Replace CoreCLR SList with NativeAOT-style design

[max-charlamb]

Note

This PR was authored with the assistance of GitHub Copilot.

Summary

Unify the CoreCLR and NativeAOT SList implementations into a single shared header at src/coreclr/inc/slist.h. NativeAOT's slist.h becomes a redirect to this shared header. The old sentinel-based SLink/SList design is replaced with the NativeAOT-style design: no sentinel node, NULL means empty, traits-based next-pointer access.

Design

SListTraits and SListMode

A single SListTraits<T, Mode> template replaces the old DefaultSListTraits/DefaultSListTraitsWithTail classes. The SListMode enum controls which operations are permitted:

Mode	Allowed Operations	Use Case
Thin (default)	InsertHead, RemoveHead, RemoveFirst, FindAndRemove, InsertAfter	Head-only lists (CacheLine, ConnectionCookie, SyncBlock, etc.)
Tail	+ InsertTail, GetTail	Tail-tracking lists (Thread, CallCountingManager, etc.)

Convenience Alias

template <typename T> using SListTail = SList<T, SListTraits<T, SListMode::Tail>>;

Other Features

• Pointer-to-pointer Iterator with Begin()/End(), Insert/Remove at position, debug validation
• SListElem<T> wrapper for non-intrusive list usage (available to both CoreCLR and NativeAOT)
• SListTailBase conditional storage — m_pTail only exists when HasTail is true

Consumer Migrations

All method names preserved: InsertHead, InsertTail, RemoveHead, FindAndRemove, GetHead, GetNext, IsEmpty.

Consumer	m_pNext type	List type
CacheLine	DPTR(CacheLine)	SList<CacheLine>
CallCountingManager	DPTR(CallCountingManager)	SListTail<CallCountingManager>
ConnectionCookie	DPTR(ConnectionCookie)	SList<ConnectionCookie>
StoredProfilerNode	DPTR(StoredProfilerNode)	SList<StoredProfilerNode>
Thread	PTR_Thread	SListTail<Thread>
SyncBlock	PTR_SyncBlock	Raw pointers (free/cleanup lists — global instance needs trivial construction)
HandleCleanupListItem	DPTR(...)	SListTail<HandleCleanupListItem>
FailedTypeInitCleanupListItem	DPTR(...)	SListTail<FailedTypeInitCleanupListItem>
SListElem<NativeCodeVersion>	DPTR(SListElem)	SListTail<SListElem<NativeCodeVersion>>
OsModuleEntry (NativeAOT)	PTR_OsModuleEntry	SList<OsModuleEntry>
TypeManagerEntry (NativeAOT)	TypeManagerEntry*	SList<TypeManagerEntry>

DAC / cDAC Changes

• SyncBlock_1.cs, Thread_1.cs: Removed SLink offset subtraction (links now point directly to objects)
• datadescriptor.inc: Removed SLink type definition, ThreadStore.FirstThreadLink changed from TYPE(SLink) to T_POINTER
• Mock descriptors: Removed dead LinkAddress/CleanupLinkAddress properties
• SyncBlock.md: Updated pseudocode to remove offset subtraction
• daccess.h: Updated stale SLink comments to reflect intrusive m_pNext design

NativeAOT Unification

• nativeaot/Runtime/slist.h redirects to ../../inc/slist.h
• Removed lock-free InsertHeadInterlocked — the two call sites run single-threaded; the previous volatile/CAS approach was also insufficient for weak memory models
• Deleted both slist.inl files (no longer needed without interlocked support)
• Removed rh::std::find/count helpers (single usage inlined)
• All NativeAOT call sites updated to use unified method names (InsertHead/RemoveHead/Begin/End)

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[Copilot]

Pull request overview

Replaces CoreCLR’s sentinel-based SList with a NativeAOT-style, traits-based design (no sentinel; NULL = empty) and migrates affected consumers and DAC/cDAC descriptors to point directly at the owning objects.

Changes:

• Reworked src/coreclr/inc/slist.h to a traits-based intrusive list with optional tail tracking and SListElem<T> for non-intrusive usage.
• Migrated multiple CoreCLR consumers from SLink m_Link to m_pNext (DAC-safe pointer types) and updated list operations accordingly.
• Updated DAC/cDAC contracts, descriptors, and mocks so list heads/links now point directly to objects (no offset subtraction / CONTAINING_RECORD).

