Remove redundant pinning over static fields/RVA by EgorBo · Pull Request #119674 · dotnet/runtime

EgorBo · 2025-09-12T23:10:23Z

I noticed that sometimes developers make assumptions that it's fine to use Unsafe.AsPointer over RVA fields since they're not movable, which is fairly unreliable thing to do (e.g. Unloadable ALCs). So let's just optimize the pinning overhead in JIT when we know it's safe to do so:

// Case 0: RVA fields
void Test0()
{
    ReadOnlySpan<int> rva = [1,2,3,4,5];
    fixed (int* p = rva)
        Consume(p);
}

// Case 1: Primitive static fields (unmanaged memory)
static int _primitiveFld;
void Test1()
{
    fixed (int* p = &_primitiveFld)
        Consume(p);
}


// Case 2: Struct static fields (boxed and stored on the NonGC heap)
static MyStruct _structFld;
void Test2()
{
    fixed (int* p = &_structFld.Y)
        Consume(p);
}

Codegen diff: https://www.diffchecker.com/LgIALnAn/

dotnet-policy-service · 2025-09-12T23:11:17Z

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
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EgorBo · 2025-09-13T15:30:41Z

@MihuBot

jkotas · 2025-09-14T00:41:21Z

+        private bool canOmitPinning(CORINFO_FIELD_STRUCT_* fldHnd)
+        {
+            FieldDesc field = HandleToObject(fldHnd);
+            if (!field.IsStatic || field.IsThreadStatic || field.HasGCStaticBase || field.OwningType.IsCanonicalSubtype(CanonicalFormKind.Any))


HasGCStaticBase condition should not be needed for NativeAOT.

See dotnet/runtimelab#870

I see, thanks. Removed

HasGCStaticBase condition is needed for R2R to be functionally correct. Are we seeing any crashes in GCStress+R2R with this change? If not, we may consider adding some targeted tests to validate that this optimization is valid.

It is an interesting question how conservative the contract should be for R2R. I guess we can start with the condition you had here originally. It will be one more reason why R2R is incompatible and ignored for collectible ALCs.

EgorBo · 2025-09-30T22:27:36Z

@jkotas could you please review the VM side? I guess I have a few conservative checks like ThreadStatics, but I was mostly focusing on RVA fields and structs (boxed statics)
@dotnet/jit-contrib PTAL jit side

Copilot

Pull Request Overview

This PR optimizes pinning operations in the JIT compiler by introducing a new canOmitPinning API that allows the JIT to skip pinning overhead when working with static fields that are guaranteed to be stable in memory. The optimization applies to RVA fields, primitive static fields in unmanaged memory, and struct static fields stored on the NonGC heap.

Key changes:

Introduces canOmitPinning JIT interface method to determine when pinning can be safely omitted
Enhances IsNotGcDef to consider field sequences and call the new API for static fields
Updates SuperPMI infrastructure to support the new interface method

Reviewed Changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/coreclr/vm/jitinterface.cpp	Implements `canOmitPinning` with logic to check if static fields are safe to use without pinning
src/coreclr/jit/gentree.cpp	Refactors `IsNotGcDef` to accept compiler parameter and check static field sequences
src/coreclr/jit/gentree.h	Changes `IsNotGcDef` signature to require compiler parameter
src/coreclr/jit/lclvars.cpp	Updates call to `IsNotGcDef` with compiler parameter
src/coreclr/inc/corinfo.h	Adds `canOmitPinning` method declaration to JIT interface
src/coreclr/inc/jiteeversionguid.h	Updates JIT-EE version identifier for interface change
src/coreclr/tools/Common/JitInterface/CorInfoImpl.cs	Implements simplified AOT version of `canOmitPinning`
src/coreclr/tools/Common/JitInterface/CorInfoImpl_generated.cs	Auto-generated code for new interface method
Various SuperPMI files	Adds infrastructure support for recording and replaying `canOmitPinning` calls

jkotas · 2025-10-01T04:48:46Z

+        private bool canOmitPinning(CORINFO_FIELD_STRUCT_* fldHnd)
+        {
+            FieldDesc field = HandleToObject(fldHnd);
+            if (!field.IsStatic || field.IsThreadStatic || field.HasGCStaticBase || field.OwningType.IsCanonicalSubtype(CanonicalFormKind.Any))


HasGCStaticBase condition is needed for R2R to be functionally correct. Are we seeing any crashes in GCStress+R2R with this change? If not, we may consider adding some targeted tests to validate that this optimization is valid.

