Split gc.cpp to multiple files based on the functionality - part 1

[janvorli]

This PR splits the monolithic gc.cpp (~57,000 lines) into 19 smaller files organized by functionality, reducing gc.cpp to ~8,300 lines of shared infrastructure. The new files are #included at the end of gc.cpp, preserving the
existing single-compilation-unit build model for this first step - no CMakeLists.txt changes were made.

New files

File	Lines	Scope
allocation.cpp	4,337	Allocation helpers, allocate_small/large, SOH/LOH/POH allocation
background.cpp	3,964	Background GC (concurrent mark, sweep, write watch support)
card_table.cpp	1,810	Card/brick table management, card bundling
collect.cpp	1,511	Collection entry points, garbage_collect, do_post_gc
diagnostics.cpp	1,615	Heap verification, verify_heap, ETW/diag walking
dynamic_heap_count.cpp	1,280	Dynamic heap count (server GC heap scaling)
dynamic_tuning.cpp	2,441	GC tuning heuristics, generation budgets, smoothing
finalization.cpp	520	Finalization queue management
init.cpp	1,350	GC initialization (gc_heap::initialize_gc)
interface.cpp	2,261	GCHeap interface implementation (public API surface)
mark_phase.cpp	3,728	Mark phase, mark stack, pinned object handling
memory.cpp	420	Virtual memory commit/decommit, address space management
no_gc.cpp	827	No-GC region support
plan_phase.cpp	7,374	Plan phase, plug/gap processing, generation planning
region_allocator.cpp	421	Region allocator (regions mode)
region_free_list.cpp	420	Region free list management
regions_segments.cpp	2,052	Segment/region lifecycle, segment mapping table
relocate_compact.cpp	1,977	Relocate and compact phases
sweep.cpp	527	Sweep phase, free list building

Design decisions

• Single compilation unit preserved: All new files are #included from gc.cpp, so gcwks.cpp and gcsvr.cpp continue to compile everything as one TU. No build system changes required.
• #ifdef guards maintained: Each moved method retains its original preprocessor guards (BACKGROUND_GC, USE_REGIONS, MULTIPLE_HEAPS, etc.).
  - Remaining in gc.cpp: Shared globals, macros, general-purpose helpers, and methods that don't clearly belong to one functional category stay in gc.cpp.

What's NOT changed

• No functional/behavioral changes — this is a pure code reorganization
• No changes to headers (gcpriv.h, gc.h, etc.)
• No changes to the build system (CMakeLists.txt)
• All existing #ifdef nesting is preserved

These changes were made by copilot cli with my supervision and reviewing. The next step will be to actually compile each of the files separately.

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[Copilot]

Pull request overview

This pull request appears to refactor CoreCLR GC implementation by moving substantial logic into new .cpp compilation units (which are then included from gc.cpp), separating concerns like sweeping, regions, no-GC mode, memory commit/decommit accounting, collection, initialization, and finalization.

Changes:

• Adds new GC implementation files for sweeping, regions (allocator/free list), no-GC region behavior, memory commit/decommit, collection, initialization, and finalization.
• Updates GC internals by removing an unused VERIFY_HEAP method declaration from gcpriv.h.
• (Implied by file structure) Consolidates GC implementation via #include-based composition of these new .cpp units.

Reviewed changes

Copilot reviewed 15 out of 21 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/coreclr/gc/sweep.cpp	Adds sweep/free-list building and related helpers (incl. FEATURE_BASICFREEZE RO sweep).
src/coreclr/gc/region_free_list.cpp	Adds region free-list management for USE_REGIONS.
src/coreclr/gc/region_allocator.cpp	Adds region allocation/deallocation logic for USE_REGIONS.
src/coreclr/gc/no_gc.cpp	Adds no-GC region support logic, including callback scheduling and region-extension helpers.
src/coreclr/gc/memory.cpp	Adds commit/decommit accounting and region decommit logic.
src/coreclr/gc/init.cpp	Adds GC initialization logic (including region range reservation/init under USE_REGIONS).
src/coreclr/gc/finalization.cpp	Adds finalization queue implementation and finalizer work scheduling.
src/coreclr/gc/collect.cpp	Adds GC collection logic (including region ephemeral/gc range computation under USE_REGIONS).
src/coreclr/gc/gcpriv.h	Removes set_batch_mark_array_bits declaration under VERIFY_HEAP.

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[MichalStrehovsky]

If there is interest in keeping git blame working, it might be possible to commit this in a way that preserves it: https://devblogs.microsoft.com/oldnewthing/20190916-00/?p=102892

A repo admin might be necessary to actually merge that because a squash would likely undo it.

[janvorli]

If there is interest in keeping git blame working, it might be possible to commit this in a way that preserves it: https://devblogs.microsoft.com/oldnewthing/20190916-00/?p=102892

That sounds very interesting. I'll give that a try.

[jkoritzinsky]

If necessary for perf reasons (ie lost optimizations that LTCG doesn’t figure out when you try to compile separately in the next PR), you can use the UNITY_BUILD feature in CMake to force all of the files to be included in one C++ file in Release builds to maintain the current experience while providing a better dev story.

[janvorli]

@MichalStrehovsky it seems that git blame still works without any special handling. You just need to pass it the -C -C option. Copilot says github UI doesn't use that option though, so it would only work from the command line. Not sure if the way suggested in the article you've shared would work in github too. But i'll give it a try anyways. Btw, the blame in the github UI never worked for the gc.cpp, as it always times out because the file is so huge. So I wonder if it would have any benefit even if I did that based on the article.

