JIT: Devirtualize shared generic virtual methods #123323

hez2010 · 2026-01-18T11:52:05Z

Previously in #122023, we hit an issue with GVM devirtualization when the devirtualized target is a shared generic method. GVM calls are imported with a runtime lookup that is specific to the base method. After devirtualization, the call requires the instantiation argument for the implementing method, and the existing lookup cannot be reused.

This PR unblocks devirtualization for shared generic targets by ensuring the call receives the correct instantiation parameter for the devirtualized method:

The multiple ad-hoc flags in dvInfo now have been unified into a single instParamLookup
When the target does not require a runtime lookup, we already know the exact generic context. We pass the instantiating stub as the inst param (shared with the existing array interface devirtualization path).
When the target requires a runtime lookup, we now introduced a new DictionaryEntryKind::DevirtualizedMethodDescSlot, and pass it to the instParamLookup so that later the VM knows that it needs to encode the class token from the devirtualized method instead of the original token.

To make this work for late devirtualization as well which runs as a post-import phase, impTokenToHandle has been changed so that after importation completes, runtime lookup trees are built using a way that doesn't append statements.

Also due to the instParamLookup change I implement the support for R2R as well.

Example:

IVritualGenericInterface i = new Processor();
Test(i, "test");

VirtualGenericClass c = new Processor();
Test(c, "test");

static void Test<T>(IVritualGenericInterface ifce, T item) where T : notnull
{
    ifce.Process(item);
}

static void Test<T>(VirtualGenericClass baseClass, T item) where T : notnull
{
    baseClass.Process(item);
}

public class Processor : VirtualGenericClass, IVritualGenericInterface
{
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public override void Process<T>(T item)
    {
        Console.WriteLine(typeof(T));
        Console.WriteLine(item.ToString());
    }
}

Codegen diff:

  G_M27646_IG01:
-       push     rdi
        push     rsi
        push     rbx
-       sub      rsp, 32
+       sub      rsp, 40
-                                                ;; size=7 bbWeight=1 PerfScore 3.25
+                                                ;; size=6 bbWeight=1 PerfScore 2.25
 G_M27646_IG02:
-       mov      rbx, 0xD1FFAB1E      ; Program+Processor
+       mov      rbx, 0xD1FFAB1E      ; 'System.String'
        mov      rcx, rbx
-       call     CORINFO_HELP_NEWSFAST
+       call     [System.Console:WriteLine(System.Object)]
-       mov      rsi, rax
+       mov      rsi, 0xD1FFAB1E      ; 'test'
-       mov      rdi, 0xD1FFAB1E      ; 'test'
        mov      rcx, rsi
-       mov      rdx, 0xD1FFAB1E      ; Program+IVritualGenericInterface
+       call     [System.Console:WriteLine(System.String)]
-       mov      r8, 0xD1FFAB1E      ; token handle
-       call     CORINFO_HELP_VIRTUAL_FUNC_PTR
+       mov      rcx, rbx
+       call     [System.Console:WriteLine(System.Object)]
        mov      rcx, rsi
-       mov      rdx, rdi
+       call     [System.Console:WriteLine(System.String)]
-       call     rax
+       nop
-       mov      rcx, rbx
+                                                ;; size=57 bbWeight=1 PerfScore 13.75
-       call     CORINFO_HELP_NEWSFAST
+G_M27646_IG03:
-       mov      rbx, rax
+       add      rsp, 40
-       mov      rcx, rbx
+       pop      rbx
-       mov      rdx, 0xD1FFAB1E      ; Program+VirtualGenericClass
+       pop      rsi
-       mov      r8, 0xD1FFAB1E      ; token handle
+       ret
-       call     CORINFO_HELP_VIRTUAL_FUNC_PTR
+                                                ;; size=7 bbWeight=1 PerfScore 2.25
-       mov      rcx, rbx
-       mov      rdx, rdi
-       call     rax
-       nop
-                                                ;; size=115 bbWeight=1 PerfScore 14.25
-G_M27646_IG03:
-       add      rsp, 32
-       pop      rbx
-       pop      rsi
-       pop      rdi
-       ret
-                                                ;; size=8 bbWeight=1 PerfScore 2.75

Contributes to #112596

Copilot

Pull request overview

This PR enables JIT devirtualization for shared generic virtual methods (GVM) that don't require runtime lookups. Previously, all shared GVMs were blocked from devirtualization due to concerns about having the right generic context. This change unblocks devirtualization when the instantiating stub doesn't need a runtime lookup, by checking for the presence of a GT_RUNTIMELOOKUP node before proceeding.

