Disable server GC and background GC for CoreCLR WebAssembly builds by Copilot · Pull Request #127178 · dotnet/runtime

Copilot · 2026-04-20T20:37:51Z

Description

Applies two complementary size optimizations for CoreCLR WebAssembly builds, which use the interpreter instead of the traditional JIT:

Disable server GC — FEATURE_SVR_GC is suppressed for WebAssembly in clrdefinitions.cmake. WebAssembly is single-threaded and has no use for server GC's multi-heap design.
Disable background GC — Background GC (concurrent GC) requires WRITE_WATCH support and background threads, neither of which is available on WebAssembly. BACKGROUND_GC is now suppressed with a #ifndef TARGET_WASM guard in gcpriv.h. The datadescriptor.h and datadescriptor.inc cDAC descriptor files are updated to guard the fields and globals that only exist when BACKGROUND_GC is defined.

Size measurements (`corerun.wasm`)

Configuration	Raw (bytes)	Brotli (bytes)
Original baseline	4,264,779	1,283,871
Disable server GC + background GC	4,171,449	1,258,272
Total savings	93,330 (2.2%)	25,599 (2.0%)

Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/ef405ab1-bc2a-472f-b797-52d49c5d8409 Co-authored-by: davidwrighton <10779849+davidwrighton@users.noreply.github.com>

dotnet-policy-service · 2026-04-20T20:41:19Z

Tagging subscribers to this area: @JulieLeeMSFT, @dotnet/gc
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janvorli · 2026-04-20T20:48:43Z

@copilot let's disable the background GC for WASM too.

janvorli · 2026-04-20T20:50:29Z

cc: @radekdoulik FYI

Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/05b0c780-4d5e-4177-b5ec-c07a31e050a7 Co-authored-by: davidwrighton <10779849+davidwrighton@users.noreply.github.com>

…eLoadedBeforeCodeIsRun, and reportTailCallDecision Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/05b0c780-4d5e-4177-b5ec-c07a31e050a7 Co-authored-by: davidwrighton <10779849+davidwrighton@users.noreply.github.com>

Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/7f661e48-3027-488a-8ca9-5fc83d287082 Co-authored-by: janvorli <10758568+janvorli@users.noreply.github.com>

Copilot · 2026-04-20T21:56:12Z

@copilot let's disable the background GC for WASM too.

Done in dc7b9f9. Added #ifndef TARGET_WASM guard around #define BACKGROUND_GC in gcpriv.h, and updated datadescriptor.h and datadescriptor.inc to guard the fields that only exist when BACKGROUND_GC is defined.

Additional savings from disabling background GC: 47,527 bytes raw / 15,680 bytes brotli, bringing the total vs baseline to 93,330 bytes raw (2.2%) / 25,599 bytes brotli (2.0%).

janvorli · 2026-04-20T22:08:26Z

@copilot please undo the changes in the jitinterface.cpp, these are unrelated to the GC changes.

Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/b8ad1421-bfb6-48e3-8387-923a7d1cc0dc Co-authored-by: janvorli <10758568+janvorli@users.noreply.github.com>

Copilot · 2026-04-20T22:11:37Z

@copilot please undo the changes in the jitinterface.cpp, these are unrelated to the GC changes.

Done in c6d22cb. The jitinterface.cpp changes have been reverted — the PR now only contains the GC-related changes (gcpriv.h, datadescriptor.h, datadescriptor.inc, and the clrdefinitions.cmake server GC disable).

janvorli · 2026-04-20T22:17:07Z

    GC_HEAP_FIELD(OomData, oom_info)

    /* For use in GCHeapDetails APIs */
+#ifdef BACKGROUND_GC


@davidwrighton is this ok for CDAC, assuming that disabling background GC is a WASM only thing?

Copilot

Pull request overview

This PR applies CoreCLR WebAssembly-specific build/feature gating to reduce binary size by removing GC modes that are not intended to be used/supported on WASM builds.