Reviewed changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/native/managed/cdac/tests/MockDescriptors/MockDescriptors.Thread.cs	Updates mock thread store/list wiring to use direct Thread.Address links.
src/native/managed/cdac/tests/MockDescriptors/MockDescriptors.SyncBlock.cs	Updates mock cleanup list to use direct SyncBlock.Address links.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/Thread_1.cs	Removes SLink offset logic; treats thread links as direct thread pointers.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Contracts/Contracts/SyncBlock_1.cs	Removes link-offsetof subtraction; iterates cleanup list via direct pointers.
src/coreclr/vm/tieredcompilation.h	Introduces OptimizationQueue typedef using tail-enabled SList for optimization queue.
src/coreclr/vm/threads.h	Replaces SLink m_Link with PTR_Thread m_pNext; updates ThreadList and cdac offsets.
src/coreclr/vm/threads.cpp	Updates FindAndRemove usage to new boolean-return API.
src/coreclr/vm/syncblk.h	Replaces SLink usage with PTR_SyncBlock m_pNext; updates cdac offsets and cache list types.
src/coreclr/vm/syncblk.cpp	Removes offsetof(SyncBlock, m_Link) arithmetic; cleanup/free lists now store SyncBlock* directly.
src/coreclr/vm/loaderallocator.hpp	Replaces SLink with m_pNext in cleanup list items; adds tail-enabled list typedefs.
src/coreclr/vm/loaderallocator.cpp	Updates list iteration/removal to match new next-pointer layout.
src/coreclr/vm/datadescriptor/datadescriptor.inc	Removes SLink type and switches FirstThreadLink descriptor to a pointer.
src/coreclr/vm/comconnectionpoints.h	Replaces SLink with m_pNext in ConnectionCookie; updates list typedef.
src/coreclr/vm/comconnectionpoints.cpp	Removes CONTAINING_RECORD/InsertAfter usage; performs direct pointer chaining.
src/coreclr/vm/callcounting.h	Replaces SLink with m_pNext; introduces tail-enabled CallCountingManagerList.
src/coreclr/vm/callcounting.cpp	Updates static list declaration and iteration to new list type/API.
src/coreclr/vm/cachelinealloc.h	Replaces SLink with m_pNext; updates list typedefs and initialization.
src/coreclr/vm/cachelinealloc.cpp	Removes now-unneeded SList::Init() calls in allocator ctor.
src/coreclr/inc/slist.h	Core redesign: traits-based next-pointer access, optional tail tracking, SListElem<T>, and updated iterator/API.
src/coreclr/inc/profilepriv.h	Migrates StoredProfilerNode to m_pNext; updates profiler list typedef.
src/coreclr/debug/daccess/request.cpp	Updates DAC traversal of syncblock lists to use direct m_pNext pointers.
docs/design/datacontracts/SyncBlock.md	Updates docs to reflect cleanup list head now points to SyncBlock objects directly.

[Copilot]

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[Copilot]

Pull request overview

Copilot reviewed 24 out of 24 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 24 out of 24 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 31 out of 31 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 31 out of 31 changed files in this pull request and generated 2 comments.

[jkotas]

@AaronRobinsonMSFT Any feedback about the C++ specifics?

[Copilot]

Pull request overview

Copilot reviewed 31 out of 31 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 31 out of 31 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 31 out of 31 changed files in this pull request and generated 1 comment.

[github-actions]

🤖 Copilot Code Review — PR #126949

Note

This review was generated by GitHub Copilot using multi-model analysis (Claude Opus 4.6, Claude Sonnet 4.5, GPT-5.3-Codex). All three models contributed findings.

Holistic Assessment

Motivation: The PR is well-justified. Maintaining two divergent SList implementations (CoreCLR's sentinel-based SLink/CONTAINING_RECORD design vs. NativeAOT's cleaner direct-m_pNext design) is a genuine maintenance burden and source of error-prone pointer arithmetic. Unifying them into one shared template is a clear win.

Approach: The traits-based design with SListMode enum (Thin, Tail, Interlocked) and static_assert guards to prevent mode misuse is clean and well-structured. The migration of all consumers is thorough — every SLink m_Link → DPTR(T) m_pNext, every CONTAINING_RECORD pattern removed, and the cDAC contracts simplified to eliminate offset arithmetic.

