[heap] Add read-only heap sharing behind a flag

Piggybacking of splitting heap setup, this change adds a shared
read-only heap and a flag to enable it.

Also makes CallOnce use std::function instead of a raw function
pointer so the CL can use lambdas with CallOnce.

Bug: v8:7464
Change-Id: I9a97fb1baa6badca39a7381de3fd9e01f5969340
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1518180Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
Cr-Commit-Position: refs/heads/master@{#60241}

BUILD.gn

@@ -171,6 +171,9 @@ declare_args() {
   # setting the "check_v8_header_includes" gclient variable to run a
   # specific hook).
   v8_check_header_includes = false
+
+  # Enable sharing read-only space across isolates.
+  v8_enable_shared_ro_heap = false
 }
 
 # We reuse the snapshot toolchain for building torque and other generators to
@@ -214,6 +217,9 @@ assert(!v8_enable_lite_mode || v8_enable_embedded_builtins,
 assert(!v8_enable_lite_mode || v8_use_snapshot,
        "Lite mode requires a snapshot build")
 
+assert(v8_use_snapshot || !v8_enable_shared_ro_heap,
+       "Nosnapshot builds are not supported with shared read-only heap enabled")
+
 v8_random_seed = "314159265"
 v8_toolset_for_shell = "host"
 
@@ -404,6 +410,9 @@ config("features") {
   if (v8_use_siphash) {
     defines += [ "V8_USE_SIPHASH" ]
   }
+  if (v8_enable_shared_ro_heap) {
+    defines += [ "V8_SHARED_RO_HEAP" ]
+  }
 }
 
 config("toolchain") {

src/base/once.h

@@ -74,7 +74,6 @@ enum : uint8_t {
   ONCE_STATE_DONE = 2
 };
 
-typedef void (*NoArgFunction)();
 typedef void (*PointerArgFunction)(void* arg);
 
 template <typename T>
@@ -85,13 +84,12 @@ struct OneArgFunction {
 V8_BASE_EXPORT void CallOnceImpl(OnceType* once,
                                  std::function<void()> init_func);
 
-inline void CallOnce(OnceType* once, NoArgFunction init_func) {
+inline void CallOnce(OnceType* once, std::function<void()> init_func) {
   if (once->load(std::memory_order_acquire) != ONCE_STATE_DONE) {
     CallOnceImpl(once, init_func);
   }
 }
 
-
 template <typename Arg>
 inline void CallOnce(OnceType* once,
     typename OneArgFunction<Arg*>::type init_func, Arg* arg) {

src/heap/heap.cc

@@ -3550,14 +3550,21 @@ const char* Heap::GarbageCollectionReasonToString(
 }
 
 bool Heap::Contains(HeapObject value) {
+  // Check RO_SPACE first because IsOutsideAllocatedSpace cannot account for a
+  // shared RO_SPACE.
+  // TODO(goszczycki): Exclude read-only space. Use ReadOnlyHeap::Contains where
+  // appropriate.
+  if (read_only_space_ != nullptr && read_only_space_->Contains(value)) {
+    return true;
+  }
   if (memory_allocator()->IsOutsideAllocatedSpace(value->address())) {
     return false;
   }
   return HasBeenSetUp() &&
          (new_space_->ToSpaceContains(value) || old_space_->Contains(value) ||
           code_space_->Contains(value) || map_space_->Contains(value) ||
-          lo_space_->Contains(value) || read_only_space_->Contains(value) ||
-          code_lo_space_->Contains(value) || new_lo_space_->Contains(value));
+          lo_space_->Contains(value) || code_lo_space_->Contains(value) ||
+          new_lo_space_->Contains(value));
 }
 
 bool Heap::InSpace(HeapObject value, AllocationSpace space) {
@@ -4550,8 +4557,9 @@ void Heap::SetUp() {
 }
 
 void Heap::SetUpFromReadOnlyHeap(ReadOnlyHeap* ro_heap) {
-  DCHECK_NULL(read_only_space_);
   DCHECK_NOT_NULL(ro_heap);
+  DCHECK_IMPLIES(read_only_space_ != nullptr,
+                 read_only_space_ == ro_heap->read_only_space());
   read_only_heap_ = ro_heap;
   space_[RO_SPACE] = read_only_space_ = ro_heap->read_only_space();
 }

src/heap/read-only-heap.cc

@@ -4,29 +4,72 @@
 
 #include "src/heap/read-only-heap.h"
 
+#include <cstring>
+
+#include "src/base/once.h"
+#include "src/heap/heap-inl.h"
 #include "src/heap/spaces.h"
 #include "src/snapshot/read-only-deserializer.h"
 
