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// Copyright 2020 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/objects/string-table.h"
#include <atomic>
#include "src/base/atomicops.h"
#include "src/base/macros.h"
#include "src/common/assert-scope.h"
#include "src/common/globals.h"
#include "src/common/ptr-compr-inl.h"
#include "src/execution/isolate-utils-inl.h"
#include "src/heap/safepoint.h"
#include "src/objects/internal-index.h"
#include "src/objects/object-list-macros.h"
#include "src/objects/slots-inl.h"
#include "src/objects/slots.h"
#include "src/objects/string-inl.h"
#include "src/objects/string-table-inl.h"
#include "src/snapshot/deserializer.h"
#include "src/utils/allocation.h"
#include "src/utils/ostreams.h"
namespace v8 {
namespace internal {
namespace {
static constexpr int kStringTableMaxEmptyFactor = 4;
static constexpr int kStringTableMinCapacity = 2048;
bool StringTableHasSufficientCapacityToAdd(int capacity, int number_of_elements,
int number_of_deleted_elements,
int number_of_additional_elements) {
int nof = number_of_elements + number_of_additional_elements;
// Return true if:
// 50% is still free after adding number_of_additional_elements elements and
// at most 50% of the free elements are deleted elements.
if ((nof < capacity) &&
((number_of_deleted_elements <= (capacity - nof) / 2))) {
int needed_free = nof / 2;
if (nof + needed_free <= capacity) return true;
}
return false;
}
int ComputeStringTableCapacity(int at_least_space_for) {
// Add 50% slack to make slot collisions sufficiently unlikely.
// See matching computation in StringTableHasSufficientCapacityToAdd().
int raw_capacity = at_least_space_for + (at_least_space_for >> 1);
int capacity = base::bits::RoundUpToPowerOfTwo32(raw_capacity);
return std::max(capacity, kStringTableMinCapacity);
}
int ComputeStringTableCapacityWithShrink(int current_capacity,
int at_least_room_for) {
// Only shrink if the table is very empty to avoid performance penalty.
DCHECK_GE(current_capacity, kStringTableMinCapacity);
if (at_least_room_for > (current_capacity / kStringTableMaxEmptyFactor))
return current_capacity;
// Recalculate the smaller capacity actually needed.
int new_capacity = ComputeStringTableCapacity(at_least_room_for);
DCHECK_GE(new_capacity, at_least_room_for);
// Don't go lower than room for {kStringTableMinCapacity} elements.
if (new_capacity < kStringTableMinCapacity) return current_capacity;
return new_capacity;
}
template <typename IsolateT, typename StringTableKey>
bool KeyIsMatch(IsolateT* isolate, StringTableKey* key, String string) {
if (string.hash() != key->hash()) return false;
if (string.length() != key->length()) return false;
return key->IsMatch(isolate, string);
}
} // namespace
// Data holds the actual data of the string table, including capacity and number
// of elements.
//
// It is a variable sized structure, with a "header" followed directly in memory
// by the elements themselves. These are accessed as offsets from the elements_
// field, which itself provides storage for the first element.
//
// The elements themselves are stored as an open-addressed hash table, with
// quadratic probing and Smi 0 and Smi 1 as the empty and deleted sentinels,
// respectively.