It is an interesting question how conservative the contract should be for R2R. I guess we can start with the condition you had here originally. It will be one more reason why R2R is incompatible and ignored for collectible ALCs.

@EgorBo

Remove the GTF_ICON_STATIC_HDL arm of IsNotGcDef. As @EgorBo noted on dotnet#129110, the static-data address can live in a collectible ALC's loader heap, where pinning may be what's keeping the LoaderAllocator reachable. The safe answer is the canOmitPinning JIT-EE API in dotnet#119674, which asks the VM to verify !LoaderAllocator->CanUnload(). The stackalloc fix in PHASE_UNPIN_LOCALS is unaffected: it still picks up the GT_LCL_ADDR path that was already in IsNotGcDef. SPMI asmdiffs loses only ~3K bytes out of -190K from this revert. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Resolve the JIT-EE conflicts, incorporate review feedback, and add JIT, ReadyToRun, NativeAOT, and GC stress coverage for static pinning. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 6626f902-a2c1-4f01-a35b-0981b4dcc0e7

Copilot

Pull request overview

Copilot reviewed 29 out of 29 changed files in this pull request and generated 2 comments.

EgorBo · 2026-07-16T10:29:42Z

@MihuBot -nuget

# Conflicts: # src/coreclr/inc/corinfo.h # src/coreclr/inc/icorjitinfoimpl_generated.h # src/coreclr/inc/jiteeversionguid.h # src/coreclr/jit/ICorJitInfo_names_generated.h # src/coreclr/jit/ICorJitInfo_wrapper_generated.hpp # src/coreclr/tools/Common/JitInterface/CorInfoImpl_generated.cs # src/coreclr/tools/Common/JitInterface/ThunkGenerator/ThunkInput.txt # src/coreclr/tools/aot/jitinterface/jitinterface_generated.h # src/coreclr/tools/superpmi/superpmi-shared/lwmlist.h # src/coreclr/tools/superpmi/superpmi-shared/methodcontext.h # src/coreclr/tools/superpmi/superpmi-shim-counter/icorjitinfo_generated.cpp # src/coreclr/tools/superpmi/superpmi-shim-simple/icorjitinfo_generated.cpp

Copilot

Pull request overview

Copilot reviewed 25 out of 25 changed files in this pull request and generated 2 comments.

EgorBo · 2026-07-21T20:31:29Z

@jkotas @MichalStrehovsky anything else here on the VM side?

MichalStrehovsky

The native AOT part looks good to me.

I don't know enough about R2R/CoreCLR VM to sign off on that part. My question marks for R2R/VM:

do we have guarantees the R2R module is not loaded into unloadable ALC?
it's not clear to me under what condition the statics are immovable. per #129664 CoreCLR will not create an immovable static for primitive types even if asked nicely, but here we seem to be saying canOmitPinning is true for them.

jkotas · 2026-07-22T00:56:15Z

            void* address
            ) = 0;

+    // Returns true iff pinning of the field's address can be elided because runtime guarantees stability.


Suggested change

// Returns true iff pinning of the field's address can be elided because runtime guarantees stability.

// Returns true iff pinning of the field's address can be elided because runtime guarantees the field

// address won't move and the field storage won't be collected.

Be more specific about what this actually guarantees

jkotas · 2026-07-22T00:56:42Z

-
-        return false;
-    }
+    bool IsNotGcDef(Compiler* comp) const;


Add a comment about what this returns?

EgorBo · 2026-07-22T01:01:08Z

do we have guarantees the R2R module is not loaded into unloadable ALC?

My understanding we do not support R2R when we load code into an unloadable ALC today. if there is an API I can call to bail out unloadble ALC in R2R - I can call, but I don't see any. I have a suspicion (not based on anything) we have a lot of challenges to solve to enable r2r for that.

For CoreCLR we bail out via fieldOwnerType.GetLoaderAllocator()->CanUnload() check

it's not clear to me under what condition the statics are immovable. per FixedAddressValueType is silently ignored when not supported #129664 CoreCLR will not create an immovable static for primitive types even if asked nicely, but here we seem to be saying canOmitPinning is true for them.

