Fix transition from monomorphic to polymorphic VSD

[AndyAyersMS]

An interface call site initially uses a dispatch stub specialized for one receiver type. After enough misses, the site should be backpatched to a resolve stub to handle polymorphic receivers.

On ARM, ARM64, LoongArch64, and RISC-V64, the expired-counter path set SDF_ResolveBackPatch but then probed the inline resolve cache. A cache hit there branched directly to the target without processing the flag, leaving the site monomorphic and causing subsequent receiver-type misses to continue through the dispatch stub.

Enter the slow resolve path after setting SDF_ResolveBackPatch so the call site is backpatched as intended. Also correct the inverted miss-counter handling on LoongArch64 and RISC-V64. This matches the existing x64 behavior.

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[AndyAyersMS]

@EgorBot -linux_arm64 -windows_arm64 --envvars DOTNET_TieredCompilation:0 DOTNET_TieredPGO:0 DOTNET_JitEnableGuardedDevirtualization:0 DOTNET_ReadyToRun:0

usingSystem.Runtime.CompilerServices;usingBenchmarkDotNet.Attributes;publicinterfaceITarget{intInvoke(intvalue);}publicsealedclassA:ITarget{[MethodImpl(MethodImplOptions.NoInlining)]publicintInvoke(intvalue)=>value+1;}publicsealedclassB:ITarget{[MethodImpl(MethodImplOptions.NoInlining)]publicintInvoke(intvalue)=>value+2;}[SimpleJob(launchCount:1,warmupCount:3,iterationCount:10,invocationCount:1)]publicclassVsdBackpatchBench{privateITarget_target=null!;privatelong_checksum;[Params(false,true)]publicboolSwitchType{get;set;}[Params(16)]publicintThreads{get;set;}[GlobalSetup]publicvoidSetup(){vara=newA();_checksum=Run(a,100_000);_target=SwitchType?newB():a;}[Benchmark]publiclongInterfaceCalls(){vargate=newManualResetEventSlim();varworkers=newThread[Threads];varresults=newlong[Threads];for(intthread=0;thread<Threads;thread++){intindex=thread;workers[thread]=newThread(()=>{gate.Wait();results[index]=Run(_target,20_000_000);});workers[thread].Start();}gate.Set();foreach(Threadworkerinworkers){worker.Join();}longchecksum=_checksum;foreach(longresultinresults){checksum^=result;}returnchecksum;}[MethodImpl(MethodImplOptions.NoInlining)]privatestaticlongRun(ITargettarget,intiterations){longsum=0;for(inti=0;i<iterations;i++){sum+=target.Invoke(i);}returnsum;}}

Note

This benchmark request was created with GitHub Copilot.

[Copilot]

Pull request overview

Updates the ARM64 virtual call resolve stub’s fail path so that when the dispatch-miss counter expires, the stub routes through the slow resolve path with SDF_ResolveBackPatch observed, enabling a monomorphic interface call site to transition to a polymorphic resolve stub (matching the established x64 behavior).

Changes:

• ARM64: On counter expiration, set SDF_ResolveBackPatch and branch to _slowEntryPoint (instead of _resolveEntryPoint) so backpatching can occur.
• ARM64: Add/adjust explanatory comments around why the slow path is required for backpatch flag processing.

[AndyAyersMS]

@davidwrighton@VSadov PTAL
fyi @EgorBo

Distilled from a real-world case I've been working on.

[AndyAyersMS]

Also Arm32/RiscV/LoongArch also ave this issue (and the latter two seem to also have some inverted logic).

[AndyAyersMS]

Also fyi @jkotas -- from what I can tell arm64 behavior has been like this (different than x64) for a long time.

[EgorBo]

@EgorBot orchard -aws_arm

[jkotas]

Good catch!

Also Arm32/RiscV/LoongArch also ave this issu

Do you plan to apply the fix there as part of this PR?

[AndyAyersMS]

Good catch!

Also Arm32/RiscV/LoongArch also ave this issu

Do you plan to apply the fix there as part of this PR?

Yeah, I can fix those as well.

[jkotas]

Thanks!

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

[AndyAyersMS]

/backport to release/11.0-rc1

[github-actions]

Started backporting to release/11.0-rc1 (link to workflow run)

[AndyAyersMS]

/backport to release/10.0

[github-actions]

Started backporting to release/10.0 (link to workflow run)

[github-actions]

@AndyAyersMS backporting to release/10.0 failed, the patch most likely resulted in conflicts. Please backport manually!

git am output

$ git cherry-pick 16f3d6c9031cb4c76abedd8a12f78bd86b0b9337Auto-merging src/coreclr/vm/arm/stubs.cppCONFLICT (content): Merge conflict in src/coreclr/vm/arm/stubs.cppAuto-merging src/coreclr/vm/arm64/virtualcallstubcpu.hppAuto-merging src/coreclr/vm/loongarch64/virtualcallstubcpu.hppAuto-merging src/coreclr/vm/riscv64/virtualcallstubcpu.hppCONFLICT (content): Merge conflict in src/coreclr/vm/riscv64/virtualcallstubcpu.hpperror: could not apply 16f3d6c9031... Fix transition from monomorphic to polymorphic VSD (#132544)hint: After resolving the conflicts, mark them withhint: "git add/rm <pathspec>", then runhint: "git cherry-pick --continue".hint: You can instead skip this commit with "git cherry-pick --skip".hint: To abort and get back to the state before "git cherry-pick",hint: run "git cherry-pick --abort".hint: Disable this message with "git config set advice.mergeConflict false"
$ git am --3way --empty=keep --ignore-whitespace --keep-non-patch changes.patchApplying: Fix ARM64 transition from monomorphic to polymorphic VSDApplying: Fix VSD backpatch on non-x64 architecturesUsing index info to reconstruct a base tree...M	src/coreclr/vm/arm/stubs.cppM	src/coreclr/vm/arm64/virtualcallstubcpu.hppM	src/coreclr/vm/loongarch64/virtualcallstubcpu.hppM	src/coreclr/vm/riscv64/virtualcallstubcpu.hppFalling back to patching base and 3-way merge...Auto-merging src/coreclr/vm/arm/stubs.cppCONFLICT (content): Merge conflict in src/coreclr/vm/arm/stubs.cppAuto-merging src/coreclr/vm/arm64/virtualcallstubcpu.hppAuto-merging src/coreclr/vm/loongarch64/virtualcallstubcpu.hppAuto-merging src/coreclr/vm/riscv64/virtualcallstubcpu.hppCONFLICT (content): Merge conflict in src/coreclr/vm/riscv64/virtualcallstubcpu.hpperror: Failed to merge in the changes.hint: Use 'git am --show-current-patch=diff' to see the failed patchhint: When you have resolved this problem, run "git am --continue".hint: If you prefer to skip this patch, run "git am --skip" instead.hint: To restore the original branch and stop patching, run "git am --abort".hint: Disable this message with "git config set advice.mergeConflict false"Patch failed at 0002 Fix VSD backpatch on non-x64 architecturesError: The process '/usr/bin/git' failed with exit code 128

Link to workflow output