Simplify single-threaded CoreCLR thread and static machinery by jkoritzinsky · Pull Request #135250 · dotnet/runtime

jkoritzinsky · 2026-10-05T21:47:24Z

CoreCLR builds without FEATURE_MULTITHREADING, including browser and WASI wasm, still carry machinery for multiple native threads and per-thread static storage. This change uses one native Thread and removes multithreading-only code and data from those builds.

Approach

Simplify native thread IDs and ThreadStore around the sole native thread. Preserve the DirectOnThreadLocalData native-thread pointer and managed unstarted Thread objects, whose start operations remain unsupported.
Treat managed thread statics as ordinary statics when multithreading is disabled. Update crossgen2 and ReadyToRun metadata to encode the single-threaded model, and exclude native thread-static machinery from these builds.
Split managed Thread implementation and startup helpers into multithreading/single-threading partials. Compile out native startup, utility-thread, priority, finalization, and helper FCalls; disable multicore JIT without multithreading.
Make cDAC thread-static data optional and update its contracts, descriptors, documentation, and tests.

Wasm retains its native GC-poll implementation. The managed polling fallback still performs a GC transition. The single-threaded spin-wait cap is zero; multithreaded behavior is retained.

Validation

Browser Debug and Release native runtime/CoreLib builds, including Release R2R CoreLib.
WASI Debug native runtime/CoreLib builds.
Windows Checked runtime/CoreLib builds and Thread tests: 55 passed, 4 skipped.
Browser Thread tests: 14 passed, 33 skipped.
cDAC unit tests: 3,240 passed during the descriptor changes.
NativeAOT CoreLib build to validate the shared Thread partial split.
Inspected built CoreLib metadata/IL to verify feature-specific methods and constants.

Release size comparison

Both sides use browser-wasm Release. Baseline: b9bafb9d14ef9e02a3940f97164b6b231d3595bb; changed source: 40327fe044c. Values are bytes. Brotli uses .NET BrotliStream with CompressionLevel.SmallestSize.

Artifact	Format	Baseline	Changed	Reduction
`dotnet.native.wasm`	Uncompressed	4,553,180	4,505,348	47,832 (1.05%)
`dotnet.native.wasm`	Brotli	1,324,802	1,313,959	10,843 (0.82%)
R2R `System.Private.CoreLib.wasm`	Uncompressed	26,683,738	26,648,519	35,219 (0.13%)
R2R `System.Private.CoreLib.wasm`	Brotli	4,194,214	4,189,499	4,715 (0.11%)

Combined reduction: 83,051 bytes uncompressed and 15,558 bytes Brotli-compressed. The CoreLib measurement is the R2R wasm artifact, not the IL DLL.

Note

This pull request description and code changes were generated with GitHub Copilot assistance.

…READING On single-threaded CoreCLR targets (browser/WASI WASM), simplify the runtime to model exactly one native Thread*: - Managed thread ID is always 1; no ID dispenser is needed. - ThreadStore holds a single thread entry. - Thread statics are process-global, and the runtime publishes the main thread's managed object into Thread.s_currentThread at startup. - Split Thread.CoreCLR.cs into shared, Multithreading, and SingleThreading partial files. The single-threaded file provides managed implementations (Yield, OS thread ID, priority, IsBackground, Join, etc.) so the corresponding QCalls are compiled out under !FEATURE_MULTITHREADING. - Move WASI helpers out of Thread.Unix.cs into Thread.Wasi.cs and add Thread.SingleThreading.cs for CoreCLR single-threaded Join/Sleep. - Skip GC-time priority boosting when multithreading is unavailable. - Update DAC/cDAC and the Thread data contract docs accordingly. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

On builds without FEATURE_MULTITHREADING (browser/WASI wasm) there is only one thread, so [ThreadStatic] fields are now laid out and accessed as regular statics. - VM: classify [ThreadStatic] fields as regular statics, hard-code the managed thread ID, and compile out the thread-static machinery (TLS index allocation, in-flight TLS data, the TLS index map, per-thread data arrays, GC root scanning, the GetThreadStaticsBy* QCalls, and the thread-static JIT helpers). - CoreLib: compile out the thread-static StaticsHelpers and the wasm-only GetThreadStaticsBaseNative FCall. - crossgen2: add --target-single-threaded, which lays out thread statics as regular statics, skips rooting the thread-static helpers, and sets a new READYTORUN_FLAG (0x2000, R2R minor version 30.1). The runtime rejects images whose flag does not match its threading model. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