 namespace v8 {
 namespace internal {
 
+#ifdef V8_SHARED_RO_HEAP
+V8_DECLARE_ONCE(setup_ro_heap_once);
+ReadOnlyHeap* shared_ro_heap = nullptr;
+#endif
+
 // static
 void ReadOnlyHeap::SetUp(Isolate* isolate, ReadOnlyDeserializer* des) {
-  auto* ro_heap = new ReadOnlyHeap(new ReadOnlySpace(isolate->heap()));
-  isolate->heap()->SetUpFromReadOnlyHeap(ro_heap);
+#ifdef V8_SHARED_RO_HEAP
+  void* isolate_ro_roots = reinterpret_cast<void*>(
+      isolate->roots_table().read_only_roots_begin().address());
+  base::CallOnce(&setup_ro_heap_once, [isolate, des, isolate_ro_roots]() {
+    shared_ro_heap = Init(isolate, des);
+    if (des != nullptr) {
+      std::memcpy(shared_ro_heap->read_only_roots_, isolate_ro_roots,
+                  kEntriesCount * sizeof(Address));
+    }
+  });
+
+  isolate->heap()->SetUpFromReadOnlyHeap(shared_ro_heap);
   if (des != nullptr) {
-    des->DeserializeInto(isolate);
-    ro_heap->read_only_space_->MarkAsReadOnly();
+    std::memcpy(isolate_ro_roots, shared_ro_heap->read_only_roots_,
+                kEntriesCount * sizeof(Address));
   }
+#else
+  Init(isolate, des);
+#endif  // V8_SHARED_RO_HEAP
 }
 
 void ReadOnlyHeap::OnCreateHeapObjectsComplete() {
+  DCHECK(!deserializing_);
+#ifdef V8_SHARED_RO_HEAP
+  read_only_space_->Forget();
+#endif
   read_only_space_->MarkAsReadOnly();
 }
 
+// static
+ReadOnlyHeap* ReadOnlyHeap::Init(Isolate* isolate, ReadOnlyDeserializer* des) {
+  auto* ro_heap = new ReadOnlyHeap(new ReadOnlySpace(isolate->heap()));
+  isolate->heap()->SetUpFromReadOnlyHeap(ro_heap);
+  if (des != nullptr) {
+    des->DeserializeInto(isolate);
+    ro_heap->deserializing_ = true;
+#ifdef V8_SHARED_RO_HEAP
+    ro_heap->read_only_space_->Forget();
+#endif
+    ro_heap->read_only_space_->MarkAsReadOnly();
+  }
+  return ro_heap;
+}
+
 void ReadOnlyHeap::OnHeapTearDown() {
+#ifndef V8_SHARED_RO_HEAP
   delete read_only_space_;
   delete this;
+#endif
 }
 
 // static

src/heap/read-only-heap.h

@@ -9,7 +9,6 @@
 #include "src/heap/heap.h"
 #include "src/objects.h"
 #include "src/roots.h"
-#include "src/snapshot/read-only-deserializer.h"
 
 namespace v8 {
 namespace internal {
@@ -18,16 +17,19 @@ class ReadOnlySpace;
 class ReadOnlyDeserializer;
 
 // This class transparently manages read-only space, roots and cache creation
-// and destruction. Eventually this will allow sharing these artifacts between
-// isolates.
+// and destruction.
 class ReadOnlyHeap final {
  public:
+  static constexpr size_t kEntriesCount =
+      static_cast<size_t>(RootIndex::kReadOnlyRootsCount);
+
   // If necessary create read-only heap and initialize its artifacts (if the
   // deserializer is provided).
   // TODO(goszczycki): Ideally we'd create this without needing a heap.
   static void SetUp(Isolate* isolate, ReadOnlyDeserializer* des);
   // Indicate that all read-only space objects have been created and will not
-  // be written to.
+  // be written to. This is not thread safe, and should really only be used as
+  // part of mksnapshot or when read-only heap sharing is disabled.
   void OnCreateHeapObjectsComplete();
   // Indicate that the current isolate no longer requires the read-only heap and
   // it may be safely disposed of.
@@ -42,9 +44,16 @@ class ReadOnlyHeap final {
   ReadOnlySpace* read_only_space() const { return read_only_space_; }
 
  private:
+  static ReadOnlyHeap* Init(Isolate* isolate, ReadOnlyDeserializer* des);
+
+  bool deserializing_ = false;
   ReadOnlySpace* read_only_space_ = nullptr;
   std::vector<Object> read_only_object_cache_;
 
+#ifdef V8_SHARED_RO_HEAP
+  Address read_only_roots_[kEntriesCount];
+#endif
+
   explicit ReadOnlyHeap(ReadOnlySpace* ro_space) : read_only_space_(ro_space) {}
   DISALLOW_COPY_AND_ASSIGN(ReadOnlyHeap);
 };

src/heap/spaces.cc

@@ -3305,6 +3305,12 @@ void ReadOnlyPage::MakeHeaderRelocatable() {
   }
 }
 
+void ReadOnlySpace::Forget() {
+  for (Page* p : *this) {
+    heap()->memory_allocator()->PreFreeMemory(p);
+  }
+}
+
 void ReadOnlySpace::SetPermissionsForPages(PageAllocator::Permission access) {
   MemoryAllocator* memory_allocator = heap()->memory_allocator();
   for (Page* p : *this) {

src/heap/spaces.h

@@ -2992,6 +2992,9 @@ class ReadOnlySpace : public PagedSpace {
 
   void ClearStringPaddingIfNeeded();
   void MarkAsReadOnly();
+  // Make the heap forget the space for memory bookkeeping purposes
+  // (e.g. prevent space's memory from registering as leaked).
+  void Forget();
 
   // During boot the free_space_map is created, and afterwards we may need
   // to write it into the free list nodes that were already created.

src/roots.h

@@ -497,6 +497,7 @@ class RootsTable {
   friend class Isolate;
   friend class Heap;
   friend class Factory;
+  friend class ReadOnlyHeap;
   friend class ReadOnlyRoots;
   friend class RootsSerializer;
 };