class StringTable::Data {
public:
static std::unique_ptr<Data> New(int capacity);
static std::unique_ptr<Data> Resize(PtrComprCageBase cage_base,
std::unique_ptr<Data> data, int capacity);
OffHeapObjectSlot slot(InternalIndex index) const {
return OffHeapObjectSlot(&elements_[index.as_uint32()]);
}
Object Get(PtrComprCageBase cage_base, InternalIndex index) const {
return slot(index).Acquire_Load(cage_base);
}
void Set(InternalIndex index, String entry) {
slot(index).Release_Store(entry);
}
void ElementAdded() {
DCHECK_LT(number_of_elements_ + 1, capacity());
DCHECK(StringTableHasSufficientCapacityToAdd(
capacity(), number_of_elements(), number_of_deleted_elements(), 1));
number_of_elements_++;
}
void DeletedElementOverwritten() {
DCHECK_LT(number_of_elements_ + 1, capacity());
DCHECK(StringTableHasSufficientCapacityToAdd(
capacity(), number_of_elements(), number_of_deleted_elements() - 1, 1));
number_of_elements_++;
number_of_deleted_elements_--;
}
void ElementsRemoved(int count) {
DCHECK_LE(count, number_of_elements_);
number_of_elements_ -= count;
number_of_deleted_elements_ += count;
}
void* operator new(size_t size, int capacity);
void* operator new(size_t size) = delete;
void operator delete(void* description);
int capacity() const { return capacity_; }
int number_of_elements() const { return number_of_elements_; }
int number_of_deleted_elements() const { return number_of_deleted_elements_; }
template <typename IsolateT, typename StringTableKey>
InternalIndex FindEntry(IsolateT* isolate, StringTableKey* key,
uint32_t hash) const;
InternalIndex FindInsertionEntry(PtrComprCageBase cage_base,
uint32_t hash) const;
template <typename IsolateT, typename StringTableKey>
InternalIndex FindEntryOrInsertionEntry(IsolateT* isolate,
StringTableKey* key,
uint32_t hash) const;
// Helper method for StringTable::TryStringToIndexOrLookupExisting.
template <typename Char>
static Address TryStringToIndexOrLookupExisting(Isolate* isolate,
String string, String source,
size_t start);
void IterateElements(RootVisitor* visitor);
Data* PreviousData() { return previous_data_.get(); }
void DropPreviousData() { previous_data_.reset(); }
void Print(PtrComprCageBase cage_base) const;
size_t GetCurrentMemoryUsage() const;
private:
explicit Data(int capacity);
// Returns probe entry.
inline static InternalIndex FirstProbe(uint32_t hash, uint32_t size) {
return InternalIndex(hash & (size - 1));
}
inline static InternalIndex NextProbe(InternalIndex last, uint32_t number,
uint32_t size) {
return InternalIndex((last.as_uint32() + number) & (size - 1));
}
private:
std::unique_ptr<Data> previous_data_;
int number_of_elements_;
int number_of_deleted_elements_;
const int capacity_;
Tagged_t elements_[1];
};
void* StringTable::Data::operator new(size_t size, int capacity) {
// Make sure the size given is the size of the Data structure.
DCHECK_EQ(size, sizeof(StringTable::Data));
// Make sure that the elements_ array is at the end of Data, with no padding,
// so that subsequent elements can be accessed as offsets from elements_.
STATIC_ASSERT(offsetof(StringTable::Data, elements_) ==
sizeof(StringTable::Data) - sizeof(Tagged_t));
// Make sure that elements_ is aligned when StringTable::Data is aligned.
STATIC_ASSERT(
(alignof(StringTable::Data) + offsetof(StringTable::Data, elements_)) %
kTaggedSize ==
0);
// Subtract 1 from capacity, as the member elements_ already supplies the
// storage for the first element.
return AlignedAlloc(size + (capacity - 1) * sizeof(Tagged_t),
alignof(StringTable::Data));
}
void StringTable::Data::operator delete(void* table) { AlignedFree(table); }
size_t StringTable::Data::GetCurrentMemoryUsage() const {
size_t usage = sizeof(*this) + (capacity_ - 1) * sizeof(Tagged_t);
if (previous_data_) {
usage += previous_data_->GetCurrentMemoryUsage();
}
return usage;
}
StringTable::Data::Data(int capacity)
: previous_data_(nullptr),
number_of_elements_(0),
number_of_deleted_elements_(0),
capacity_(capacity) {
OffHeapObjectSlot first_slot = slot(InternalIndex(0));
MemsetTagged(first_slot, empty_element(), capacity);
}
std::unique_ptr<StringTable::Data> StringTable::Data::New(int capacity) {
return std::unique_ptr<Data>(new (capacity) Data(capacity));
}
std::unique_ptr<StringTable::Data> StringTable::Data::Resize(
PtrComprCageBase cage_base, std::unique_ptr<Data> data, int capacity) {
std::unique_ptr<Data> new_data(new (capacity) Data(capacity));
DCHECK_LT(data->number_of_elements(), new_data->capacity());
DCHECK(StringTableHasSufficientCapacityToAdd(
new_data->capacity(), new_data->number_of_elements(),
new_data->number_of_deleted_elements(), data->number_of_elements()));
// Rehash the elements.
