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Custom STL Allocators Pete Isensee Xbox Advanced Technology Group

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1 Custom STL Allocators Pete Isensee Xbox Advanced Technology Group

2 Topics Allocators: What are They Good For? Writing Your First Allocator The Devil in the Details Allocator Pitfalls –State –Syntax –Testing Case Study

3 Containers and Allocators STL containers allocate memory –e.g. vector (contiguous), list (nodes) –string is a container, for this talk Allocators provide a standard interface for container memory use If you don’t provide an allocator, one is provided for you

4 Example Default Allocator list b; // same as: list > b; Custom Allocator #include “MyAlloc.h” list > c;

5 The Good Original idea: abstract the notion of near and far memory pointers Expanded idea: allow customization of container allocation Good for –Size: Optimizing memory usage (pools, fixed-size allocators) –Speed: Reducing allocation time (single- threaded, one-time free)

6 Example Allocators No heap locking (single thread) Avoiding fragmentation Aligned allocations (_aligned_malloc) Fixed-size allocations Custom free list Debugging Custom heap Specific memory type

7 The Bad No realloc() Requires advanced C++ compilers C++ Standard hand-waving Generally library-specific –If you change STL libraries you may need to rewrite allocators Generally not cross-platform –If you change compilers you may need to rewrite allocators

8 The Ugly Not quite real objects –Allocators with state may not work as expected Gnarly syntax –map m; –map, MyAlloc > > m;

9 Pause to Reflect “Premature optimization is the root of all evil” – Donald Knuth Allocators are a last resort and low- level optimization Especially for games, allocators can be the perfect optimization Written correctly, they can be introduced w/o many code changes

10 Writing Your First Allocator Create MyAlloc.h #include Copy or derive from the default allocator Rename “allocator” to “MyAlloc” Resolve any helper functions Replace some code with your own

11 Writing Your First Allocator Demo Visual C++ Pro 7.0 ( ) Dinkumware STL (V3.10:0009) 933MHz PIII w/ 512MB Windows XP Pro 2002 Launch Visual Studio Visual Studio

12 Two key functions Allocate Deallocate That’s all!

13 Conventions template class allocator { typedef size_t size_type; typedef T* pointer; typedef const T* const_pointer; typedef T value_type; };

14 Allocate Function pointer allocate( size_type n, allocator ::const_pointer p = 0) –n is the number of items T, NOT bytes –returns pointer to enough memory to hold n * sizeof(T) bytes –returns raw bytes; NO construction –may throw an exception (std::bad_alloc) –default calls ::operator new –p is optional hint; avoid

15 Deallocate function void deallocate( pointer p, size_type n ) –p must come from allocate() –p must be raw bytes; already destroyed –n must match the n passed to allocate() –default calls ::operator delete(void*) –Most implementations allow and ignore NULL p; you should too

16 A Custom Allocator Demo That’s it! Not quite: the devil is in the details –Construction –Destruction –Example STL container code –Rebind

17 Construction Allocate() doesn’t call constructors Why? Performance Allocators provide construct function void construct(pointer p, const T& t) { new( (void*)p ) T(t); } Placement new –Doesn’t allocate memory –Calls copy constructor

18 Destruction Deallocate() doesn’t call destructors Allocators provide a destroy function void destroy( pointer p ) { ((T*)p)->~T(); } Direct destructor invocation –Doesn’t deallocate memory –Calls destructor

19 Example: Vector template class vector { A a; // allocator pointer pFirst; // first object pointer pEnd; // 1 beyond end pointer pLast; // 1 beyond last };

20 Example: Reserve vector::reserve( size_type n ) { pointer p = a.allocate( n, 0 ); // loop on a.construct() to copy // loop on a.destroy() to tear down a.deallocate( pFirst, capacity() ); pFirst = p; pLast = p + size(); pEnd = p + n; }

21 Performance is paramount Reserve –Single allocation –Doesn’t default construct anything –Deals properly with real objects No memcpy Copy constructs new objects Destroys old objects –Single deallocation

