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// Copyright 2021 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.
#include "src/snapshot/shared-heap-serializer.h"
#include "src/heap/read-only-heap.h"
#include "src/objects/objects-inl.h"
#include "src/snapshot/read-only-serializer.h"
namespace v8 {
namespace internal {
// static
bool SharedHeapSerializer::CanBeInSharedOldSpace(HeapObject obj) {
if (ReadOnlyHeap::Contains(obj)) return false;
if (obj.IsString()) {
return obj.IsInternalizedString() ||
String::IsInPlaceInternalizable(String::cast(obj));
}
return false;
}
// static
bool SharedHeapSerializer::ShouldBeInSharedHeapObjectCache(HeapObject obj) {
// To keep the shared heap object cache lean, only include objects that should
// not be duplicated. Currently, that is only internalized strings. In-place
// internalizable strings will still be allocated in the shared heap by the
// deserializer, but do not need to be kept alive forever in the cache.
if (CanBeInSharedOldSpace(obj)) {
if (obj.IsInternalizedString()) return true;
}
return false;
}
SharedHeapSerializer::SharedHeapSerializer(
Isolate* isolate, Snapshot::SerializerFlags flags,
ReadOnlySerializer* read_only_serializer)
: RootsSerializer(isolate, flags, RootIndex::kFirstStrongRoot),
read_only_serializer_(read_only_serializer)
#ifdef DEBUG
,
serialized_objects_(isolate->heap())
#endif
{
if (ShouldReconstructSharedHeapObjectCacheForTesting()) {
ReconstructSharedHeapObjectCacheForTesting();
}
}
SharedHeapSerializer::~SharedHeapSerializer() {
OutputStatistics("SharedHeapSerializer");
}
void SharedHeapSerializer::FinalizeSerialization() {
// This is called after serialization of the startup and context snapshots
// which entries are added to the shared heap object cache. Terminate the
// cache with an undefined.
Object undefined = ReadOnlyRoots(isolate()).undefined_value();
VisitRootPointer(Root::kSharedHeapObjectCache, nullptr,
FullObjectSlot(&undefined));
// When v8_flags.shared_string_table is true, all internalized and
// internalizable-in-place strings are in the shared heap.
SerializeStringTable(isolate()->string_table());
SerializeDeferredObjects();
Pad();
#ifdef DEBUG
// Check that all serialized object are in shared heap and not RO. RO objects
// should be in the RO snapshot.
IdentityMap<int, base::DefaultAllocationPolicy>::IteratableScope it_scope(
&serialized_objects_);
for (auto it = it_scope.begin(); it != it_scope.end(); ++it) {
HeapObject obj = HeapObject::cast(it.key());
CHECK(CanBeInSharedOldSpace(obj));
CHECK(!ReadOnlyHeap::Contains(obj));
}
#endif
}
bool SharedHeapSerializer::SerializeUsingReadOnlyObjectCache(
SnapshotByteSink* sink, Handle<HeapObject> obj) {
return read_only_serializer_->SerializeUsingReadOnlyObjectCache(sink, obj);
}
bool SharedHeapSerializer::SerializeUsingSharedHeapObjectCache(
SnapshotByteSink* sink, Handle<HeapObject> obj) {
if (!ShouldBeInSharedHeapObjectCache(*obj)) return false;
int cache_index = SerializeInObjectCache(obj);
// When testing deserialization of a snapshot from a live Isolate where there
// is also a shared Isolate, the shared object cache needs to be extended
// because the live isolate may have had new internalized strings that were
// not present in the startup snapshot to be serialized.
if (ShouldReconstructSharedHeapObjectCacheForTesting()) {
std::vector<Object>* existing_cache =
isolate()->shared_heap_isolate()->shared_heap_object_cache();
const size_t existing_cache_size = existing_cache->size();
// This is strictly < because the existing cache contains the terminating
// undefined value, which the reconstructed cache does not.
