heap: Fix --minor-mc ephemeron processing

Minor MC does not support processing the specialized remembered set
for ephemeron tables.

Temporarily delegate to the regular write barrier for correctness
until the other barrier is supported.

Bug: v8:12262
Change-Id: Iad74b27f8738237dcc1e146b2df3aa6ed8c9a505
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3195895Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77170}

src/heap/heap.cc

@@ -7087,11 +7087,18 @@ void Heap::GenerationalBarrierSlow(HeapObject object, Address slot,
 
 void Heap::RecordEphemeronKeyWrite(EphemeronHashTable table, Address slot) {
   DCHECK(ObjectInYoungGeneration(HeapObjectSlot(slot).ToHeapObject()));
-  int slot_index = EphemeronHashTable::SlotToIndex(table.address(), slot);
-  InternalIndex entry = EphemeronHashTable::IndexToEntry(slot_index);
-  auto it =
-      ephemeron_remembered_set_.insert({table, std::unordered_set<int>()});
-  it.first->second.insert(entry.as_int());
+  if (FLAG_minor_mc) {
+    // Minor MC lacks support for specialized generational ephemeron barriers.
+    // The regular write barrier works as well but keeps more memory alive.
+    MemoryChunk* chunk = MemoryChunk::FromHeapObject(table);
+    RememberedSet<OLD_TO_NEW>::Insert<AccessMode::NON_ATOMIC>(chunk, slot);
+  } else {
+    int slot_index = EphemeronHashTable::SlotToIndex(table.address(), slot);
+    InternalIndex entry = EphemeronHashTable::IndexToEntry(slot_index);
+    auto it =
+        ephemeron_remembered_set_.insert({table, std::unordered_set<int>()});
+    it.first->second.insert(entry.as_int());
+  }
 }
 
 void Heap::EphemeronKeyWriteBarrierFromCode(Address raw_object,

src/heap/mark-compact.cc

@@ -4797,6 +4797,9 @@ class MinorMarkCompactCollector::RootMarkingVisitor : public RootVisitor {
 };
 
 void MinorMarkCompactCollector::CollectGarbage() {
+  // Minor MC does not support processing the ephemeron remembered set.
+  DCHECK(heap()->ephemeron_remembered_set_.empty());
+
   {
     TRACE_GC(heap()->tracer(), GCTracer::Scope::MINOR_MC_SWEEPING);
     heap()->mark_compact_collector()->sweeper()->EnsureIterabilityCompleted();