22 Rebind Allocators don’t always allocate T list ObjList; // allocates nodes How? Rebind template struct rebind { typedef allocator other; } To allocate an N given type T Alloc a; T* t = a.allocate(1); // allocs sizeof(T) Alloc ::rebind ::other na; N* n = na.allocate(1); // allocs sizeof(N)

23 Allocator Pitfalls To Derive or Not to Derive State –Copy ctor and template copy ctor –Allocator comparison Syntax issues Testing Case Study

24 To Derive or Not To Derive Deriving from std::allocator –Dinkumware derives (see ) –Must provide rebind, allocate, deallocate –Less code; easier to see differences Writing from scratch –Allocator not designed as base class –Josuttis and Austern write from scratch –Better understanding Personal preference

25 Allocators with State State = allocator member data Default allocator has no data C++ Std says (paraphrasing ): –Vendors encouraged to support allocators with state –Containers may assume that allocators don’t have state

26 State Recommendations Be aware of compatibility issues across STL vendors list::splice() or C::swap() will indicate if your vendor supports stateful allocators –Dinkumware: yes –STLport: no Test carefully

27 State Implications Container size increase Must provide allocator: –Constructor(s) Default may be private if parameters required –Copy constructor –Template copy constructor –Global comparison operators (==, !=) No assignment operators required Avoid static data; generates one per T

28 Heap Allocator Example template class Halloc { Halloc(); // could be private explicit Halloc( HANDLE hHeap ); Halloc( const Halloc& ); // copy template // templatized Halloc( const Halloc & ); // copy };

29 Template Copy Constructor Can’t see private data template Halloc( const Halloc & a ) : m_hHeap( a.m_hHeap ) {} // error Solutions –Provide public data accessor function –Or allow access to other types U template friend class Halloc;

30 Allocator comparison Example template bool operator==( const Alloc & a, const Alloc & b ) { return a.state == b.state; } Provide both == and != Should be global fucns, not members May require accessor functions

31 Syntax: Typedefs Prefer typedefs Offensive list > b; Better //.h typedef Alloc IAlloc; typedef list IntList; //.cpp IntList v;

32 Syntax: Construction Containers accept allocators via ctors IntList b( IAlloc( x,y,z ) ); If none specified, you get the default IntList b; // calls IAlloc() Map/multimap requires pairs Alloc > a; map, Alloc > > m( less (), a );

33 Syntax: Other Containers Container adaptors accept containers via constructors, not allocators Alloc a; deque > d(a); stack > > s(d); String example Alloc a; basic_string, Alloc > s(a);

34 Testing Test the normal case Test with all containers (don’t forget string, hash containers, stack, etc.) Test with different objects T, particularly those w/ non-trivial dtors Test edge cases like list::splice Verify that your version is better! Allocator test framework:

35 Case Study In-place allocator –Hand off existing memory block –Dole out allocations from the block –Never free Example usage typedef InPlaceAlloc IPA; void* p = malloc( 1024 ); list x( IPA( p, 1024 ) ); x.push_back( 1 ); free( p ); View code

36 In-Place Allocator Problems –Fails w/ multiple concurrent copies –No copy constructor –Didn’t support comparison –Didn’t handle containers of void* Correct implementationimplementation –Reference counted –Copy constructor implemented –Comparison operators –Void specialization

37 In-Place Summary Speed –Scenario: add x elements, remove half –About 50x faster than default allocator! Advantages –Fast; no overhead; no fragmentation –Whatever memory you want Disadvantages –Proper implementation isn’t easy –Limited use

38 Recommendations Allocators: a last resort optimization Base your allocator on Beware porting issues (both compilers and STL vendor libraries) Beware allocators with state Test thoroughly Verify speed/size improvements

39 Recommendations part II Use typedefs to simplify life Don’t forget to write –Rebind –Copy constructor –Templatized copy constructor –Comparison operators –Void specialization

40 References C++ Standard section , Your STL implementation: GDC Proceedings: References section Game Gems III

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