Changes:

Suppress FEATURE_SVR_GC for WebAssembly targets in CoreCLR CMake definitions.
Disable BACKGROUND_GC when compiling for TARGET_WASM.
Update GC cDAC descriptors to only describe background-GC-only fields/globals when BACKGROUND_GC is defined.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/coreclr/gc/gcpriv.h	Guards `BACKGROUND_GC` so it is not enabled for `TARGET_WASM`.
src/coreclr/gc/datadescriptor/datadescriptor.h	Wraps background-GC-only `gc_heap` field descriptors in `#ifdef BACKGROUND_GC`.
src/coreclr/gc/datadescriptor/datadescriptor.inc	Wraps background-GC-only cDAC globals in `#ifdef BACKGROUND_GC`.
src/coreclr/clrdefinitions.cmake	Avoids defining `FEATURE_SVR_GC` for WASM target builds.

Copilot · 2026-04-21T06:55:26Z


+#ifndef TARGET_WASM
 #define BACKGROUND_GC   //concurrent background GC (requires WRITE_WATCH)
+#endif //!TARGET_WASM


The #endif trailing comment style here is inconsistent with the surrounding codebase (typically #endif // !TARGET_WASM with a space after //). Consider adjusting this for consistency/readability.

Suggested change

#endif //!TARGET_WASM

#endif // !TARGET_WASM

Disable FEATURE_SVR_GC for WebAssembly CoreCLR builds

e85d3f7

Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/ef405ab1-bc2a-472f-b797-52d49c5d8409 Co-authored-by: davidwrighton <10779849+davidwrighton@users.noreply.github.com>

Copilot AI assigned Copilot and davidwrighton Apr 20, 2026

Copilot AI requested review from Copilot and removed request for Copilot April 20, 2026 20:37

github-actions bot added the area-GC-coreclr label Apr 20, 2026

Copilot created this pull request from a session on behalf of davidwrighton April 20, 2026 20:39 View session

Copilot AI requested a review from davidwrighton April 20, 2026 20:39

Copilot finished work on behalf of davidwrighton April 20, 2026 20:39

davidwrighton requested review from janvorli and pavelsavara April 20, 2026 20:41

Copilot started work on behalf of davidwrighton April 20, 2026 20:45 View session

Stub unused JIT interface methods for interpreter-only WASM builds

b5b5b6d

Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/05b0c780-4d5e-4177-b5ec-c07a31e050a7 Co-authored-by: davidwrighton <10779849+davidwrighton@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot April 20, 2026 21:19

Copilot AI requested review from Copilot and removed request for Copilot April 20, 2026 21:37

Copilot finished work on behalf of davidwrighton April 20, 2026 21:38

Copilot started work on behalf of janvorli April 20, 2026 21:38 View session

Disable background GC for WASM builds (#ifndef TARGET_WASM in gcpriv.h)

dc7b9f9

Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/7f661e48-3027-488a-8ca9-5fc83d287082 Co-authored-by: janvorli <10758568+janvorli@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot April 20, 2026 21:56

Copilot AI changed the title ~~Disable server GC for CoreCLR WebAssembly builds~~ Reduce CoreCLR WebAssembly binary size: stub unused JIT interface methods and disable background GC Apr 20, 2026

Copilot finished work on behalf of janvorli April 20, 2026 21:57

Copilot started work on behalf of janvorli April 20, 2026 22:08 View session

Revert jitinterface.cpp changes (JIT interface stubs are out of scope)

c6d22cb

Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/b8ad1421-bfb6-48e3-8387-923a7d1cc0dc Co-authored-by: janvorli <10758568+janvorli@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot April 20, 2026 22:11

Copilot AI changed the title ~~Reduce CoreCLR WebAssembly binary size: stub unused JIT interface methods and disable background GC~~ Disable server GC and background GC for CoreCLR WebAssembly builds Apr 20, 2026

Copilot finished work on behalf of janvorli April 20, 2026 22:12

janvorli reviewed Apr 20, 2026

View reviewed changes

Merge branch 'main' into copilot/analyze-coreclr-gc-build-config

b5305fa

Copilot AI review requested due to automatic review settings April 21, 2026 06:49

Copilot started reviewing on behalf of pavelsavara April 21, 2026 06:50 View session

Copilot AI reviewed Apr 21, 2026

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Disable server GC and background GC for CoreCLR WebAssembly builds#127178

Disable server GC and background GC for CoreCLR WebAssembly builds#127178
Copilot wants to merge 6 commits intomainfrom
copilot/analyze-coreclr-gc-build-config

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Size measurements (corerun.wasm)

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Copilot AI commented Apr 20, 2026 •

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Size measurements (`corerun.wasm`)