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/*
* Copyright 2014 Google, Inc
*
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
*/
#ifndef SkSmallAllocator_DEFINED
#define SkSmallAllocator_DEFINED
#include "SkTDArray.h"
#include "SkTypes.h"
#include <new>
/*
* Template class for allocating small objects without additional heap memory
* allocations. kMaxObjects is a hard limit on the number of objects that can
* be allocated using this class. After that, attempts to create more objects
* with this class will assert and return nullptr.
*
* kTotalBytes is the total number of bytes provided for storage for all
* objects created by this allocator. If an object to be created is larger
* than the storage (minus storage already used), it will be allocated on the
* heap. This class's destructor will handle calling the destructor for each
* object it allocated and freeing its memory.
*
* Current the class always aligns each allocation to 16-bytes to be safe, but future
* may reduce this to only the alignment that is required per alloc.
*/
template<uint32_t kMaxObjects, size_t kTotalBytes>
class SkSmallAllocator : SkNoncopyable {
public:
SkSmallAllocator()
: fStorageUsed(0)
, fNumObjects(0)
{}
~SkSmallAllocator() {
// Destruct in reverse order, in case an earlier object points to a
// later object.
while (fNumObjects > 0) {
fNumObjects--;
Rec* rec = &fRecs[fNumObjects];
rec->fKillProc(rec->fObj);
// Safe to do if fObj is in fStorage, since fHeapStorage will
// point to nullptr.
sk_free(rec->fHeapStorage);
}
}
/*
* Create a new object of type T. Its lifetime will be handled by this
* SkSmallAllocator.
* Note: If kMaxObjects have been created by this SkSmallAllocator, nullptr
* will be returned.
*/
template<typename T, typename... Args>
T* createT(const Args&... args) {
void* buf = this->reserveT<T>();
if (nullptr == buf) {
return nullptr;
}
return new (buf) T(args...);
}
/*
* Reserve a specified amount of space (must be enough space for one T).
* The space will be in fStorage if there is room, or on the heap otherwise.
* Either way, this class will call ~T() in its destructor and free the heap
* allocation if necessary.
* Unlike createT(), this method will not call the constructor of T.
*/
template<typename T> void* reserveT(size_t storageRequired = sizeof(T)) {
SkASSERT(fNumObjects < kMaxObjects);
SkASSERT(storageRequired >= sizeof(T));
if (kMaxObjects == fNumObjects) {
return nullptr;
}
const size_t storageRemaining = sizeof(fStorage) - fStorageUsed;
Rec* rec = &fRecs[fNumObjects];
if (storageRequired > storageRemaining) {
// Allocate on the heap. Ideally we want to avoid this situation,
// but we're not sure we can catch all callers, so handle it but
// assert false in debug mode.
SkASSERT(false);
rec->fStorageSize = 0;
rec->fHeapStorage = sk_malloc_throw(storageRequired);
rec->fObj = static_cast<void*>(rec->fHeapStorage);
} else {
// There is space in fStorage.
rec->fStorageSize = storageRequired;
rec->fHeapStorage = nullptr;
rec->fObj = static_cast<void*>(fStorage.fBytes + fStorageUsed);
fStorageUsed += storageRequired;
}
rec->fKillProc = DestroyT<T>;
fNumObjects++;
return rec->fObj;
}
/*
* Free the memory reserved last without calling the destructor.
* Can be used in a nested way, i.e. after reserving A and B, calling
* freeLast once will free B and calling it again will free A.
*/
void freeLast() {
SkASSERT(fNumObjects > 0);
Rec* rec = &fRecs[fNumObjects - 1];
sk_free(rec->fHeapStorage);
fStorageUsed -= rec->fStorageSize;
fNumObjects--;
}
private:
struct Rec {
size_t fStorageSize; // 0 if allocated on heap
void* fObj;
void* fHeapStorage;
void (*fKillProc)(void*);
};
// Used to call the destructor for allocated objects.
template<typename T>
static void DestroyT(void* ptr) {
static_cast<T*>(ptr)->~T();
}
struct SK_STRUCT_ALIGN(16) Storage {
// we add kMaxObjects * 15 to account for the worst-case slop, where each allocation wasted
// 15 bytes (due to forcing each to be 16-byte aligned)
char fBytes[kTotalBytes + kMaxObjects * 15];
};
Storage fStorage;
// Number of bytes used so far.
size_t fStorageUsed;
uint32_t fNumObjects;
Rec fRecs[kMaxObjects];
};
#endif // SkSmallAllocator_DEFINED
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