Reland "[heap] Allow shared references in WeakMap"

This is a reland of a1838956

Now that https://crrev.com/c/3485678 landed and fixed the deadlock
in the linked bug, we can reland this CL without changes.

Original change's description:
> [heap] Allow shared references in WeakMap
>
> Shared references can also be stored in WeakMaps and during marking we
> need to be able to deal with such references. In a client GC shared
> objects are treated as live, so we don't need to update or check mark
> bits for such objects.
>
> Bug: v8:11708
> Change-Id: I0dbf797472c4779f462750dab63cc9b012aad091
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3447365
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#79153}

Bug: v8:11708, v8:12642
Change-Id: I5945a16255647c897a1df834267137bf73b6207f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3485679
Auto-Submit: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79267}

src/heap/mark-compact.cc

@@ -2917,12 +2917,16 @@ void MarkCompactCollector::ClearWeakCollections() {
       if (FLAG_verify_heap) {
         Object value = table.ValueAt(i);
         if (value.IsHeapObject()) {
-          CHECK_IMPLIES(non_atomic_marking_state()->IsBlackOrGrey(key),
-                        non_atomic_marking_state()->IsBlackOrGrey(
-                            HeapObject::cast(value)));
+          HeapObject heap_object = HeapObject::cast(value);
+          CHECK_IMPLIES(
+              (!is_shared_heap_ && key.InSharedHeap()) ||
+                  non_atomic_marking_state()->IsBlackOrGrey(key),
+              (!is_shared_heap_ && heap_object.InSharedHeap()) ||
+                  non_atomic_marking_state()->IsBlackOrGrey(heap_object));
         }
       }
 #endif
+      if (!is_shared_heap_ && key.InSharedHeap()) continue;
       if (!non_atomic_marking_state()->IsBlackOrGrey(key)) {
         table.RemoveEntry(i);
       }

src/heap/marking-visitor-inl.h

@@ -355,7 +355,8 @@ int MarkingVisitorBase<ConcreteVisitor, MarkingState>::VisitEphemeronHashTable(
     ObjectSlot value_slot =
         table.RawFieldOfElementAt(EphemeronHashTable::EntryToValueIndex(i));
 
-    if (concrete_visitor()->marking_state()->IsBlackOrGrey(key)) {
+    if ((!is_shared_heap_ && key.InSharedHeap()) ||
+        concrete_visitor()->marking_state()->IsBlackOrGrey(key)) {
       VisitPointer(table, value_slot);
     } else {
       Object value_obj = table.ValueAt(i);
@@ -366,6 +367,8 @@ int MarkingVisitorBase<ConcreteVisitor, MarkingState>::VisitEphemeronHashTable(
         concrete_visitor()->RecordSlot(table, value_slot, value);
         AddWeakReferenceForReferenceSummarizer(table, value);
 
+        if (!is_shared_heap_ && value.InSharedHeap()) continue;
+
         // Revisit ephemerons with both key and value unreachable at end
         // of concurrent marking cycle.
         if (concrete_visitor()->marking_state()->IsWhite(value)) {

test/mjsunit/shared-memory/shared-string-in-weak-map.js

+// Copyright 2022 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+//
+// Flags: --shared-string-table --allow-natives-syntax --stress-compaction --expose-gc
+
+const val1 = "some value";
+assertTrue(%IsSharedString(val1));
+
+const wm = new WeakMap();
+const key1 = {};
+
+wm.set(key1, val1);
+assertTrue(wm.get(key1) == val1);
+assertTrue(%IsSharedString(wm.get(key1)));
+
+gc();
+assertTrue(wm.get(key1) == val1);
+assertTrue(%IsSharedString(wm.get(key1)));
+
+%SharedGC();
+assertTrue(wm.get(key1) == val1);
+assertTrue(%IsSharedString(wm.get(key1)));