…EADING Compile out Thread::m_ThreadLocalDataPtr and the TLS data descriptors (ThreadStaticsInfo, ThreadLocalData, InFlightTLSData, TLSIndex, NumberOfTlsOffsetsNotUsedInNoncollectibleArray) in single-threaded builds. The cDAC treats Thread.ThreadLocalDataPtr as optional and skips thread-static base lookups for types without thread-static fields. Update contract docs and add a unit test. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Conditionally build threadstatics.cpp and threadstatics.h using FEATURE_MULTITHREADING, and guard their VM consumers and MethodTable layouts. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Guard Thread priority declarations and definitions, initialize single-threaded managed priority to Normal, and guard the finalizer priority boost. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Split shared Thread startup logic into feature-specific partials, remove single-threaded native startup and finalization paths, disable multicore JIT without multithreading, and inline single-threaded helper behavior. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

dotnet-policy-service · 2026-10-05T21:49:00Z

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

dotnet-policy-service · 2026-10-05T22:07:02Z

Tagging subscribers to 'arch-wasm': @lewing, @pavelsavara
See info in area-owners.md if you want to be subscribed.

Adapt single-threaded helper rooting to the upstream managed-helper table and advance the single-threaded ReadyToRun format extension to version 31.1. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Remove unreachable dead-thread branches from Join and IsThreadPoolThread, and allocate managed IDs without synchronization while retaining the exhaustion guard. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Integrate shared Thread spin/yield helpers and platform OS thread IDs while retaining single-threaded stubs. Keep the WASM thread-static bootstrap FCall limited to multithreaded builds. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Preserve the feature-specific threading implementation while incorporating main's dead-code cleanup. Remove staticallocationhelpers.inl, which has no remaining consumers. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Make single-threaded Thread.PollGC a no-op and restrict PollGCInternal and its native QCall to FEATURE_MULTITHREADING. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Compile the native thread-exit helper only with FEATURE_MULTITHREADING, leaving single-threaded setup-failure and detach paths unchanged. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Remove the unused managed WASM GC polling fallback, simplify Thread properties and source selection, and ignore ThreadStatic attributes on RVA fields. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Pass the single-threaded target setting in WASM layout tests and regenerate browser and WASI call helpers. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Gate the GC-to-EE thread creation contract and server/background GC on FEATURE_MULTITHREADING. Propagate the feature to NativeAOT and the GC sample, and guard background-only region diagnostics. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

jkotas · 2026-10-10T23:55:09Z

The test failures look related, e.g.:

Unhandled exception. System.PlatformNotSupportedException: Operation is not supported on this platform.
at System.Threading.Thread.JoinInternal(Int32 millisecondsTimeout)

This reverts commit e343bf7. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Simplify threading feature guards, make profile optimization a no-op without multithreading, and implement finite self-joins as timed sleeps. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

jkotas · 2026-10-11T19:37:03Z

Revert "Exclude GC thread creation without multithreading"

I think the commit had a bunch of good changes like disabling BACKGROUND GC based of FEATURE_MULTITHREADING or not building ServerGC with FEATURE_MULTITHREADING . I would keep those.

… commit

jkoritzinsky · 2026-10-14T17:34:40Z

/ba-g MSBuild-CMake leg broken by prexisting issue.

jkoritzinsky and others added 6 commits October 2, 2026 13:51

Exclude thread-static machinery from single-threaded CoreCLR builds

7664c1e

Conditionally build threadstatics.cpp and threadstatics.h using FEATURE_MULTITHREADING, and guard their VM consumers and MethodTable layouts. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Compile native thread priority methods only with multithreading

9547c5f

Guard Thread priority declarations and definitions, initialize single-threaded managed priority to Normal, and guard the finalizer priority boost. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

jkoritzinsky requested a review from MichalStrehovsky as a code owner October 5, 2026 21:47

github-actions Bot added the area-VM-coreclr label Oct 5, 2026

dotnet-policy-service Bot assigned jkoritzinsky Oct 5, 2026

github-project-automation Bot added this to AppModel Oct 5, 2026

jkoritzinsky requested a review from davidwrighton October 5, 2026 21:49

jkoritzinsky added the arch-wasm WebAssembly architecture label Oct 5, 2026

jkotas reviewed Oct 5, 2026

View reviewed changes

Comment thread src/coreclr/System.Private.CoreLib/src/System/Threading/Thread.CoreCLR.SingleThreading.cs Outdated