Summary: ⚠️ Needs Human Review. All code changes are correct across all three review models. One documentation pseudocode section was not updated to match the new semantics, and there are stale comments from the removed SLink type. A human reviewer should confirm the SyncBlock.md pseudocode issue and decide whether the minor suggestions merit fixing in this PR or follow-up.

---

Detailed Findings

✅ InsertHeadInterlocked CAS loop — Correct

The CAS loop in slist.inl correctly reads the volatile m_pHead into pItem->m_pNext, captures expected, and performs CAS. The static_assert guards (!HasTail, IsInterlocked) are appropriate. Verified equivalent to the old NativeAOT PushHeadInterlocked. (Confirmed by all 3 models)

✅ Tail pointer maintenance — Correct

All four mutation operations correctly handle m_pTail:

• InsertHead: sets tail when list transitions empty→non-empty
• RemoveHead: clears tail when list transitions non-empty→empty
• InsertTail: updates both head (if empty) and tail
• RemoveFirst: updates tail when removed item was the tail element (this->m_pTail = prev)

Edge cases (empty, single-element, multi-element) all handled. (Confirmed by all 3 models)

✅ SyncBlock free/cleanup list migration — Correct

The elimination of SLink/CONTAINING_RECORD from syncblk.h/cpp and request.cpp is correct. m_pCleanupBlockList and m_FreeBlockList are now directly PTR_SyncBlock, and traversal via m_pNext gives the next SyncBlock directly without offset arithmetic. (Confirmed by all 3 models)

✅ cDAC contract simplification — Correct

SyncBlock_1.cs and Thread_1.cs correctly remove _syncBlockLinkOffset and _threadLinkOffset adjustments. ThreadStore.FirstThreadLink now uses offsetof(ThreadList, m_pHead) which is correct for the new layout where SListTailBase::m_pTail precedes m_pHead. (Confirmed by all 3 models)

✅ NativeAOT redirect — Complete

slist.h → #include "../../inc/slist.h" and slist.inl → #include "../../inc/slist.inl". All NativeAOT consumers migrated: PushHead→InsertHead, PopHead→RemoveHead, PushHeadInterlocked→InsertHeadInterlocked. The rh::std helpers (count, find) correctly removed. (Confirmed by all 3 models)

✅ static_assert guards — Comprehensive

All mode-incompatible operations are gated: InsertHead/RemoveHead/InsertTail/RemoveFirst/FindAndRemove/InsertAfter/Insert/Remove all block IsInterlocked lists. InsertAfter/Insert/Remove also block HasTail lists. InsertHeadInterlocked requires IsInterlocked and blocks HasTail.

⚠️ SyncBlock.md pseudocode not updated — Documentation bug

docs/design/datacontracts/SyncBlock.md lines 152 and 161 still show:

return cleanupBlockList - /* SyncBlock::LinkNext offset */;
return linkNext - /* SyncBlock::LinkNext offset */;

With the new design, CleanupBlockList and LinkNext point directly to SyncBlock objects — no offset subtraction is needed. The PR correctly updated the table description at line 27 but missed updating this pseudocode. The actual C# implementation in SyncBlock_1.cs is correct (it returns the pointers directly without subtraction). (Flagged by Opus 4.6 and Sonnet 4.5)

💡 Stale SLink comments in daccess.h — Follow-up

src/coreclr/inc/daccess.h lines 241–250 still describe the old SLink-based SList pattern. These comments are now misleading since SLink no longer exists. Consider updating in this PR or a follow-up. (Flagged by Opus 4.6 and GPT-5.3-Codex)

💡 Consider static_assert for m_pNext field validation

The old SList used a member pointer template parameter (SLink T::*LinkPtr = &T::m_Link) which gave compile-time validation that T has the expected link field. The new SListTraits::GetNextPtr uses offsetof(T, m_pNext) which will fail to compile if m_pNext doesn't exist, but doesn't validate its type. A static_assert like static_assert(sizeof(T::m_pNext) == sizeof(void*)) could add an extra safety layer. Not blocking. (Flagged by GPT-5.3-Codex)

Generated by Code Review for issue #126949 · ◷

[Copilot]

Pull request overview

Copilot reviewed 32 out of 32 changed files in this pull request and generated no new comments.

[jkotas]

LGTM otherwise

[max-charlamb]

/ba-g

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