Changes:

Introduced needsMethodContext flag to track when a method context is needed for devirtualization
For shared generic methods, obtain the instantiating stub and set needsMethodContext = true
Unified handling of array interface and generic virtual method devirtualization paths
Added runtime lookup check in the JIT to bail out when a lookup is needed but context is unavailable

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/coreclr/vm/jitinterface.cpp	Added logic to detect shared generic methods and obtain instantiating stubs, unified array and GVM devirtualization handling
src/coreclr/jit/importercalls.cpp	Updated assertions to allow GVM in AOT scenarios, added runtime lookup check to prevent devirtualization when context is unavailable

src/coreclr/vm/jitinterface.cpp

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

hez2010 · 2026-01-18T12:51:29Z

@MihuBot

src/coreclr/jit/importercalls.cpp

hez2010 · 2026-01-18T15:27:20Z

Failures seem to be caused by missing context during spmi replay. Otherwise all tests are passing.

src/coreclr/vm/jitinterface.cpp

Copilot

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

src/coreclr/jit/importercalls.cpp

src/coreclr/vm/jitinterface.cpp

Copilot

Pull request overview

Copilot reviewed 17 out of 17 changed files in this pull request and generated 1 comment.

src/coreclr/vm/jitinterface.cpp

Copilot

Pull request overview

Copilot reviewed 17 out of 17 changed files in this pull request and generated 2 comments.

src/coreclr/tools/Common/Compiler/DevirtualizationManager.cs

src/coreclr/tools/Common/JitInterface/CorInfoImpl.cs

hez2010 · 2026-02-10T19:14:04Z

I realised that the existing runtime lookup tests are not actually doing runtime lookup, they are const lookup.
So I updated the runtime lookup tests to make them really perform a runtime lookup.

Copilot

Pull request overview

Copilot reviewed 18 out of 18 changed files in this pull request and generated 1 comment.

Copilot · 2026-02-10T19:23:10Z

src/coreclr/jit/importercalls.cpp

+            // If we don't know the array type exactly we may have the wrong interface type here.
+            // Bail out.
+            //
+            if (!isExact)


The needsCompileTimeLookup path unconditionally bails out when !isExact with an "Array interface devirt" message, but needsCompileTimeLookup can now also be true for shared generic virtual method devirtualization (not just array-interface devirt). This can prevent devirtualization in cases where objClassIsFinal/derivedMethodIsFinal would otherwise allow it. Consider gating the !isExact bailout (and the diagnostic text) to the array-interface scenario only, and allow the GVM path to proceed without requiring isExact.

Suggested change

// If we don't know the array type exactly we may have the wrong interface type here.

// Bail out.

//

if (!isExact)

// For array interface devirtualization, if we don't know the array type exactly

// we may have the wrong interface type here. Bail out.

//

if (!call->IsGenericVirtual(this) && !isExact)

Copilot

Pull request overview

Copilot reviewed 18 out of 18 changed files in this pull request and generated no new comments.

Copilot

Pull request overview

Copilot reviewed 18 out of 18 changed files in this pull request and generated 1 comment.

Copilot · 2026-02-11T09:37:22Z

src/coreclr/tools/Common/JitInterface/CorInfoImpl.cs

+                impl.IsCanonicalMethod(CanonicalFormKind.Specific))
+            {
+#if READYTORUN
+                MethodWithToken originalImplWithToken = new MethodWithToken(originalImpl, resolver.GetModuleTokenForMethod(originalImpl.GetTypicalMethodDefinition(), allowDynamicallyCreatedReference: false, throwIfNotFound: false), null, false, null, null);


In the const instParamLookup path, originalImplWithToken is created with GetModuleTokenForMethod(... throwIfNotFound: false) but the returned ModuleToken isn’t checked for IsNull before constructing MethodWithToken / creating the ReadyToRunHelperId.MethodHandle node. If the method isn’t representable in the R2R image, this can lead to asserts/NREs rather than gracefully failing devirtualization. Consider using throwIfNotFound: true here, or checking IsNull and returning CORINFO_DEVIRTUALIZATION_FAILED_DECL_NOT_REPRESENTABLE (or the appropriate detail) before building the helper node.

