JIT: use type argument as the exact type for Activator.CreateInstance<T> by hez2010 · Pull Request #130074 · dotnet/runtime

hez2010 · 2026-07-01T10:02:18Z

Activator.CreateInstance<T> can only result in an instance of exact T. Teach the JIT so that when the JIT sees a call to Activator.CreateInstance<T>, it will know the result must be the exact T. Then any subsequent calls against the allocated object instance can be devirtualized.

This also applies to any where T : new() patterns.

Example:

class Program
{
    static T Get<T>() where T : new()
    {
        return new T();
    }

    class A
    {
        public virtual int Value => 1;
    }

    class B : A
    {
        public override int Value => 2;
    }

    static void Main()
    {
        Console.WriteLine(Get<A>().Value);
        Console.WriteLine(Get<B>().Value);
    }
}

Before:

G_M27646_IG01:  ;; offset=0x0000
       sub      rsp, 40
G_M27646_IG02:  ;; offset=0x0004
       mov      rcx, 0x7FF92B6AB428      ; System.Activator:CreateInstance[Program+A]():Program+A
       call     [System.Activator:CreateInstance[System.__Canon]():System.__Canon]
       mov      rcx, rax
       mov      rax, qword ptr [rax]
       mov      rax, qword ptr [rax+0x48]
       call     [rax+0x20]Program+A:get_Value():int:this
       mov      ecx, eax
       call     [System.Console:WriteLine(int)]
       mov      rcx, 0x7FF92B6AB890      ; System.Activator:CreateInstance[Program+B]():Program+B
       call     [System.Activator:CreateInstance[System.__Canon]():System.__Canon]
       mov      rcx, rax
       mov      rax, qword ptr [rax]
       mov      rax, qword ptr [rax+0x48]
       call     [rax+0x20]Program+A:get_Value():int:this
       mov      ecx, eax
       call     [System.Console:WriteLine(int)]
       nop
G_M27646_IG03:  ;; offset=0x004F
       add      rsp, 40
       ret

After:

G_M27646_IG01:  ;; offset=0x0000
       sub      rsp, 40
G_M27646_IG02:  ;; offset=0x0004
       mov      rcx, 0x7FF92C79B428      ; System.Activator:CreateInstance[Program+A]():Program+A
       call     [System.Activator:CreateInstance[System.__Canon]():System.__Canon]
       mov      ecx, 1
       call     [System.Console:WriteLine(int)]
       mov      rcx, 0x7FF92C79B890      ; System.Activator:CreateInstance[Program+B]():Program+B
       call     [System.Activator:CreateInstance[System.__Canon]():System.__Canon]
       mov      ecx, 2
       call     [System.Console:WriteLine(int)]
       nop
G_M27646_IG03:  ;; offset=0x003B
       add      rsp, 40
       ret

In the future we can apply escape analysis against CreateInstance<T> as well to omit unescaped object allocations.

dotnet-policy-service · 2026-07-01T10:03:29Z

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

hez2010 · 2026-07-01T10:04:28Z

@MihuBot -nuget

Copilot

Pull request overview

This PR extends CoreCLR JIT intrinsic recognition so calls to System.Activator.CreateInstance<T>() can be treated as producing an instance of the exact type argument T, enabling downstream optimizations (e.g., devirtualization) when the type argument is known.

Changes:

Adds a new named intrinsic NI_System_Activator_CreateInstance_T.
Recognizes Activator.CreateInstance with a type argument in lookupNamedIntrinsic and marks it as a “special intrinsic” call site.
Teaches gtGetClassHandle to recover the exact T for CreateInstance<T> calls to sharpen return type information.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/coreclr/jit/namedintrinsiclist.h	Adds a new `NamedIntrinsic` enum value for `Activator.CreateInstance<T>`.
src/coreclr/jit/importercalls.cpp	Recognizes `Activator.CreateInstance<T>` as a named intrinsic and flags it as “special” for later optimizations.
src/coreclr/jit/gentree.cpp	Attempts to infer the exact return type of `Activator.CreateInstance<T>` in `gtGetClassHandle`.