DCHECK_LT(base::checked_cast<size_t>(cache_index), existing_cache_size);
if (base::checked_cast<size_t>(cache_index) == existing_cache_size - 1) {
ReadOnlyRoots roots(isolate());
DCHECK(existing_cache->back().IsUndefined(roots));
existing_cache->back() = *obj;
existing_cache->push_back(roots.undefined_value());
}
}
sink->Put(kSharedHeapObjectCache, "SharedHeapObjectCache");
sink->PutInt(cache_index, "shared_heap_object_cache_index");
return true;
}
void SharedHeapSerializer::SerializeStringTable(StringTable* string_table) {
// A StringTable is serialized as:
//
// N : int
// string 1
// string 2
// ...
// string N
//
// Notably, the hashmap structure, including empty and deleted elements, is
// not serialized.
sink_.PutInt(string_table->NumberOfElements(),
"String table number of elements");
// Custom RootVisitor which walks the string table, but only serializes the
// string entries. This is an inline class to be able to access the non-public
// SerializeObject method.
class SharedHeapSerializerStringTableVisitor : public RootVisitor {
public:
explicit SharedHeapSerializerStringTableVisitor(
SharedHeapSerializer* serializer)
: serializer_(serializer) {}
void VisitRootPointers(Root root, const char* description,
FullObjectSlot start, FullObjectSlot end) override {
UNREACHABLE();
}
void VisitRootPointers(Root root, const char* description,
OffHeapObjectSlot start,
OffHeapObjectSlot end) override {
DCHECK_EQ(root, Root::kStringTable);
Isolate* isolate = serializer_->isolate();
for (OffHeapObjectSlot current = start; current < end; ++current) {
Object obj = current.load(isolate);
if (obj.IsHeapObject()) {
DCHECK(obj.IsInternalizedString());
serializer_->SerializeObject(handle(HeapObject::cast(obj), isolate));
}
}
}
private:
SharedHeapSerializer* serializer_;
};
SharedHeapSerializerStringTableVisitor string_table_visitor(this);
isolate()->string_table()->IterateElements(&string_table_visitor);
}
void SharedHeapSerializer::SerializeObjectImpl(Handle<HeapObject> obj) {
// Objects in the shared heap cannot depend on per-Isolate roots but can
// depend on RO roots since sharing objects requires sharing the RO space.
DCHECK(CanBeInSharedOldSpace(*obj) || ReadOnlyHeap::Contains(*obj));
{
DisallowGarbageCollection no_gc;
HeapObject raw = *obj;
if (SerializeHotObject(raw)) return;
if (IsRootAndHasBeenSerialized(raw) && SerializeRoot(raw)) return;
}
if (SerializeUsingReadOnlyObjectCache(&sink_, obj)) return;
{
DisallowGarbageCollection no_gc;
HeapObject raw = *obj;
if (SerializeBackReference(raw)) return;
CheckRehashability(raw);
DCHECK(!ReadOnlyHeap::Contains(raw));
}
ObjectSerializer object_serializer(this, obj, &sink_);
object_serializer.Serialize();
#ifdef DEBUG
CHECK_NULL(serialized_objects_.Find(obj));
// There's no "IdentitySet", so use an IdentityMap with a value that is
// later ignored.
serialized_objects_.Insert(obj, 0);
#endif
}
bool SharedHeapSerializer::ShouldReconstructSharedHeapObjectCacheForTesting()
const {
// When the live Isolate being serialized is not a client Isolate, there's no
// need to reconstruct the shared heap object cache because it is not actually
// shared.
return reconstruct_read_only_and_shared_object_caches_for_testing() &&
isolate()->has_shared_heap();
}
void SharedHeapSerializer::ReconstructSharedHeapObjectCacheForTesting() {
std::vector<Object>* cache =
isolate()->shared_heap_isolate()->shared_heap_object_cache();
// Don't reconstruct the final element, which is always undefined and marks
// the end of the cache, since serializing the live Isolate may extend the
// shared object cache.
for (size_t i = 0, size = cache->size(); i < size - 1; i++) {
Handle<HeapObject> obj(HeapObject::cast(cache->at(i)), isolate());
DCHECK(ShouldBeInSharedHeapObjectCache(*obj));
int cache_index = SerializeInObjectCache(obj);
USE(cache_index);
DCHECK_EQ(cache_index, i);
}
DCHECK(cache->back().IsUndefined(isolate()));
}
} // namespace internal
} // namespace v8
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