[offthread] Allow ReadOnlyRoots from OffThreadHeap

Allow ReadOnlyRoots initialization from an OffThreadHeap, by creating
a FromHeap getter on OffThreadIsolate analogous to the one on Isolate.

Bug: chromium:1075999
Change-Id: Ie00e1547160e24d35bd7b0dd36d1b7eead87341e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2184289Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67600}

src/execution/off-thread-isolate.h

@@ -78,6 +78,11 @@ class V8_EXPORT_PRIVATE OffThreadIsolate final
   explicit OffThreadIsolate(Isolate* isolate, Zone* zone);
   ~OffThreadIsolate();
 
+  static OffThreadIsolate* FromHeap(OffThreadHeap* heap) {
+    return reinterpret_cast<OffThreadIsolate*>(
+        reinterpret_cast<Address>(heap) - OFFSET_OF(OffThreadIsolate, heap_));
+  }
+
   OffThreadHeap* heap() { return &heap_; }
 
   inline Address isolate_root() const;

src/heap/off-thread-heap.cc

@@ -228,7 +228,7 @@ HeapObject OffThreadHeap::AllocateRaw(int size, AllocationType allocation,
 
 HeapObject OffThreadHeap::CreateFillerObjectAt(
     Address addr, int size, ClearFreedMemoryMode clear_memory_mode) {
-  ReadOnlyRoots roots(lo_space_.heap());
+  ReadOnlyRoots roots(this);
   HeapObject filler =
       Heap::CreateFillerObjectAt(roots, addr, size, clear_memory_mode);
   return filler;

src/roots/roots-inl.h

@@ -62,6 +62,9 @@ bool RootsTable::IsRootHandle(Handle<T> handle, RootIndex* index) const {
 ReadOnlyRoots::ReadOnlyRoots(Heap* heap)
     : ReadOnlyRoots(Isolate::FromHeap(heap)) {}
 
+ReadOnlyRoots::ReadOnlyRoots(OffThreadHeap* heap)
+    : ReadOnlyRoots(OffThreadIsolate::FromHeap(heap)) {}
+
 ReadOnlyRoots::ReadOnlyRoots(Isolate* isolate)
     : read_only_roots_(reinterpret_cast<Address*>(
           isolate->roots_table().read_only_roots_begin().address())) {}

src/roots/roots.h

@@ -19,6 +19,7 @@ namespace internal {
 
 // Forward declarations.
 enum ElementsKind : uint8_t;
+class OffThreadHeap;
 class OffThreadIsolate;
 template <typename T>
 class Handle;
@@ -498,6 +499,7 @@ class ReadOnlyRoots {
       static_cast<size_t>(RootIndex::kReadOnlyRootsCount);
 
   V8_INLINE explicit ReadOnlyRoots(Heap* heap);
+  V8_INLINE explicit ReadOnlyRoots(OffThreadHeap* heap);
   V8_INLINE explicit ReadOnlyRoots(Isolate* isolate);
   V8_INLINE explicit ReadOnlyRoots(OffThreadIsolate* isolate);