaults to the build configuration: on for debug, off for release). * Remove `ProcessEnsureUIThread` (and its case) from the linker's OptimizeGeneratedCode step. * Emit the `ObjCRuntime.Runtime.CheckForIllegalCrossThreadCalls` RuntimeHostConfigurationOption from Xamarin.Shared.Sdk.targets and stub the `EnsureUIThread` method body via ILLink substitutions on all four platforms when the feature is off. * Document the new `$(CheckForIllegalCrossThreadCalls)` property. Also make `[NS|UI]Application.CheckForIllegalCrossThreadCalls` trimmable when the checks are off, without an API break: the field's `= true` initializer rooted it permanently. Instead, add an internal feature-switched `Runtime.CheckForIllegalCrossThreadCalls` getter (with matching substitutions) and set the public field to true from the application startup path, guarded by `if (Runtime.CheckForIllegalCrossThreadCalls)`. When the feature is off the getter substitutes to a constant `false`, so the guarded assignment is dead-code-eliminated and — with `EnsureUIThread` also stubbed — the field can be trimmed away entirely. Add EnsureUIThreadChecksTest (with a dedicated EnsureUIThreadApp project that keeps EnsureUIThread reachable) verifying the feature switch value and that, in the app bundle, EnsureUIThread is stubbed and the field is trimmed when the checks are off (iOS + macOS). Fixes #26104 Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

The EnsureUIThread IL rewrite (its only consumer) is gone, so the `--optimize=remove-uithread-checks` mtouch/mmp flag no longer does anything. Follow the existing `inline-intptr-size` convention: keep the option name registered (so passing it doesn't error) but empty its valid platforms so it degrades to an MT2003 warning, and drop the now-dead default. Update optimizations.md to point at the replacement `CheckForIllegalCrossThreadCalls` MSBuild property. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot

Pull request overview

This PR moves the [NS|UI]Application.EnsureUIThread “remove UI thread checks” behavior from a hand-authored linker IL rewrite to an ILLink feature switch (ObjCRuntime.Runtime.CheckForIllegalCrossThreadCalls) driven by a new $(CheckForIllegalCrossThreadCalls) MSBuild property (defaulting to enabled in Debug and disabled in Release). This aligns the behavior with other trimmer feature switches and enables ILLink to stub/check-eliminate more effectively.

Changes:

Remove the linker’s ProcessEnsureUIThread rewrite and mark --optimize=remove-uithread-checks as no longer applicable.
Add the ObjCRuntime.Runtime.CheckForIllegalCrossThreadCalls runtime feature switch, plus ILLink substitutions to stub EnsureUIThread when disabled.
Add an end-to-end unit test app + test to validate the emitted feature switch and the resulting linked assembly shape; update docs to describe the new MSBuild property and deprecate the old optimize flag.

Reviewed changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
tools/linker/OptimizeGeneratedCode.cs	Removes the special-case IL rewrite for `EnsureUIThread`.
tools/common/Optimizations.cs	Keeps the old optimize flag name but makes it non-applicable (removed) and removes its old defaulting logic.
tests/dotnet/UnitTests/EnsureUIThreadChecksTest.cs	Adds an end-to-end test validating the feature switch and that `EnsureUIThread` is stubbed / field trimmed when disabled.
tests/dotnet/EnsureUIThreadApp/iOS/EnsureUIThreadApp.csproj	Adds a minimal test app project for iOS.
tests/dotnet/EnsureUIThreadApp/tvOS/EnsureUIThreadApp.csproj	Adds a minimal test app project for tvOS.
tests/dotnet/EnsureUIThreadApp/macOS/EnsureUIThreadApp.csproj	Adds a minimal test app project for macOS.
tests/dotnet/EnsureUIThreadApp/MacCatalyst/EnsureUIThreadApp.csproj	Adds a minimal test app project for Mac Catalyst.
tests/dotnet/EnsureUIThreadApp/shared.csproj	Shared project settings + compilation inputs for the test app.
tests/dotnet/EnsureUIThreadApp/EnsureUIThreadApp.cs	References `EnsureUIThread` to keep it reachable for inspection in linked output.
tests/dotnet/EnsureUIThreadApp/Directory.Build.props	Imports the parent Directory.Build.props for consistent test infrastructure settings.
tests/assembly-preparer/OptimizeGeneratedCodeHandlerTests.cs	Removes tests for the deleted IL rewrite and adjusts coverage to other optimizations.
src/UIKit/UIApplication.cs	Makes `CheckForIllegalCrossThreadCalls` trimmable and ties initialization to the feature switch.
src/AppKit/NSApplication.cs	Makes `CheckForIllegalCrossThreadCalls` trimmable and ties initialization to the feature switch.
src/ObjCRuntime/Runtime.cs	Adds an optimizable getter wrapping the feature switch for ILLink substitution.
src/ILLink.Substitutions.iOS.xml	Adds substitutions to constant-fold the feature switch and stub `UIApplication.EnsureUIThread` when disabled.
src/ILLink.Substitutions.tvOS.xml	Same substitutions for tvOS.
src/ILLink.Substitutions.MacCatalyst.xml	Same substitutions for Mac Catalyst.
src/ILLink.Substitutions.macOS.xml	Same substitutions for macOS (`NSApplication.EnsureUIThread`).
dotnet/targets/Xamarin.Shared.Sdk.targets	Emits the `RuntimeHostConfigurationOption` based on `$(CheckForIllegalCrossThreadCalls)` with Debug/Release defaults.
docs/website/optimizations.md	Notes the optimize flag is replaced and points users to the MSBuild property.
docs/building-apps/build-properties.md	Documents the new `CheckForIllegalCrossThreadCalls` property and its trimming/runtime effects.