Copilot

Pull request overview

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

Comments suppressed due to low confidence (1)

src/coreclr/jit/importercalls.cpp:10706

lookupNamedIntrinsic now unconditionally calls eeGetMethodSig(method, &sig) even though the signature is only used for the Activator.CreateInstance check. Since lookupNamedIntrinsic runs for every intrinsic call import, this adds extra EE work on a hot import path. Consider fetching the signature only inside the Activator/CreateInstance branch (or using a cheaper generic-method check) so most intrinsics avoid the extra eeGetMethodSig call.

        info.compCompHnd->getMethodNameFromMetadata(method, &className, &namespaceName, enclosingClassNames,
                                                    ArrLen(enclosingClassNames));

    JITDUMP("Named Intrinsic ");

Copilot

Pull request overview

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

Copilot

Pull request overview

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

Copilot

Pull request overview

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

Copilot

Pull request overview

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

huoyaoyuan · 2026-07-01T11:29:07Z

Activator.CreateInstance<T> can only result in an instance of exact T.

Is this also applicable to COM imported types?

hez2010 · 2026-07-01T12:09:27Z

Is this also applicable to COM imported types?

Unlike new T(), Activator.CreateInstance<T> doesn't allow instantiating an interface type even if the interface type has a CoClass attribute.

For example:

using System.Runtime.InteropServices;

internal static class Program
{
    [STAThread]
    static void Main()
    {
        ShellWindows windows = new ShellWindows(); // changing to `Activator.CreateInstance<ShellWindows>()` will result in a runtime exception
        Console.WriteLine(windows.Count);
    }
}

[ComImport]
[Guid("9BA05972-F6A8-11CF-A442-00A0C90A8F39")]
internal class ShellWindowsClass
{
}

[ComImport]
[Guid("85CB6900-4D95-11CF-960C-0080C7F4EE85")]
[CoClass(typeof(ShellWindowsClass))]
[InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
internal interface ShellWindows
{
    [DispId(0x60020000)]
    int Count { get; }
}

jakobbotsch

This LGTM. @EgorBo what is your opinion?

EgorBo · 2026-07-01T16:08:13Z

LGTM, but to be honest, 29 total methods with Code Size differences accross a huge -nuget collection sounds very small to me

EgorBo · 2026-07-01T16:11:12Z

BTW, it looks like there are no hits in BCL so it's basically an untested bit of code?

hez2010 · 2026-07-01T16:25:49Z

We do have many coverages: https://github.com/search?q=repo%3Adotnet%2Fruntime+%22Activator.CreateInstance%3C%22+OR+repo%3Adotnet%2Fruntime+%2Fwhere.*%3F%3A.*%3Fnew%5C%28%5C%29%2F&type=code

It's not showing up in the spmi because it enables more devirtualization so we end up with context mismatches.

JulieLeeMSFT · 2026-07-02T18:21:35Z

Waiting on #130099 before merging.

hez2010 · 2026-07-03T00:06:16Z

Waiting on #130099 before merging.

I believe #130099 solves a different issue (which expands CreateInstance to newobj) which is parallel to this PR. And it was only opened for checking the diff from what I have seen in the PR description.

hez2010 · 2026-07-07T01:28:40Z

I think instead of walking through the call tree, I can just simply spill the call and mark the store as single def local with the exact class.

@jakobbotsch What do you think?

jakobbotsch · 2026-07-07T13:15:10Z

I think instead of walking through the call tree, I can just simply spill the call and mark the store as single def local with the exact class.

@jakobbotsch What do you think?

Would it simplify anything? I think in the end it will look about the same but move the recognition one higher level up than necessary. Ideally places that need this information use gtGetClassHandle for it (e.g. on the def if SSA is available and it wasn't propagated to the local when the local was created).

hez2010 · 2026-07-07T13:24:01Z

Would it simplify anything? I think in the end it will look about the same but move the recognition one higher level up than necessary. Ideally places that need this information use gtGetClassHandle for it (e.g. on the def if SSA is available and it wasn't propagated to the local when the local was created).

