[ext-code-space] Fix issues with shared GC

Bug: v8:11880, v8:12478
Change-Id: I47f5384195aee8d6f251ca141714bdaec272f62f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3318659
Auto-Submit: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78266}

src/heap/mark-compact.cc

@@ -1130,9 +1130,7 @@ class MarkCompactCollector::CustomRootBodyMarkingVisitor final
 
   void VisitCodePointer(HeapObject host, CodeObjectSlot slot) override {
     CHECK(V8_EXTERNAL_CODE_SPACE_BOOL);
-    // At the moment, custom roots cannot contain CodeDataContainers - the only
-    // objects that can contain Code pointers.
-    UNREACHABLE();
+    MarkObject(host, slot.load(code_cage_base()));
   }
 
   void VisitPointers(HeapObject host, MaybeObjectSlot start,
@@ -1191,9 +1189,7 @@ class MarkCompactCollector::SharedHeapObjectVisitor final
 
   void VisitCodePointer(HeapObject host, CodeObjectSlot slot) override {
     CHECK(V8_EXTERNAL_CODE_SPACE_BOOL);
-    // At the moment, custom roots cannot contain CodeDataContainers - the only
-    // objects that can contain Code pointers.
-    UNREACHABLE();
+    MarkObject(host, slot.load(code_cage_base()));
   }
 
   void VisitPointers(HeapObject host, MaybeObjectSlot start,
@@ -1861,9 +1857,9 @@ void MarkCompactCollector::MarkObjectsFromClientHeaps() {
       [&visitor](Isolate* client) {
         Heap* heap = client->heap();
         HeapObjectIterator iterator(heap, HeapObjectIterator::kNoFiltering);
+        PtrComprCageBase cage_base(client);
         for (HeapObject obj = iterator.Next(); !obj.is_null();
              obj = iterator.Next()) {
-          PtrComprCageBase cage_base = GetPtrComprCageBase(obj);
           obj.IterateFast(cage_base, &visitor);
         }
       });
@@ -4450,9 +4446,9 @@ void MarkCompactCollector::UpdatePointersInClientHeaps() {
       [&visitor](Isolate* client) {
         Heap* heap = client->heap();
         HeapObjectIterator iterator(heap, HeapObjectIterator::kNoFiltering);
+        PtrComprCageBase cage_base(client);
         for (HeapObject obj = iterator.Next(); !obj.is_null();
              obj = iterator.Next()) {
-          PtrComprCageBase cage_base = GetPtrComprCageBase(obj);
           obj.IterateFast(cage_base, &visitor);
         }
       });