I am not sure I fully understand the issue, my understanding that they're always allocated on non-gc memory (nongc primitives). NonGC non-primitives are typically allocated on FOH (boxed statics).

jkotas · 2026-07-22T01:32:49Z

My understanding we do not support R2R when we load code into an unloadable ALC today.

Right.

we have a lot of challenges to solve to enable r2r for that.

Known issues would not be that hard to solve. The testing/reliability (finding all issues) is the harder part.

CoreCLR will not create an immovable static for primitive types even if asked nicely

Primitive statics are movable only in collectible assemblies in the current main.

jkotas · 2026-07-22T01:37:42Z

+
+#if READYTORUN
+            // RVA data and reference-type static slots are always stable. Keep value-type GC
+            // statics conservative because ReadyToRun can store them in movable boxes.


Suggested change

// statics conservative because ReadyToRun can store them in movable boxes.

// statics conservative because the runtime can store them in movable boxes.

jkotas · 2026-07-22T01:40:15Z

The VM side looks fine to me.

Speaking about pinning, I wish for #63397. It should be able to produce similar codegen diffs in a lot more cases.

Remove redundant pinning over static fields/RVA

1555ada

github-actions Bot added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Sep 12, 2025

dotnet-policy-service Bot assigned EgorBo Sep 12, 2025

less conservative

59e75d3

EgorBo added the reduce-unsafe label Sep 12, 2025

fix build

cc4a8f5

build-analysis Bot mentioned this pull request Sep 13, 2025

Intermittent build failure in AfterSourceBuild: "Could not write state file" #76488

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MihuBot mentioned this pull request Sep 13, 2025

[JitDiff X64] [EgorBo] Remove redundant pinning over static fields/RVA MihuBot/runtime-utils#1484

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EgorBo added 2 commits September 13, 2025 17:02

Implement R2R/NAOT

1297bc2

formatting

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MihuBot mentioned this pull request Sep 13, 2025

[JitDiff X64] [EgorBo] Remove redundant pinning over static fields/RVA MihuBot/runtime-utils#1485

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fix diffs

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build-analysis Bot mentioned this pull request Sep 13, 2025

[android] Android.Device_Emulator.JIT.Test failing on emulators with CoreCLR #112633

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jkotas reviewed Sep 14, 2025

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EgorBo added 2 commits October 1, 2025 00:26

Update CorInfoImpl.cs

8c9e82d

Merge branch 'main' into avoid-pinning-rva

3caad0b

EgorBo marked this pull request as ready for review September 30, 2025 22:27

EgorBo requested a review from MichalStrehovsky as a code owner September 30, 2025 22:27

Copilot AI review requested due to automatic review settings September 30, 2025 22:27

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EgorBo requested a review from jkotas September 30, 2025 22:27

EgorBo commented Sep 30, 2025

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Comment thread src/coreclr/jit/gentree.cpp

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MichalPetryka mentioned this pull request May 3, 2026

Elide pinning over spans from stackalloc #127691

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EgorBo mentioned this pull request Jun 8, 2026

JIT: unpin locals whose value is provably non-movable #129110

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Merge branch 'main' into avoid-pinning-rva

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Comment thread src/tests/JIT/Regression/JitBlue/Runtime_119674/Runtime_119674_Disasm.cs

Comment thread src/tests/JIT/Regression/JitBlue/Runtime_119674/Runtime_119674_Disasm.cs

MihuBot mentioned this pull request Jul 16, 2026

[JitDiff X64] [EgorBo] Remove redundant pinning over static fields/RVA MihuBot/runtime-utils#2093

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build-analysis Bot mentioned this pull request Jul 16, 2026

slow macOS - "##[error]The job running on agent Azure Pipelines 9 ran longer than the maximum time of 60 minutes." dotnet/dnceng#1883

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Comment thread src/coreclr/tools/aot/ILCompiler.Compiler.Tests/JitInterfaceTests.cs Outdated

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Test failure: A COM object used through FreeThreadedStrategy is not free threaded (agile). #130961

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EgorBo added this to the 12.0.0 milestone Jul 30, 2026

	// Returns true iff pinning of the field's address can be elided because runtime guarantees stability.
	// Returns true iff pinning of the field's address can be elided because runtime guarantees the field
	// address won't move and the field storage won't be collected.

	// statics conservative because ReadyToRun can store them in movable boxes.
	// statics conservative because the runtime can store them in movable boxes.

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