for (InternalIndex i : InternalIndex::Range(data->capacity())) {
Object element = data->Get(cage_base, i);
if (element == empty_element() || element == deleted_element()) continue;
String string = String::cast(element);
uint32_t hash = string.hash();
InternalIndex insertion_index =
new_data->FindInsertionEntry(cage_base, hash);
new_data->Set(insertion_index, string);
}
new_data->number_of_elements_ = data->number_of_elements();
new_data->previous_data_ = std::move(data);
return new_data;
}
template <typename IsolateT, typename StringTableKey>
InternalIndex StringTable::Data::FindEntry(IsolateT* isolate,
StringTableKey* key,
uint32_t hash) const {
uint32_t count = 1;
// EnsureCapacity will guarantee the hash table is never full.
for (InternalIndex entry = FirstProbe(hash, capacity_);;
entry = NextProbe(entry, count++, capacity_)) {
// TODO(leszeks): Consider delaying the decompression until after the
// comparisons against empty/deleted.
Object element = Get(isolate, entry);
if (element == empty_element()) return InternalIndex::NotFound();
if (element == deleted_element()) continue;
String string = String::cast(element);
if (KeyIsMatch(isolate, key, string)) return entry;
}
}
InternalIndex StringTable::Data::FindInsertionEntry(PtrComprCageBase cage_base,
uint32_t hash) const {
uint32_t count = 1;
// EnsureCapacity will guarantee the hash table is never full.
for (InternalIndex entry = FirstProbe(hash, capacity_);;
entry = NextProbe(entry, count++, capacity_)) {
// TODO(leszeks): Consider delaying the decompression until after the
// comparisons against empty/deleted.
Object element = Get(cage_base, entry);
if (element == empty_element() || element == deleted_element())
return entry;
}
}
template <typename IsolateT, typename StringTableKey>
InternalIndex StringTable::Data::FindEntryOrInsertionEntry(
IsolateT* isolate, StringTableKey* key, uint32_t hash) const {
InternalIndex insertion_entry = InternalIndex::NotFound();
uint32_t count = 1;
// EnsureCapacity will guarantee the hash table is never full.
for (InternalIndex entry = FirstProbe(hash, capacity_);;
entry = NextProbe(entry, count++, capacity_)) {
// TODO(leszeks): Consider delaying the decompression until after the
// comparisons against empty/deleted.
Object element = Get(isolate, entry);
if (element == empty_element()) {
// Empty entry, it's our insertion entry if there was no previous Hole.
if (insertion_entry.is_not_found()) return entry;
return insertion_entry;
}
if (element == deleted_element()) {
// Holes are potential insertion candidates, but we continue the search
// in case we find the actual matching entry.
if (insertion_entry.is_not_found()) insertion_entry = entry;
continue;
}
String string = String::cast(element);
if (KeyIsMatch(isolate, key, string)) return entry;
}
}
void StringTable::Data::IterateElements(RootVisitor* visitor) {
OffHeapObjectSlot first_slot = slot(InternalIndex(0));
OffHeapObjectSlot end_slot = slot(InternalIndex(capacity_));
visitor->VisitRootPointers(Root::kStringTable, nullptr, first_slot, end_slot);
}
void StringTable::Data::Print(PtrComprCageBase cage_base) const {
OFStream os(stdout);
os << "StringTable {" << std::endl;
for (InternalIndex i : InternalIndex::Range(capacity_)) {
os << " " << i.as_uint32() << ": " << Brief(Get(cage_base, i))
<< std::endl;
}
os << "}" << std::endl;
}
StringTable::StringTable(Isolate* isolate)
: data_(Data::New(kStringTableMinCapacity).release())
#ifdef DEBUG
,
isolate_(isolate)
#endif
{
}
StringTable::~StringTable() { delete data_; }
int StringTable::Capacity() const {
return data_.load(std::memory_order_acquire)->capacity();
}
int StringTable::NumberOfElements() const {
{
base::MutexGuard table_write_guard(&write_mutex_);
return data_.load(std::memory_order_relaxed)->number_of_elements();
}
}
// InternalizedStringKey carries a string/internalized-string object as key.
