[Mono.Android] Reduce JNI overhead in the CoreCLR GC bridge

[simonrozsival]

Note

This pull request was created with the assistance of GitHub Copilot. The code and description were AI-generated and reviewed by the author.

Summary

Reduces JNI overhead and fixes local-reference hygiene in the CoreCLR GC
bridge host (src/native/clr/host, shared with the NativeAOT host). No
behavioral change to which objects survive a collection — these are
efficiency/robustness improvements to how bridge processing talks to JNI.

Changes

• Cache mono.android.IGCUserPeer method IDs. add_reference /
  clear_references previously did a GetObjectClass + GetMethodID
  lookup (plus a transient local reference) for every reference edge
  and every cleared node, on every bridge cycle. The interface method IDs
  (monodroidAddReference / monodroidClearReferences) are now resolved
  once at runtime init and dispatched via an IsInstanceOf guard. This
  mirrors how the NativeAOT host already caches the
  net.dot.jni.GCUserPeerable method IDs, roughly halves the JNI calls
  per edge, and removes a per-edge local reference. The IsInstanceOf
  guard preserves the previous "missing method" fallback and the
  add_circular_references abort-on-failure semantics.

• Reserve JNI local capacity for empty-SCC temporary peers. Each
  strongly connected component with no IGCUserPeers is represented by a
  temporary peer held as a JNI local reference until all cross references
  are added. prepare_for_java_collection now reserves local reference
  capacity up front so a large number of such SCCs cannot overflow the
  JNI local reference table (which only guarantees 16 slots by default).

• Fix a local-reference leak in GCBridge::log_handle_context. The
  jclass from GetObjectClass was never deleted, leaking one local per
  logged handle on the long-lived, attached bridge thread whenever GC
  spew is enabled.

• Fix the inverted GC-spew guard in log_weak_ref_collected. The
  "was collected by a Java GC" message was emitted when GC spew was
  disabled and suppressed when it was enabled — the opposite of every
  sibling logging helper (e.g. log_gc_summary).

Testing

Built the CoreCLR android-arm64 runtime with these changes and ran the
full Mono.Android.NET-Tests suite on an API 36 arm64-v8a emulator with
-p:UseMonoRuntime=false (CoreCLR):

• 915 passed, 0 failed, 55 skipped.
• With debug.mono.log=gc enabled, the GC bridge ran 12 times during the
  suite (graphs up to ~389 objects, tens of objects resurrected per cycle),
  exercising the cached-method-ID add_reference/clear_references paths
  and the temporary-peer capacity path, with no CheckJNI / JNI errors /
  local-reference-table overflows (CheckJNI is active for the debuggable
  test app).
• One unrelated timing-sensitive test (JavaObjectTest.UnregisterFromRuntime,
  a registered-peer count assertion) flaked once under heavy GC-spew logging
  and passed on re-run; it is not affected by these changes.

[Copilot]

Pull request overview

This PR reduces JNI overhead and improves local-reference hygiene in the CoreCLR GC bridge host (shared with the NativeAOT host), focusing on making bridge processing more efficient and robust without changing GC liveness semantics.

Changes:

• Cache mono.android.IGCUserPeer interface method IDs once at runtime init, avoiding per-edge GetObjectClass/GetMethodID and associated local refs during bridge cycles.
• Proactively call EnsureLocalCapacity() to accommodate temporary peers created for SCCs with Count == 0, mitigating JNI local-ref-table overflow risk.
• Fix local-ref management and GC-spew logging guards (DeleteLocalRef() for GetObjectClass() in log_handle_context, and correct the gc_spew_enabled() early-return logic).

Show a summary per file

File	Description
src/native/clr/include/host/bridge-processing-shared.hh	Adds cached IGCUserPeer JNI handles (class + method IDs) to support per-edge JNI call reductions.
src/native/clr/host/gc-bridge.cc	Deletes the jclass local ref created by GetObjectClass() in log_handle_context, preventing a local-ref leak on the bridge thread when logging.
src/native/clr/host/bridge-processing.cc	Initializes cached IGCUserPeer method IDs, reserves local-ref capacity for temporary peers, and switches reference add/clear to cached interface dispatch with an IsInstanceOf guard; fixes GC-spew guard inversion in log_weak_ref_collected.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 0