Fix nested exception handler reachability in ILLink by sbomer · Pull Request #131236 · dotnet/runtime

sbomer · 2026-07-22T21:53:28Z

Fix exception-handler reachability analysis in the unreachable blocks optimizer by rescanning handlers until no newly reachable handler bodies expose additional nested protected regions.

Previously, handlers were scanned only once. A nested handler could therefore be classified as unreachable before its enclosing handler body was traversed, causing required cleanup code to be removed.

Add a regression test covering an async method with nested finally blocks.

Validation:

Mono.Linker UnreachableBlock tests: 22 passed
Full Mono.Linker test suite: 1,134 passed, 30 skipped
Original standalone repro behaves identically with ipconstprop enabled and disabled

Fixes #131088

Note

This content was created with assistance from AI (GitHub Copilot).

Rescan exception handlers until no newly reachable handler bodies expose additional nested protected regions. Add an async nested-finally regression test for the trimming failure. Assisted-by: GitHub Copilot:gpt-5.6-sol Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 2cee0e07-8e06-4275-b1de-508fe931bf95

azure-pipelines · 2026-07-22T21:54:06Z

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dotnet-policy-service · 2026-07-22T21:54:40Z

Tagging subscribers to this area: @agocke, @dotnet/illink
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Pull request overview

This PR fixes ILLink’s unreachable-block analysis for exception handlers by repeatedly discovering newly reachable handler bodies until reaching a fixed point, preventing nested handlers (e.g., nested finally blocks in async state machines) from being misclassified as unreachable and incorrectly removed.

Changes:

Update UnreachableBlocksOptimizer reachability analysis to iteratively enqueue newly reachable exception handler (and filter) blocks until no additional handlers become reachable.
Add a regression test case exercising an async method with nested finally blocks under ipconstprop.
Add a dependency library used by the new test to reproduce the nested-handler shape.

Show a summary per file

File	Description
src/tools/illink/src/linker/Linker.Steps/UnreachableBlocksOptimizer.cs	Reworks exception-handler reachability to rescan handlers and traverse newly reachable handler bodies, enabling correct discovery of nested protected regions.
src/tools/illink/test/Mono.Linker.Tests.Cases/UnreachableBlock/NestedFinallyInAsyncMethod.cs	New test driver enabling `ipconstprop` and asserting nested-cleanup executes (via a counter).
src/tools/illink/test/Mono.Linker.Tests.Cases/UnreachableBlock/Dependencies/NestedFinallyInAsyncMethod_Lib.cs	New dependency library containing an async method with nested `finally` blocks to reproduce the scenario.

Copilot's findings

Files reviewed: 3/3 changed files
Comments generated: 0

jtschuster

Source changes look good to me, just wanted some clarifications on why tests are the way they are.

Replace the async dependency-based repro with a synchronous nested-finally case that exercises the same handler reachability bug. Track the test as an expected ILTrim limitation because ILTrim does not implement ipconstprop. Assisted-by: GitHub Copilot:gpt-5.6-sol Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 2cee0e07-8e06-4275-b1de-508fe931bf95

Copilot

Copilot's findings

Comments suppressed due to low confidence (1)

src/tools/illink/test/Mono.Linker.Tests.Cases/UnreachableBlock/NestedFinallyInFinallyHandler.cs:52

After removing CleanupCount, Cleanup should no longer reference it. Keeping Cleanup as a no-op still allows the test to validate that the nested finally body remains reachable (via the existing [Kept] expectation on Cleanup).

        static void Cleanup() => CleanupCount++;

Files reviewed: 3/3 changed files
Comments generated: 2

+        [Kept]
+        static int CleanupCount;
+
+        public static void Main()
+        {
+            Test();
+
+            if (CleanupCount != 1)
+                throw new InvalidOperationException();
+        }


 UnreachableBlock.MultiStageRemoval
+UnreachableBlock.NestedFinallyInFinallyHandler
 UnreachableBlock.ReplacedJumpTarget


hydraxman

Thanks for fixing this. The change looks good to me. Left 2 small comments. Also the PR text still says the test is async, but its sync now.

hydraxman · 2026-07-23T07:08:43Z

+                {
+                    Reached();
+                }
+                finally


Could we add one more nested finally here? With only 2 levels, an impl that just scans twice would still pass, so this does not fully test the fixed point loop.

hydraxman · 2026-07-23T07:08:43Z

+                        // Newly reachable handlers can contain protected regions for nested handlers.
                        var instrs = Instructions;
-                        foreach (var handler in ExceptionHandlers)
+                        for (int handlerIndex = 0; handlerIndex < ExceptionHandlers.Count; handlerIndex++)


Should we cache the try ranges before the loop? IndexOf is linear and now runs for every handler on each pass. Probably minor, but easy to avoid.

Copilot AI review requested due to automatic review settings July 22, 2026 21:53

github-actions Bot added the area-Tools-ILLink .NET linker development as well as trimming analyzers label Jul 22, 2026

dotnet-policy-service Bot added the linkable-framework Issues associated with delivering a linker friendly framework label Jul 22, 2026

dotnet-policy-service Bot assigned sbomer Jul 22, 2026

github-project-automation Bot added this to AppModel Jul 22, 2026

Copilot started reviewing on behalf of sbomer July 22, 2026 21:53 View session

Copilot AI reviewed Jul 22, 2026

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sbomer requested a review from a team July 22, 2026 22:11

jtschuster reviewed Jul 22, 2026

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Comment thread ...test/Mono.Linker.Tests.Cases/UnreachableBlock/Dependencies/NestedFinallyInAsyncMethod_Lib.cs Outdated

jtschuster reviewed Jul 22, 2026

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Comment thread ...test/Mono.Linker.Tests.Cases/UnreachableBlock/Dependencies/NestedFinallyInAsyncMethod_Lib.cs Outdated

jtschuster reviewed Jul 22, 2026

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Comment thread src/tools/illink/test/Mono.Linker.Tests.Cases/UnreachableBlock/NestedFinallyInAsyncMethod.cs Outdated

jtschuster approved these changes Jul 22, 2026

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build-analysis Bot mentioned this pull request Jul 23, 2026

XHarness package install failure on iOS due to devicectl NSPOSIXErrorDomain error 49 #123796

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Copilot AI review requested due to automatic review settings July 23, 2026 02:02

Copilot started reviewing on behalf of sbomer July 23, 2026 02:03 View session

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hydraxman reviewed Jul 23, 2026

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Fix nested exception handler reachability in ILLink#131236

Fix nested exception handler reachability in ILLink#131236
sbomer wants to merge 2 commits into
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