[janvorli]

Hmm, I take it back, the git -C -C doesn't work.

[Copilot]

Pull request overview

This PR appears to split previously monolithic GC implementation code into multiple focused .cpp files under src/coreclr/gc/ (e.g., sweep/regions/no-GC/memory/init/finalization/collect) and updates .git-blame-ignore-revs to keep git blame attribution useful after the mechanical refactor.

Changes:

• Extract GC implementation areas into new compilation-unit fragments (included from gc.cpp), such as sweeping, region allocation/free-list management, no-GC region logic, memory commit/decommit accounting, GC init, finalization, and collection logic.
• Add region allocator/free-list implementations behind USE_REGIONS.
• Add blame-ignore entries for the mechanical “split gc.cpp” trimming commits.

Reviewed changes

Copilot reviewed 15 out of 21 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
src/coreclr/gc/sweep.cpp	New split-out sweeping/free-list threading logic (plus RO segment sweeping under BASICFREEZE).
src/coreclr/gc/region_free_list.cpp	New region free-list bookkeeping, ordering, and sorting utilities (USE_REGIONS).
src/coreclr/gc/region_allocator.cpp	New region allocator implementation (USE_REGIONS). Contains correctness issues noted in PR comments.
src/coreclr/gc/no_gc.cpp	New split-out no-GC region and callback logic.
src/coreclr/gc/memory.cpp	New split-out commit/decommit accounting and decommit stepping logic (incl. regions).
src/coreclr/gc/init.cpp	New split-out GC initialization logic (incl. regions initial reservation).
src/coreclr/gc/finalization.cpp	New split-out finalization queue logic and finalizer work scheduling.
src/coreclr/gc/collect.cpp	New split-out GC collection driver logic and region ephemeral-range computation.
.git-blame-ignore-revs	Adds mechanical split/trim commits to blame-ignore list to preserve attribution.

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[janvorli]

Copilot cli has reworked the PR based on the article mentioned. It also added a .git-blame-ignore-revs file that enables github blame to work properly by ignoring the changes in this PR in the blame.

To make local git blame work the best, git config diff.algorithm histogram can be used. The default diff is not quite good in properly detecting all moves.

[janvorli]

It requires git version >= 2.40 (older git doesn't seem to use the algorithm)

[janvorli]

And this PR needs to be merged without squashing. Otherwise the tracking would get lost

[jkotas]

Let me know when you are ready to merge it. I can elevate to merge it without squashing.

[Copilot]

Pull request overview

This PR reorganizes CoreCLR GC implementation by splitting the previously monolithic gc.cpp into multiple functionality-focused .cpp files, which are still #included from gc.cpp to preserve the single translation unit build model.

Changes:

• Split GC implementation into 19 category-specific .cpp files (allocation, marking, planning, sweeping, background GC, regions, etc.).
• Updated gc.cpp to include the new files at the end, keeping shared infrastructure in gc.cpp.
• Added .git-blame-ignore-revs entries to make git blame ignore the mechanical “trim” commits for the split files.

Reviewed changes

Copilot reviewed 15 out of 21 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
.git-blame-ignore-revs	Adds ignore-revs entries for the mechanical trimming commits associated with the split.
src/coreclr/gc/gc.cpp	Retains shared GC infrastructure and #includes the newly split implementation files.
src/coreclr/gc/allocation.cpp	Allocation helpers and SOH/LOH/POH allocation paths.
src/coreclr/gc/background.cpp	Background GC (concurrent mark/sweep, write watch support).
src/coreclr/gc/card_table.cpp	Card/brick table management and related helpers.
src/coreclr/gc/collect.cpp	Collection entry points and post-GC processing.
src/coreclr/gc/diagnostics.cpp	Heap verification and diagnostics/ETW walking support.
src/coreclr/gc/dynamic_heap_count.cpp	Dynamic heap count (server GC heap scaling).
src/coreclr/gc/dynamic_tuning.cpp	GC tuning heuristics, budgets, smoothing.
src/coreclr/gc/finalization.cpp	Finalization queue management and related helpers.
src/coreclr/gc/init.cpp	GC initialization logic (gc_heap::initialize_gc, related init helpers).
src/coreclr/gc/interface.cpp	GCHeap interface implementation (public API surface).
src/coreclr/gc/mark_phase.cpp	Mark phase implementation (mark stack, pinned handling).
src/coreclr/gc/memory.cpp	Virtual memory commit/decommit and address space management.
src/coreclr/gc/no_gc.cpp	No-GC region support.
src/coreclr/gc/plan_phase.cpp	Plan phase implementation (plug/gap processing, generation planning).
src/coreclr/gc/region_allocator.cpp	Region allocator implementation (regions mode).
src/coreclr/gc/region_free_list.cpp	Region free list management.
src/coreclr/gc/regions_segments.cpp	Segment/region lifecycle and mapping table management.
src/coreclr/gc/relocate_compact.cpp	Relocate/compact phases.
src/coreclr/gc/sweep.cpp	Sweep phase and free list building helpers.

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[janvorli]

@jkotas I think it is ready for merging. The CI failures are either known issues or Mono related ones.

[janvorli]

@jkotas please wait a bit with the merging, I have tried to test clrgc.dll from this build and there seems to be some issue, I need to verify it first.