Reland: Move files in wtf/ to platform/wtf/ (Part 12).

third_party/WebKit/Source/wtf/Allocator.md

Index: third_party/WebKit/Source/wtf/Allocator.md
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-# Memory management in Blink
-
-This document gives a high-level overview of the memory management in Blink.
-
-[TOC]
-
-## Memory allocators
-
-Blink objects are allocated by one of the following four memory allocators.
-
-### Oilpan
-
-Oilpan is a garbage collection system in Blink.
-The lifetime of objects allocated by Oilpan is automatically managed.
-The following objects are allocated by Oilpan:
-
-* Objects that inherit from GarbageCollected<T> or GarbageCollectedFinalized<T>.
-
-* HeapVector<T>, HeapHashSet<T>, HeapHashMap<T, U> etc
-
-The implementation is in platform/heap/.
-See [BlinkGCDesign.md](../platform/heap/BlinkGCDesign.md) to learn the design.
-
-### PartitionAlloc
-
-PartitionAlloc is Blink's default memory allocator.
-PartitionAlloc is highly optimized for performance and security requirements
-in Blink. All Blink objects that don't need a GC or discardable memory should be
-allocated by PartitionAlloc (instead of malloc).
-The following objects are allocated by PartitionAlloc:
-
-* Objects that have a USING_FAST_MALLOC macro.
-
-* Nodes (which will be moved to Oilpan in the near future)
-
-* LayoutObjects
-
-* Strings, Vectors, HashTables, ArrayBuffers and other primitive containers.
-
-The implementation is in /base/allocator/partition_allocator.
-See [PartitionAlloc.md](/base/allocator/partition_allocator/PartitionAlloc.md)
-to learn the design.
-
-### Discardable memory
-
-Discardable memory is a memory allocator that automatically discards
-(not-locked) objects under memory pressure. Currently SharedBuffers
-(which are mainly used as backing storage of Resource objects) are the only
-user of the discardable memory.
-
-The implementation is in src/base/memory/discardable_memory.*.
-See [this document](https://docs.google.com/document/d/1aNdOF_72_eG2KUM_z9kHdbT_fEupWhaDALaZs5D8IAg/edit)
-to learn the design.
-
-### malloc
-
-When you simply call 'new' or 'malloc', the object is allocated by malloc.
-As mentioned above, malloc is discouraged and should be replaced with
-PartitionAlloc. PartitionAlloc is faster, securer and more instrumentable
-than malloc.
-
-The actual implementation of malloc is platform-dependent.
-As of 2015 Dec, Linux uses tcmalloc. Mac and Windows use their system allocator.
-Android uses device-dependent allocators such as dlmalloc.
-
-## Basic allocation rules
-
-In summary, Blink objects (except several special objects) should be allocated
-using Oilpan or PartitionAlloc. malloc is discouraged.
-
-The following is a basic rule to determine which of Oilpan or PartitionAlloc
-you should use when allocating a new object:
-
-* Use Oilpan if you want a GC to manage the lifetime of the object.
-You need to make the object inherit from GarbageCollected<T> or
-GarbageCollectedFinalized<T>. See
-[BlinkGCAPIReference.md](../platform/heap/BlinkGCAPIReference.md) to learn
-programming with Oilpan.
-
-```c++
-class X : public GarbageCollected<X> {
-  ...;
-};
-
-void func() {
-  X* x = new X;  // This is allocated by Oilpan.
-}
-```
-
-* Use PartitionAlloc if you don't need a GC to manage the lifetime of
-the object (i.e., if RefPtr or OwnPtr is enough to manage the lifetime
-of the object). You need to add a USING_FAST_MALLOC macro to the object.
-
-```c++
-class X {
-  USING_FAST_MALLOC(X);
-  ...;
-};
-
-void func() {
-  RefPtr<X> x = adoptRefPtr(new X);  // This is allocated by PartitionAlloc.
-}
-```
-
-It is not a good idea to unnecessarily increase the number of objects
-managed by Oilpan. Although Oilpan implements an efficient GC, the more objects
-you allocate on Oilpan, the more pressure you put on Oilpan, leading to
-a longer pause time.
-
-Here is a guideline for when you ought to allocate an object using Oilpan,
-and when you probably shouldn't:
-
-* Use Oilpan for all script exposed objects (i.e., derives from
-ScriptWrappable).
-
-* Use Oilpan if managing its lifetime is usually simpler with Oilpan.
-But see the next bullet.
-
-* If the allocation rate of the object is very high, that may put unnecessary
-strain on the Oilpan's GC infrastructure as a whole. If so and the object can be
-allocated not using Oilpan without creating cyclic references or complex
-lifetime handling, then use PartitionAlloc. For example, we allocate Strings
-and LayoutObjects on PartitionAlloc.
-
-For objects that don't need an operator new, you need to use either of the
-following macros:
-
-* If the object is only stack allocated, use STACK_ALLOCATED().
-
-```c++
-class X {
-  STACK_ALLOCATED();
-  ...;
-};
-
-void func() {
-  X x;  // This is allowed.
-  X* x2 = new X;  // This is forbidden.
-}
-```
-
-* If the object can be allocated only as a part of object, use DISALLOW_NEW().
-
-```c++
-class X {
-  DISALLOW_NEW();
-  ...;
-};
-
-class Y {
-  USING_FAST_MALLOC(Y);
-  X m_x;  // This is allowed.
-};
-
-void func() {
-  X x;  // This is allowed.
-  X* x = new X;  // This is forbidden.
-}
-```
-
-* If the object can be allocated only as a part of object or by a placement new
-(e.g., the object is used as a value object in a container),
-use DISALLOW_NEW_EXCEPT_PLACEMENT_NEW().
-
-```c++
-class X {
-  DISALLOW_NEW_EXCEPT_PLACEMENT_NEW();
-  ...;
-};
-
-class Y {
-  USING_FAST_MALLOC(Y);
-  X m_x;  // This is allowed.
-  Vector<X> m_vector;  // This is allowed.
-};
-
-void func() {
-  X x;  // This is allowed.
-  X* x = new X;  // This is forbidden.
-}
-```
-
-Note that these macros are inherited. See a comment in wtf/Allocator.h
-for more details about the relationship between the macros and Oilpan.
-
-If you have any question, ask oilpan-reviews@chromium.org.