Suggested change

MethodWithToken originalImplWithToken = new MethodWithToken(originalImpl, resolver.GetModuleTokenForMethod(originalImpl.GetTypicalMethodDefinition(), allowDynamicallyCreatedReference: false, throwIfNotFound: false), null, false, null, null);

var originalImplModuleToken = resolver.GetModuleTokenForMethod(originalImpl.GetTypicalMethodDefinition(), allowDynamicallyCreatedReference: false, throwIfNotFound: false);

if (originalImplModuleToken.IsNull)

{

info->detail = CORINFO_DEVIRTUALIZATION_DETAIL.CORINFO_DEVIRTUALIZATION_FAILED_DECL_NOT_REPRESENTABLE;

return false;

}

MethodWithToken originalImplWithToken = new MethodWithToken(originalImpl, originalImplModuleToken, null, false, null, null);

hez2010 · 2026-02-11T12:32:19Z

Test failures seem unrelated.
@jakobbotsch Can I have another pri1 test run?

jakobbotsch · 2026-02-11T13:01:55Z

/azp run runtime-coreclr outerloop

azure-pipelines · 2026-02-11T13:02:16Z

Azure Pipelines successfully started running 1 pipeline(s).

Support for shared GVM devirt that doesn't require a runtime lookup

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github-actions bot added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Jan 18, 2026

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Update src/coreclr/vm/jitinterface.cpp

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

MihuBot mentioned this pull request Jan 18, 2026

[JitDiff X64] [hez2010] JIT: Devirtualize shared GVM that doesn't need a run ... MihuBot/runtime-utils#1717

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hez2010 commented Jan 18, 2026

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A better way to check runtime lookup

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hez2010 requested a review from MichalStrehovsky as a code owner January 18, 2026 13:15

Add a TODO

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hez2010 mentioned this pull request Jan 18, 2026

JIT: Devirtualization for generic virtual methods #112596

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hez2010 commented Jan 20, 2026

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Merge branch 'main' into gvm-devirt-shared

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This was referenced Jan 29, 2026

[android-arm64] The Operation will be canceled. The next steps may not contain expected logs. dotnet/dnceng#6408

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The Operation will be canceled. The next steps may not contain expected logs. dotnet/dnceng#3008

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hez2010 changed the title ~~JIT: Devirtualize shared GVM that doesn't need a runtime lookup~~ JIT: Devirtualize shared generic virtual methods Feb 1, 2026

Add support for runtime lookup as well

c8b7f69

hez2010 force-pushed the gvm-devirt-shared branch from bf5b09a to c8b7f69 Compare February 1, 2026 18:12

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hez2010 marked this pull request as draft February 1, 2026 18:23

hez2010 added 2 commits February 2, 2026 04:04

Try another approach

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Nit

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Guard lookupToken

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hez2010 added 2 commits February 11, 2026 02:48

More fixes

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Fix

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hez2010 added 2 commits February 11, 2026 04:12

Bail NativeAOT

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Make runtime lookup tests really do runtime lookup

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hez2010 force-pushed the gvm-devirt-shared branch from e48f6a7 to 583277b Compare February 10, 2026 19:12

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hez2010 added 2 commits February 11, 2026 04:47

Fix R2R const lookup

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Bail on runtime lookup for R2R

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R2R runtime lookup support

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Merge branch 'main' into gvm-devirt-shared

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System.IO.Tests.Directory_Create_Tests.FileSystemWatcher_Directory_Create_InNestedDirectory fails with "Created event did not occur as expected" #103630

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-                MethodWithToken originalImplWithToken = new MethodWithToken(originalImpl, resolver.GetModuleTokenForMethod(originalImpl.GetTypicalMethodDefinition(), allowDynamicallyCreatedReference: false, throwIfNotFound: false), null, false, null, null);
+                var originalImplModuleToken = resolver.GetModuleTokenForMethod(originalImpl.GetTypicalMethodDefinition(), allowDynamicallyCreatedReference: false, throwIfNotFound: false);
+                if (originalImplModuleToken.IsNull)
+                {
+                    info->detail = CORINFO_DEVIRTUALIZATION_DETAIL.CORINFO_DEVIRTUALIZATION_FAILED_DECL_NOT_REPRESENTABLE;
+                    return false;
+                }
+                MethodWithToken originalImplWithToken = new MethodWithToken(originalImpl, originalImplModuleToken, null, false, null, null);

JIT: Devirtualize shared generic virtual methods #123323

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