Comment thread src/coreclr/System.Private.CoreLib/src/System/Threading/Thread.CoreCLR.SingleThreading.cs Outdated

jkoritzinsky and others added 3 commits October 5, 2026 15:16

Merge main and preserve single-threaded CoreCLR implementation

367770c

Adapt single-threaded helper rooting to the upstream managed-helper table and advance the single-threaded ReadyToRun format extension to version 31.1. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Simplify single-threaded managed Thread state

013042e

Remove unreachable dead-thread branches from Join and IsThreadPoolThread, and allocate managed IDs without synchronization while retaining the exhaustion guard. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

davidwrighton reviewed Oct 8, 2026

View reviewed changes

jkoritzinsky and others added 2 commits October 8, 2026 13:57

Exclude managed GC polling interop from single-threaded builds

8ad7dc3

Make single-threaded Thread.PollGC a no-op and restrict PollGCInternal and its native QCall to FEATURE_MULTITHREADING. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Restrict DestroyThread to multithreaded builds

a96823e

Compile the native thread-exit helper only with FEATURE_MULTITHREADING, leaving single-threaded setup-failure and detach paths unchanged. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

build-analysis Bot mentioned this pull request Oct 8, 2026

SslStreamTlsResumeTests.DifferentEncryptionPolicy_NoResume failing #133418

Closed

build-analysis Bot mentioned this pull request Oct 9, 2026

Crash: access violation in coreclr!PgoManager::getPgoInstrumentationResults during tier-1 background JIT (Windows x86) #134695

Open

jkoritzinsky and others added 2 commits October 9, 2026 10:53

Allow ThreadStatic RVA fields in single-threaded builds

e870d75

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Address single-threaded CoreCLR review feedback

a96e31e

Remove the unused managed WASM GC polling fallback, simplify Thread properties and source selection, and ignore ThreadStatic attributes on RVA fields. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

jkoritzinsky requested review from steveisok and vitek-karas as code owners October 9, 2026 18:47

jkoritzinsky and others added 2 commits October 9, 2026 14:04

Fix single-threaded WASM CI regressions

66d8307

Pass the single-threaded target setting in WASM layout tests and regenerate browser and WASI call helpers. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

jkoritzinsky requested review from davidwrighton and jkotas October 9, 2026 22:17

jkotas reviewed Oct 10, 2026

View reviewed changes

Comment thread src/coreclr/gc/gcinterface.ee.h Outdated

jkoritzinsky and others added 2 commits October 10, 2026 22:36

Revert "Exclude GC thread creation without multithreading"

efc95c7

This reverts commit e343bf7. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Address single-threading review feedback and CI failures

5a3980a

Simplify threading feature guards, make profile optimization a no-op without multithreading, and implement finite self-joins as timed sleeps. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

jkotas reviewed Oct 11, 2026

View reviewed changes

Comment thread src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems Outdated

build-analysis Bot mentioned this pull request Oct 11, 2026

Runtime_133587 SIGSEGV in mono #133910

Open

jkoritzinsky added 2 commits October 12, 2026 10:53

Bring back the good changes from the exlude thread creation interface…

653eaee

… commit

Address suggestion

0fea5d1

jkoritzinsky requested a review from jkotas October 12, 2026 18:21

This was referenced Oct 12, 2026

HttpMetricsTest_Http20.TimeInQueue_RecordedForNewConnectionsOnly test failure #122522

Open

System.Net.Quic.QuicException : Connection handshake was canceled due to the configured timeout #133645

Open

jkotas approved these changes Oct 13, 2026

View reviewed changes

jkotas mentioned this pull request Oct 14, 2026

Do not attempt to start finalizer thread on WASM #135769

Merged

jkoritzinsky merged commit c5e1d63 into main Oct 14, 2026
182 of 188 checks passed

jkoritzinsky deleted the dev/jkoritzinsky/single-thread-coreclr branch October 14, 2026 17:34

github-project-automation Bot moved this to Done in AppModel Oct 14, 2026

xen2 mentioned this pull request Oct 15, 2026

Browser/Web as target platform stride3d/stride#70

Open

dotnet-maestro Bot mentioned this pull request Oct 19, 2026

[main] Source code updates from dotnet/runtime dotnet/dotnet#10269

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jkoritzinsky commented Oct 5, 2026

Approach

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Release size comparison

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jkotas commented Oct 10, 2026

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jkotas commented Oct 11, 2026 •

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