class InternalizedStringKey final : public StringTableKey {
public:
explicit InternalizedStringKey(Handle<String> string)
: StringTableKey(0, string->length()), string_(string) {
DCHECK(!string->IsInternalizedString());
DCHECK(string->IsFlat());
// Make sure hash_field is computed.
string->EnsureHash();
set_raw_hash_field(string->raw_hash_field());
}
bool IsMatch(Isolate* isolate, String string) {
DCHECK(!SharedStringAccessGuardIfNeeded::IsNeeded(string));
return string_->SlowEquals(string);
}
Handle<String> AsHandle(Isolate* isolate) {
// Internalize the string if possible.
MaybeHandle<Map> maybe_map =
isolate->factory()->InternalizedStringMapForString(string_);
Handle<Map> map;
if (maybe_map.ToHandle(&map)) {
string_->set_map_no_write_barrier(*map);
DCHECK(string_->IsInternalizedString());
return string_;
}
// External strings get special treatment, to avoid copying their
// contents as long as they are not uncached.
StringShape shape(*string_);
if (shape.IsExternalOneByte() && !shape.IsUncachedExternal()) {
return isolate->factory()
->InternalizeExternalString<ExternalOneByteString>(string_);
} else if (shape.IsExternalTwoByte() && !shape.IsUncachedExternal()) {
return isolate->factory()
->InternalizeExternalString<ExternalTwoByteString>(string_);
} else {
// Otherwise allocate a new internalized string.
return isolate->factory()->NewInternalizedStringImpl(
string_, string_->length(), string_->raw_hash_field());
}
}
private:
Handle<String> string_;
};
Handle<String> StringTable::LookupString(Isolate* isolate,
Handle<String> string) {
string = String::Flatten(isolate, string);
if (string->IsInternalizedString()) return string;
InternalizedStringKey key(string);
Handle<String> result = LookupKey(isolate, &key);
if (!string->IsInternalizedString()) {
string->MakeThin(isolate, *result);
}
return result;
}
template <typename StringTableKey, typename IsolateT>
Handle<String> StringTable::LookupKey(IsolateT* isolate, StringTableKey* key) {
// String table lookups are allowed to be concurrent, assuming that:
//
// - The Heap access is allowed to be concurrent (using LocalHeap or
// similar),
// - All writes to the string table are guarded by the Isolate string table
// mutex,
// - Resizes of the string table first copies the old contents to the new
// table, and only then sets the new string table pointer to the new
// table,
// - Only GCs can remove elements from the string table.
//
// These assumptions allow us to make the following statement:
//
// "Reads are allowed when not holding the lock, as long as false negatives
// (misses) are ok. We will never get a false positive (hit of an entry no
// longer in the table)"
//
// This is because we _know_ that if we find an entry in the string table, any
// entry will also be in all reallocations of that tables. This is required
// for strong consistency of internalized string equality implying reference
// equality.
//
// We therefore try to optimistically read from the string table without
// taking the lock (both here and in the NoAllocate version of the lookup),
// and on a miss we take the lock and try to write the entry, with a second
// read lookup in case the non-locked read missed a write.
//
// One complication is allocation -- we don't want to allocate while holding
// the string table lock. This applies to both allocation of new strings, and
// re-allocation of the string table on resize. So, we optimistically allocate
// (without copying values) outside the lock, and potentially discard the
// allocation if another write also did an allocation. This assumes that
// writes are rarer than reads.
// Load the current string table data, in case another thread updates the
// data while we're reading.
const Data* data = data_.load(std::memory_order_acquire);
// First try to find the string in the table. This is safe to do even if the
// table is now reallocated; we won't find a stale entry in the old table
// because the new table won't delete it's corresponding entry until the
// string is dead, in which case it will die in this table too and worst
// case we'll have a false miss.
