Make base::WeakPtr::Get() fast

base/memory/weak_ptr.cc

Index: base/memory/weak_ptr.cc
diff --git a/base/memory/weak_ptr.cc b/base/memory/weak_ptr.cc
index 591b955136a3bafaa0f37720461f48e14c9b220a..393cfb8ee50b7534c156095feb4c9d56cb73e715 100644
--- a/base/memory/weak_ptr.cc
+++ b/base/memory/weak_ptr.cc
@@ -4,75 +4,53 @@
 
 #include "base/memory/weak_ptr.h"
 
+#include "base/debug/leak_annotations.h"
+
 namespace base {
 namespace internal {
 
+static constexpr uintptr_t kTrueMask = ~static_cast<uintptr_t>(0);
+
+WeakReference::Flag::Flag() : is_valid_(kTrueMask) {
 #if DCHECK_IS_ON()
-WeakReference::NewFlag::NewFlag(WeakReference::NewFlag::NullFlagTag)
-    : is_valid_(false), ref_count_(1) {
   // Flags only become bound when checked for validity, or invalidated,
   // so that we can check that later validity/invalidation operations on
   // the same Flag take place on the same sequenced thread.
   sequence_checker_.DetachFromSequence();
-}
-#else
-constexpr WeakReference::NewFlag::NewFlag(WeakReference::NewFlag::NullFlagTag)
-    : is_valid_(false), ref_count_(1) {}
 #endif
-
-WeakReference::NewFlag WeakReference::NewFlag::g_null_flag(
-    WeakReference::NewFlag::kNullFlagTag);
-
-#if 0
-WeakReference::Flag::Flag() : is_valid_(true) {
-  // Flags only become bound when checked for validity, or invalidated,
-  // so that we can check that later validity/invalidation operations on
-  // the same Flag take place on the same sequenced thread.
-  sequence_checker_.DetachFromSequence();
 }
 
-void WeakReference::Flag::Invalidate() {
-  // The flag being invalidated with a single ref implies that there are no
-  // weak pointers in existence. Allow deletion on other thread in this case.
-  DCHECK(sequence_checker_.CalledOnValidSequence() || HasOneRef())
-      << "WeakPtrs must be invalidated on the same sequenced thread.";
-  is_valid_ = false;
-}
+WeakReference::Flag::Flag(WeakReference::Flag::NullFlagTag) : is_valid_(false) {
+  // There is no need for sequence_checker_.DetachFromSequence() because the
+  // null flag doesn't participate in the sequence checks.
 
-bool WeakReference::Flag::IsValid() const {
-  DCHECK(sequence_checker_.CalledOnValidSequence())
-      << "WeakPtrs must be checked on the same sequenced thread.";
-  return is_valid_;
+  AddRef();  // Stayin' alive.
 }
 
-WeakReference::Flag::~Flag() {
+WeakReference::Flag* WeakReference::Flag::nullFlag() {
+  ANNOTATE_SCOPED_MEMORY_LEAK;
+  static Flag* g_null_flag = new Flag(kNullFlagTag);
+  return g_null_flag;
 }
-#endif
 
-WeakReference::WeakReference() : new_flag_(NewFlag::NullFlag()) {
-}
+WeakReference::Flag::~Flag() {}
 
-WeakReference::WeakReference(/*const Flag* flag,*/ const NewFlag* newFlag)
-    : /*flag_(flag),*/ new_flag_(newFlag) {}
+WeakReference::WeakReference() : flag_(Flag::nullFlag()) {}
 
 WeakReference::~WeakReference() {
 }
 
-WeakReference::WeakReference(WeakReference&& other) : new_flag_(std::move(other.new_flag_)) {
-  other.new_flag_ = NewFlag::NullFlag();
+WeakReference::WeakReference(const Flag* flag) : flag_(flag) {}
+
+WeakReference::WeakReference(WeakReference&& other)
+    : flag_(std::move(other.flag_)) {
+  other.flag_ = Flag::nullFlag();
 }
 
 WeakReference::WeakReference(const WeakReference& other) = default;
 
-#if 0
-bool WeakReference::is_valid() const {
-  //return flag_.get() && flag_->IsValid();
-  return new_flag_->IsValid();
-}
-#endif
-
-WeakReferenceOwner::WeakReferenceOwner() : new_flag_(WeakReference::NewFlag::NullFlag()) {
-}
+WeakReferenceOwner::WeakReferenceOwner()
+    : flag_(WeakReference::Flag::nullFlag()) {}
 
 WeakReferenceOwner::~WeakReferenceOwner() {
   Invalidate();
@@ -80,36 +58,29 @@ WeakReferenceOwner::~WeakReferenceOwner() {
 
 WeakReference WeakReferenceOwner::GetRef() const {
   // If we hold the last reference to the Flag then create a new one.
-  if (!HasRefs()) {
-    //flag_ = new WeakReference::Flag();
-    new_flag_ = new WeakReference::NewFlag();
-  }
+  if (!HasRefs())
+    flag_ = new WeakReference::Flag();
 
-  return WeakReference(/*flag_.get(),*/ new_flag_.get());
+  return WeakReference(flag_.get());
 }
 
 void WeakReferenceOwner::Invalidate() {
-#if 0
-  if (flag_.get()) {
-    flag_->Invalidate();
-    flag_ = NULL;
-  }
-#endif
-  new_flag_->Invalidate();
-  new_flag_ = WeakReference::NewFlag::NullFlag();
+  flag_->Invalidate();
+  flag_ = WeakReference::Flag::nullFlag();
 }
 
-WeakPtrBase::WeakPtrBase() : ptr_(0) { }
+WeakPtrBase::WeakPtrBase() : ptr_(0) {}
 
-WeakPtrBase::~WeakPtrBase() { }
+WeakPtrBase::~WeakPtrBase() {}
 
 WeakPtrBase::WeakPtrBase(const WeakReference& ref) : ref_(ref) {
 }
 
-}  // namespace internal
-
 WeakPtrFactoryBase::WeakPtrFactoryBase(uintptr_t ptr) : ptr_(ptr) {}
 
-WeakPtrFactoryBase::~WeakPtrFactoryBase() { ptr_ = 0; }
+WeakPtrFactoryBase::~WeakPtrFactoryBase() {
+  ptr_ = 0;
+}
 
+}  // namespace internal
 }  // namespace base