cppgc: young-gen: Enable generational GC before weak processing

During weak processing we remember weak callbacks for objects in the old
generation. We should check the young-gc flag and enable generational GC
before weak processing, as otherwise we would miss the callbacks and
forget to update the weak refs.

Bug: chromium:1029379
Change-Id: I72c98d4926b57c36af6cc503ce34712f67d50f42
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3616721Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80286}

src/heap/cppgc-js/cpp-heap.cc

@@ -637,6 +637,15 @@ bool CppHeap::FinishConcurrentMarkingIfNeeded() {
 void CppHeap::TraceEpilogue() {
   CHECK(in_atomic_pause_);
   CHECK(marking_done_);
+
+#if defined(CPPGC_YOUNG_GENERATION)
+  // Check if the young generation was enabled via flag. We must enable young
+  // generation before calling the custom weak callbacks to make sure that the
+  // callbacks for old objects are registered in the remembered set.
+  if (FLAG_cppgc_young_generation) {
+    EnableGenerationalGC();
+  }
+#endif  // defined(CPPGC_YOUNG_GENERATION)
   {
     cppgc::subtle::DisallowGarbageCollectionScope disallow_gc_scope(*this);
     marker_->LeaveAtomicPause();
@@ -662,9 +671,6 @@ void CppHeap::TraceEpilogue() {
   USE(bytes_allocated_in_prefinalizers);
 
 #if defined(CPPGC_YOUNG_GENERATION)
-  // Check if the young generation was enabled via flag.
-  if (FLAG_cppgc_young_generation) EnableGenerationalGC();
-
   ResetRememberedSet();
 #endif  // defined(CPPGC_YOUNG_GENERATION)
 

src/heap/cppgc/heap.cc

@@ -170,6 +170,15 @@ void Heap::FinalizeGarbageCollection(Config::StackState stack_state) {
   config_.stack_state = stack_state;
   SetStackEndOfCurrentGC(v8::base::Stack::GetCurrentStackPosition());
   in_atomic_pause_ = true;
+
+#if defined(CPPGC_YOUNG_GENERATION)
+  // Check if the young generation was enabled. We must enable young generation
+  // before calling the custom weak callbacks to make sure that the callbacks
+  // for old objects are registered in the remembered set.
+  if (generational_gc_enabled_) {
+    HeapBase::EnableGenerationalGC();
+  }
+#endif  // defined(CPPGC_YOUNG_GENERATION)
   {
     // This guards atomic pause marking, meaning that no internal method or
     // external callbacks are allowed to allocate new objects.
@@ -190,9 +199,6 @@ void Heap::FinalizeGarbageCollection(Config::StackState stack_state) {
   USE(bytes_allocated_in_prefinalizers);
 
 #if defined(CPPGC_YOUNG_GENERATION)
-  if (generational_gc_enabled_) {
-    HeapBase::EnableGenerationalGC();
-  }
   ResetRememberedSet();
 #endif  // defined(CPPGC_YOUNG_GENERATION)