[wasm] Fix R2R codegen trap on async try/return/catch normal-completion branch

[lewing]

Fixes #131285

Problem

On WebAssembly R2R (crossgen), an async method with a try { ...; return; } catch { } where the try completes normally by branching out early (a return after an await) generates a valid wasm module that traps at runtime with RuntimeError: unreachable on the normal-completion path.

This is distinct from #131279 (dropped terminal end → invalid module) and is not a reopening of #129335 — here the module is well-formed and validates; it simply traps at runtime.

Root cause

In Compiler::fgWasmControlFlow(), a non-contiguous forward branch that exits a try/catch region started its enclosing wasm Block at the innermost try's begin. When the branch also escapes an outer catch-try that shares the same end cursor — the wasm catch-resumption / resume-after-catch shape, where the paired ExnRefWrapper is stretched past the try-end to cover the resumption dispatcher — the Block ends up nested inside that outer try. The normal-completion branch then resolves to depth 0 and lands on the outer try's trailing validation unreachable (emitted right after its end by genEmitStartBlock) instead of the continuation.

Fix

Walk the enclosing try regions outward and start the Block at the outermost escaped catch-try's begin, so the Block encloses those trys' ends and the branch resolves to the continuation (br 0 → br 1). For a single-level try/catch the outermost escaped region is also the innermost, so behavior there is unchanged.

Test

Adds src/tests/JIT/Regression/JitBlue/Runtime_131285, a sync [Fact] that drives an async Task MoveNext through the normal-completion return path. The csproj sets <AlwaysUseCrossGen2 Condition="'$(TargetOS)' == 'browser'">true</AlwaysUseCrossGen2> so the browser leg is actually compiled to wasm R2R and run under node — without it a plain browser leg runs the test as ordinary JIT and silently false-passes.

Validation

• Runtime A/B on the exact CLRTest node command: unfixed JIT traps (exit=1), fixed JIT passes (exit=100).
• Composite-mode crossgen A/B: unfixed emits trapping br 0, fixed emits br 1.
• 194-assembly sweep: 0 new invalid modules, 185 byte-identical; single-level try/catch provably unaffected.

Note

This pull request was generated with the assistance of GitHub Copilot.

[azure-pipelines]

Azure Pipelines:
Successfully started running 6 pipeline(s).
10 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[dotnet-policy-service]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

This PR adjusts wasm control-flow interval construction in fgWasmControlFlow() so forward branches that exit multiple enclosing try/catch regions create their enclosing Block interval starting at the outermost escaped catch-try’s try begin, avoiding mis-targeted branches that can land on the post-end validation unreachable and trap at runtime in wasm R2R async EH shapes.

Changes:

• Update wasm interval selection logic to walk outward across enclosing EH regions and choose a safer Block start for non-contiguous forward branches that exit catch-try regions.
• Add a wasm R2R-focused regression test that forces CrossGen2 on the browser leg to ensure the problematic codegen path is actually exercised.
• Introduce a dedicated JitBlue regression test project for the repro.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/coreclr/jit/fgwasm.cpp	Walks enclosing EH regions to pick the outermost escaped catch-try begin as the Block interval start for affected forward branches.
src/tests/JIT/Regression/JitBlue/Runtime_131285/Runtime_131285.csproj	Forces CrossGen2 only on TargetOS=browser to ensure wasm R2R coverage for the regression.
src/tests/JIT/Regression/JitBlue/Runtime_131285/Runtime_131285.cs	Adds an async try/return/catch repro driven synchronously to validate normal-completion behavior under wasm R2R.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

[azure-pipelines]

Azure Pipelines:
Successfully started running 6 pipeline(s).
10 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[AndyAyersMS]

There's probably a more elegant way to do this, but this fix seems reasonable enough.

[am11]

/__w/1/s/src/tests/JIT/Regression/JitBlue/Runtime_131285/Runtime_131285.csproj(7,59): error MSB4025: The project file could not be loaded. An XML comment cannot contain '--', and '-' cannot be the last character. Line 7, position 59. [/__w/1/s/src/tests/build.proj]
/__w/1/s/src/tests/JIT/Regression/JitBlue/Runtime_131285/Runtime_131285.csproj(7,59): error MSB4025: The project file could not be loaded. An XML comment cannot contain '--', and '-' cannot be the last character. Line 7, position 59. [/__w/1/s/src/tests/build.proj]
/__w/1/s/src/tests/Directory.Build.targets(508,5): error MSB4181: The "MSBuild" task returned false but did not log an error. [/__w/1/s/src/tests/JIT/Regression/Regression_2.csproj] [/__w/1/s/src/tests/build.proj]
/__w/1/s/src/tests/Common/dir.traversal.targets(25,5): error : (No message specified) [/__w/1/s/src/tests/build.proj] [/__w/1/s/src/tests/build.proj]
/__w/1/s/src/tests/build.proj(266,5): error MSB3073: The command ""/__w/1/s/dotnet.sh" msbuild /__w/1/s/src/tests/build.proj /t:Build "/p:TargetArchitecture=x64" "/p:Configuration=Checked" "/p:LibrariesConfiguration=Release" "/p:TasksConfiguration=Checked" "/p:TargetOS=AnyOS" "/p:ToolsOS=" "/p:PackageOS=" "/p:RuntimeFlavor=coreclr" "/p:RuntimeVariant=" "/p:CLRTestBuildAllTargets=allTargets" "/p:UseCodeFlowEnforcement=" "/p:__TestGroupToBuild=3" "/p:__SkipRestorePackages=1" /nodeReuse:false /maxcpucount "/p:UseMonoRuntime=false" /bl:/__w/1/s/artifacts//log/Checked/InnerManagedTestBuild.3.binlog "/p:CrossBuild=true" /p:ContinuousIntegrationBuild=true "/p:DevTeamProvisioning=-"" exited with code 1.
    6 Warning(s)
    5 Error(s)

[am11]

Suggested change

	actually compiled to wasm R2R and run under node -- otherwise a plain
	actually compiled to wasm R2R and run under node; otherwise a plain