[webkit-changes] [WebKit/WebKit] 0ea98b: Optimize the creation of WeakPtrs for many object ...

[Chris Dumez]

  Branch: refs/heads/main
  Home:   https://github.com/WebKit/WebKit
  Commit: 0ea98bdecabcba02abd3154aef5a17d11f35d9ca
      https://github.com/WebKit/WebKit/commit/0ea98bdecabcba02abd3154aef5a17d11f35d9ca
  Author: Chris Dumez <cdumez at apple.com>
  Date:   2023-12-07 (Thu, 07 Dec 2023)

  Changed paths:
    M Source/WTF/wtf/WeakHashMap.h
    M Source/WTF/wtf/WeakHashSet.h
    M Source/WTF/wtf/WeakListHashSet.h
    M Source/WTF/wtf/WeakPtr.h
    M Source/WTF/wtf/WeakRef.h
    M Source/WebCore/dom/EventTarget.cpp
    M Source/WebCore/dom/EventTarget.h

  Log Message:
  -----------
  Optimize the creation of WeakPtrs for many object types
https://bugs.webkit.org/show_bug.cgi?id=265981

Reviewed by Darin Adler.

The performance of WeakPtrs is increasingly important as we adopt it
more and more in our code base. As I am working to use WeakPtr more
in rendering code, I noticed that the construction of WeakPtr was
impact by the fact that WeakPtrFactory was using a data member of
type `CompactRefPtrTuple<WeakPtrImpl, uint16_t>` to store a
packed WeakPtrImpl pointer in order to free up space for a bitfield.
This means that every time we want to construct a WeakPtr from an
object, the WeakPtrFactory has to extract the WeakPtrImpl pointer
from the CompactRefPtrTuple, which can be costly, especially in
Intel hardware.

The only type actually leveraging the WeakPtrFactory's bit field
is EventTarget. All the other classes (like RenderObject) are just
paying this cost without actually getting any benefit from the
pointer packing.

To address the issue, I renamed WeakPtrFactory to
WeakPtrFactoryWithBitField and had EventTarget use this one
instead. I introduced a simpler WeakPtrFactory which is using a
straight `RefPtr<WeakPtrImpl>` instead, which every other type
is now using.

Based on my A/B testing, this should allow us to replace
CheckedPtr / CheckedRef with WeakPtr / WeakRef in rendering code
without regressing our benchmarks.

* Source/WTF/wtf/WeakHashMap.h:
* Source/WTF/wtf/WeakHashSet.h:
* Source/WTF/wtf/WeakListHashSet.h:
* Source/WTF/wtf/WeakPtr.h:
(WTF::WeakPtr::implForObject):
(WTF::WeakPtrFactory::~WeakPtrFactory):
(WTF::WeakPtrFactory::impl const):
(WTF::WeakPtrFactory::initializeIfNeeded const):
(WTF::WeakPtrFactory::createWeakPtr const):
(WTF::WeakPtrFactory::revokeAll):
(WTF::WeakPtrFactory::weakPtrCount const):
(WTF::WeakPtrFactory::isInitialized const):
(WTF::WeakPtrFactoryWithBitField::WeakPtrFactoryWithBitField):
(WTF::WeakPtrFactoryWithBitField::~WeakPtrFactoryWithBitField):
(WTF::WeakPtrFactoryWithBitField::impl const):
(WTF::CanMakeWeakPtrBase::weakPtrFactory const):
(WTF::CanMakeWeakPtrBase::weakPtrFactory):
(WTF::CanMakeWeakPtrBase::CanMakeWeakPtrBase):
(WTF::CanMakeWeakPtrBase::operator=):
(WTF::CanMakeWeakPtrBase::initializeWeakPtrFactory):
(WTF::WeakPtrFactory::impl): Deleted.
(WTF::WeakPtrFactory::bitfield const): Deleted.
(WTF::WeakPtrFactory::setBitfield const): Deleted.
(WTF::CanMakeWeakPtr::weakPtrFactory const): Deleted.
(WTF::CanMakeWeakPtr::weakPtrFactory): Deleted.
(WTF::CanMakeWeakPtr::CanMakeWeakPtr): Deleted.
(WTF::CanMakeWeakPtr::operator=): Deleted.
(WTF::CanMakeWeakPtr::initializeWeakPtrFactory): Deleted.
* Source/WTF/wtf/WeakRef.h:
(WTF::WeakRef::implForObject):
* Source/WebCore/dom/EventTarget.h:

Canonical link: https://commits.webkit.org/271701@main