Okay, let's keep it as is.

Is there anything else I need to do before this can be merged?

jakobbotsch · 2026-07-07T13:27:39Z

Is there anything else I need to do before this can be merged?

No, I think this looks good. Thanks!

…<T> (#130074) `Activator.CreateInstance<T>` can only result in an instance of exact `T`. Teach the JIT so that when the JIT sees a call to `Activator.CreateInstance<T>`, it will know the result must be the exact `T`. Then any subsequent calls against the allocated object instance can be devirtualized. This also applies to any `where T : new()` patterns. Example: ```cs class Program { static T Get<T>() where T : new() { return new T(); } class A { public virtual int Value => 1; } class B : A { public override int Value => 2; } static void Main() { Console.WriteLine(Get<A>().Value); Console.WriteLine(Get<B>().Value); } } ``` Before: ```nasm G_M27646_IG01: ;; offset=0x0000 sub rsp, 40 G_M27646_IG02: ;; offset=0x0004 mov rcx, 0x7FF92B6AB428 ; System.Activator:CreateInstance[Program+A]():Program+A call [System.Activator:CreateInstance[System.__Canon]():System.__Canon] mov rcx, rax mov rax, qword ptr [rax] mov rax, qword ptr [rax+0x48] call [rax+0x20]Program+A:get_Value():int:this mov ecx, eax call [System.Console:WriteLine(int)] mov rcx, 0x7FF92B6AB890 ; System.Activator:CreateInstance[Program+B]():Program+B call [System.Activator:CreateInstance[System.__Canon]():System.__Canon] mov rcx, rax mov rax, qword ptr [rax] mov rax, qword ptr [rax+0x48] call [rax+0x20]Program+A:get_Value():int:this mov ecx, eax call [System.Console:WriteLine(int)] nop G_M27646_IG03: ;; offset=0x004F add rsp, 40 ret ``` After: ```nasm G_M27646_IG01: ;; offset=0x0000 sub rsp, 40 G_M27646_IG02: ;; offset=0x0004 mov rcx, 0x7FF92C79B428 ; System.Activator:CreateInstance[Program+A]():Program+A call [System.Activator:CreateInstance[System.__Canon]():System.__Canon] mov ecx, 1 call [System.Console:WriteLine(int)] mov rcx, 0x7FF92C79B890 ; System.Activator:CreateInstance[Program+B]():Program+B call [System.Activator:CreateInstance[System.__Canon]():System.__Canon] mov ecx, 2 call [System.Console:WriteLine(int)] nop G_M27646_IG03: ;; offset=0x003B add rsp, 40 ret ``` In the future we can apply escape analysis against `CreateInstance<T>` as well to omit unescaped object allocations.

Give Activator.CreateInstance an exact type

049c59e

Copilot AI review requested due to automatic review settings July 1, 2026 10:02

github-actions Bot added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Jul 1, 2026

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dotnet-policy-service Bot added the community-contribution Indicates that the PR has been added by a community member label Jul 1, 2026

MihuBot mentioned this pull request Jul 1, 2026

[JitDiff X64] [hez2010] JIT: use type argument as the exact type for Activat ... MihuBot/runtime-utils#2056

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hez2010 added 3 commits July 1, 2026 19:13

Use GetCompileTimeHandle

3e14c52

Merge into impGetSpecialIntrinsicExactReturnType

07a0d52

Address feedbacks

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Avoid unconditionally calling eeGetMethodSig

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Nit

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Don't fail for Canon

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Address more feedbacks

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Oops

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Nit

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MichalPetryka mentioned this pull request Jul 1, 2026

Add CreateInstance JIT intrinsics #130099

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jakobbotsch merged commit 8ffcfe6 into dotnet:main Jul 7, 2026
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[main] Source code updates from dotnet/runtime dotnet/dotnet#7592

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