InternalIndex entry = data->FindEntry(isolate, key, key->hash());
if (entry.is_found()) {
return handle(String::cast(data->Get(isolate, entry)), isolate);
}
// No entry found, so adding new string.
// Allocate the string before the first insertion attempt, reuse this
// allocated value on insertion retries. If another thread concurrently
// allocates the same string, the insert will fail, the lookup above will
// succeed, and this string will be discarded.
Handle<String> new_string = key->AsHandle(isolate);
{
base::MutexGuard table_write_guard(&write_mutex_);
Data* data = EnsureCapacity(isolate, 1);
// Check one last time if the key is present in the table, in case it was
// added after the check.
entry = data->FindEntryOrInsertionEntry(isolate, key, key->hash());
Object element = data->Get(isolate, entry);
if (element == empty_element()) {
// This entry is empty, so write it and register that we added an
// element.
data->Set(entry, *new_string);
data->ElementAdded();
return new_string;
} else if (element == deleted_element()) {
// This entry was deleted, so overwrite it and register that we
// overwrote a deleted element.
data->Set(entry, *new_string);
data->DeletedElementOverwritten();
return new_string;
} else {
// Return the existing string as a handle.
return handle(String::cast(element), isolate);
}
}
}
template Handle<String> StringTable::LookupKey(Isolate* isolate,
OneByteStringKey* key);
template Handle<String> StringTable::LookupKey(Isolate* isolate,
TwoByteStringKey* key);
template Handle<String> StringTable::LookupKey(Isolate* isolate,
SeqOneByteSubStringKey* key);
template Handle<String> StringTable::LookupKey(Isolate* isolate,
SeqTwoByteSubStringKey* key);
template Handle<String> StringTable::LookupKey(LocalIsolate* isolate,
OneByteStringKey* key);
template Handle<String> StringTable::LookupKey(LocalIsolate* isolate,
TwoByteStringKey* key);
template Handle<String> StringTable::LookupKey(Isolate* isolate,
StringTableInsertionKey* key);
template Handle<String> StringTable::LookupKey(LocalIsolate* isolate,
StringTableInsertionKey* key);
StringTable::Data* StringTable::EnsureCapacity(PtrComprCageBase cage_base,
int additional_elements) {
// This call is only allowed while the write mutex is held.
write_mutex_.AssertHeld();
// This load can be relaxed as the table pointer can only be modified while
// the lock is held.
Data* data = data_.load(std::memory_order_relaxed);
// Grow or shrink table if needed. We first try to shrink the table, if it
// is sufficiently empty; otherwise we make sure to grow it so that it has
// enough space.
int current_capacity = data->capacity();
int current_nof = data->number_of_elements();
int capacity_after_shrinking =
ComputeStringTableCapacityWithShrink(current_capacity, current_nof + 1);
int new_capacity = -1;
if (capacity_after_shrinking < current_capacity) {
DCHECK(StringTableHasSufficientCapacityToAdd(capacity_after_shrinking,
current_nof, 0, 1));
new_capacity = capacity_after_shrinking;
} else if (!StringTableHasSufficientCapacityToAdd(
current_capacity, current_nof,
data->number_of_deleted_elements(), 1)) {
new_capacity = ComputeStringTableCapacity(current_nof + 1);
}
if (new_capacity != -1) {
std::unique_ptr<Data> new_data =
Data::Resize(cage_base, std::unique_ptr<Data>(data), new_capacity);
// `new_data` is the new owner of `data`.
DCHECK_EQ(new_data->PreviousData(), data);
// Release-store the new data pointer as `data_`, so that it can be
// acquire-loaded by other threads. This string table becomes the owner of
// the pointer.
data = new_data.release();
data_.store(data, std::memory_order_release);
}
return data;
}
// static
template <typename Char>
Address StringTable::Data::TryStringToIndexOrLookupExisting(Isolate* isolate,
String string,
String source,
size_t start) {
// TODO(leszeks): This method doesn't really belong on StringTable::Data.
// Ideally it would be a free function in an anonymous namespace, but that
// causes issues around method and class visibility.