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

…convert-ensureuithread-optimization-to-a-5f492b

…nstant folding The DeadCodeElimination and NoOptimizationWithoutBindingAttributes tests used `if (true) { return 1; } return 2;` as the dead-code sample, but the C# compiler itself constant-folds `if (true)` and never emits the `return 2` (ldc.i4.2) in the IL. As a result NoOptimizationWithoutBindingAttributes failed (there was no dead code to preserve) and DeadCodeElimination passed only trivially (the dead code was already gone before the optimizer ran). Use `if (Runtime.IsARM64CallingConvention)` instead: Roslyn can't fold it, so the `return 2` survives compilation. The assembly-preparer's InlineIsARM64CallingConvention optimization inlines the field access to a constant, after which dead-code elimination removes the unreachable branch - but only when the method is optimizable ([BindingImpl(Optimizable)]). This exercises both the inline and the dead-code-elimination passes and the [BindingImpl] gating. The test assembly is now built with <Optimize>true</Optimize> (so the condition is a direct `ldsfld; brfalse` rather than a debug-mode stloc/ldloc round-trip that defeats constant folding), and with TargetArchitectures=ARM64 so the ARM64 calling-convention value is known. Add extraCsproj/extraConfig passthrough to BaseClass.AssertPrepareCode to support this. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 4fdcea25-bb9d-4fd5-b750-c47bff394b74

vs-mobiletools-engineering-service2 · 2026-07-17T17:29:20Z

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vs-mobiletools-engineering-service2 · 2026-07-17T20:07:05Z

🔥 [PR Build #ca53f18] Build failed (Build macOS tests) 🔥

Build failed for the job 'Build macOS tests' (with job status 'Canceled')

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Hash: ca53f180fc2220779b34eca98fa21de78d0556be [PR build]

vs-mobiletools-engineering-service2 · 2026-07-17T20:20:34Z

🔥 [CI Build #ca53f18] Test results 🔥

Test results

❌ Tests failed on VSTS: test results

8 tests crashed, 3 tests failed, 141 tests passed.

Failures

❌ dotnettests tests (iOS)

1 tests failed, 0 tests passed.

Failed tests

DotNet tests: Failed (Execution failed with exit code 1)
- Xamarin.Tests.AppSizeTest.MonoVM_Interpreter(iOS,"ios-arm64"): Multiple failures or warnings in test:

Unexpected APIs were added to the preserved set. The updated expected file is avai...
* Xamarin.Tests.AppSizeTest.MonoVM(iOS,"ios-arm64"): Multiple failures or warnings in test:
Unexpected APIs were added to the preserved set. The updated expected file is avai...

Html Report (VSDrops) Download

❌ dotnettests tests (MacCatalyst)

🔥 Failed catastrophically on VSTS: test results - dotnettests_maccatalyst (no summary found).

Html Report (VSDrops) Download

❌ dotnettests tests (macOS)

1 tests failed, 0 tests passed.

Failed tests

DotNet tests: Failed (Execution failed with exit code 1)
- Xamarin.Tests.AppSizeTest.NativeAOT_TrimmableStatic(MacOSX,"osx-...: App size changed significantly (+16,362 bytes (16.0 KB = 0.0 MB) different > tolerance of +-10,240 bytes (10.0 KB = 0.0 MB)). Ex...

Html Report (VSDrops) Download

❌ dotnettests tests (tvOS)

1 tests failed, 0 tests passed.

Failed tests

DotNet tests: Failed (Execution failed with exit code 1)
- Xamarin.Tests.AppSizeTest.MonoVM_Interpreter(TVOS,"tvos-arm64"): Multiple failures or warnings in test:

Unexpected APIs were added to the preserved set. The updated expected file is avai...
* Xamarin.Tests.AppSizeTest.MonoVM(TVOS,"tvos-arm64"): Multiple failures or warnings in test:
Unexpected APIs were added to the preserved set. The updated expected file is avai...
* Xamarin.Tests.AppSizeTest.NativeAOT_TrimmableStatic(TVOS,"tvos-a...: App size changed significantly (+16,378 bytes (16.0 KB = 0.0 MB) different > tolerance of +-10,240 bytes (10.0 KB = 0.0 MB)). Ex...

Html Report (VSDrops) Download

❌ linker tests (iOS)

🔥 Failed catastrophically on VSTS: test results - linker_ios (no summary found).

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