DisallowGarbageCollection no_gc;
uint64_t seed = HashSeed(isolate);
int length = string.length();
std::unique_ptr<Char[]> buffer;
const Char* chars;
if (source.IsConsString()) {
DCHECK(!source.IsFlat());
buffer.reset(new Char[length]);
String::WriteToFlat(source, buffer.get(), 0, length);
chars = buffer.get();
} else {
chars = source.GetChars<Char>(no_gc) + start;
}
// TODO(verwaest): Internalize to one-byte when possible.
SequentialStringKey<Char> key(base::Vector<const Char>(chars, length), seed);
// String could be an array index.
uint32_t raw_hash_field = key.raw_hash_field();
if (Name::ContainsCachedArrayIndex(raw_hash_field)) {
return Smi::FromInt(String::ArrayIndexValueBits::decode(raw_hash_field))
.ptr();
}
if ((raw_hash_field & Name::kIsNotIntegerIndexMask) == 0) {
// It is an index, but it's not cached.
return Smi::FromInt(ResultSentinel::kUnsupported).ptr();
}
Data* string_table_data =
isolate->string_table()->data_.load(std::memory_order_acquire);
InternalIndex entry = string_table_data->FindEntry(isolate, &key, key.hash());
if (entry.is_not_found()) {
// A string that's not an array index, and not in the string table,
// cannot have been used as a property name before.
return Smi::FromInt(ResultSentinel::kNotFound).ptr();
}
String internalized = String::cast(string_table_data->Get(isolate, entry));
string.MakeThin(isolate, internalized);
return internalized.ptr();
}
// static
Address StringTable::TryStringToIndexOrLookupExisting(Isolate* isolate,
Address raw_string) {
String string = String::cast(Object(raw_string));
DCHECK(!string.IsInternalizedString());
// Valid array indices are >= 0, so they cannot be mixed up with any of
// the result sentinels, which are negative.
STATIC_ASSERT(
!String::ArrayIndexValueBits::is_valid(ResultSentinel::kUnsupported));
STATIC_ASSERT(
!String::ArrayIndexValueBits::is_valid(ResultSentinel::kNotFound));
size_t start = 0;
String source = string;
if (source.IsSlicedString()) {
SlicedString sliced = SlicedString::cast(source);
start = sliced.offset();
source = sliced.parent();
} else if (source.IsConsString() && source.IsFlat()) {
source = ConsString::cast(source).first();
}
if (source.IsThinString()) {
source = ThinString::cast(source).actual();
if (string.length() == source.length()) {
return source.ptr();
}
}
if (source.IsOneByteRepresentation()) {
return StringTable::Data::TryStringToIndexOrLookupExisting<uint8_t>(
isolate, string, source, start);
}
return StringTable::Data::TryStringToIndexOrLookupExisting<uint16_t>(
isolate, string, source, start);
}
void StringTable::Print(PtrComprCageBase cage_base) const {
data_.load(std::memory_order_acquire)->Print(cage_base);
}
size_t StringTable::GetCurrentMemoryUsage() const {
return sizeof(*this) +
data_.load(std::memory_order_acquire)->GetCurrentMemoryUsage();
}
void StringTable::IterateElements(RootVisitor* visitor) {
// This should only happen during garbage collection when background threads
// are paused, so the load can be relaxed.
DCHECK(isolate_->heap()->safepoint()->IsActive());
data_.load(std::memory_order_relaxed)->IterateElements(visitor);
}
void StringTable::DropOldData() {
// This should only happen during garbage collection when background threads
// are paused, so the load can be relaxed.
DCHECK(isolate_->heap()->safepoint()->IsActive());
DCHECK_NE(isolate_->heap()->gc_state(), Heap::NOT_IN_GC);
data_.load(std::memory_order_relaxed)->DropPreviousData();
}
void StringTable::NotifyElementsRemoved(int count) {
// This should only happen during garbage collection when background threads
// are paused, so the load can be relaxed.
DCHECK(isolate_->heap()->safepoint()->IsActive());
DCHECK_NE(isolate_->heap()->gc_state(), Heap::NOT_IN_GC);
data_.load(std::memory_order_relaxed)->ElementsRemoved(count);
}
} // namespace internal
} // namespace v8
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