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/*
* Copyright 2016 Google Inc.
*
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
*/
#ifndef SkFixedAlloc_DEFINED
#define SkFixedAlloc_DEFINED
#include "SkRefCnt.h"
#include "SkTFitsIn.h"
#include "SkTypes.h"
#include <cstddef>
#include <new>
#include <type_traits>
#include <utility>
#include <vector>
// SkArenaAlloc allocates object and destroys the allocated objects when destroyed. It's designed
// to minimize the number of underlying block allocations. SkArenaAlloc allocates first out of an
// (optional) user-provided block of memory, and when that's exhausted it allocates on the heap,
// starting with an allocation of extraSize bytes. If your data (plus a small overhead) fits in
// the user-provided block, SkArenaAlloc never uses the heap, and if it fits in extraSize bytes,
// it'll use the heap only once. If you pass extraSize = 0, it allocates blocks for each call to
// make<T>.
//
// Examples:
//
// char block[mostCasesSize];
// SkArenaAlloc arena(block, almostAllCasesSize);
//
// If mostCasesSize is too large for the stack, you can use the following pattern.
//
// std::unique_ptr<char[]> block{new char[mostCasesSize]};
// SkArenaAlloc arena(block.get(), mostCasesSize, almostAllCasesSize);
//
// If the program only sometimes allocates memory, use the following.
//
// SkArenaAlloc arena(nullptr, 0, almostAllCasesSize);
//
// The storage does not necessarily need to be on the stack. Embedding the storage in a class also
// works.
//
// class Foo {
// char storage[mostCasesSize];
// SkArenaAlloc arena (storage, almostAllCasesSize);
// };
//
// In addition, the system is optimized to handle POD data including arrays of PODs (where
// POD is really data with no destructors). For POD data it has zero overhead per item, and a
// typical block overhead of 8 bytes. For non-POD objects there is a per item overhead of 4 bytes.
// For arrays of non-POD objects there is a per array overhead of typically 8 bytes. There is an
// addition overhead when switching from POD data to non-POD data of typically 8 bytes.
//
// If additional blocks are needed they are increased exponentially. This strategy bounds the
// recursion of the RunDtorsOnBlock to be limited to O(log size-of-memory). Block size grow using
// the Fibonacci sequence which means that for 2^32 memory there are 48 allocations, and for 2^48
// there are 71 allocations.
class SkArenaAlloc {
public:
SkArenaAlloc(char* block, size_t size, size_t extraSize = 0);
template <size_t kSize>
SkArenaAlloc(char (&block)[kSize], size_t extraSize = kSize)
: SkArenaAlloc(block, kSize, extraSize)
{}
SkArenaAlloc(size_t extraSize)
: SkArenaAlloc(nullptr, 0, extraSize)
{}
~SkArenaAlloc();
template <typename T, typename... Args>
T* make(Args&&... args) {
uint32_t size = SkTo<uint32_t>(sizeof(T));
uint32_t alignment = SkTo<uint32_t>(alignof(T));
char* objStart;
if (skstd::is_trivially_destructible<T>::value) {
objStart = this->allocObject(size, alignment);
fCursor = objStart + size;
} else {
objStart = this->allocObjectWithFooter(size + sizeof(Footer), alignment);
// Can never be UB because max value is alignof(T).
uint32_t padding = SkTo<uint32_t>(objStart - fCursor);
// Advance to end of object to install footer.
fCursor = objStart + size;
FooterAction* releaser = [](char* objEnd) {
char* objStart = objEnd - (sizeof(T) + sizeof(Footer));
((T*)objStart)->~T();
return objStart;
};
this->installFooter(releaser, padding);
}
// This must be last to make objects with nested use of this allocator work.
return new(objStart) T(std::forward<Args>(args)...);
}
template <typename T, typename... Args>
sk_sp<T> makeSkSp(Args&&... args) {
SkASSERT(SkTFitsIn<uint32_t>(sizeof(T)));
// The arena takes a ref for itself to account for the destructor. The sk_sp count can't
// become zero or the sk_sp will try to call free on the pointer.
return sk_sp<T>(SkRef(this->make<T>(std::forward<Args>(args)...)));
}
template <typename T>
T* makeArrayDefault(size_t count) {
uint32_t safeCount = SkTo<uint32_t>(count);
T* array = (T*)this->commonArrayAlloc<T>(safeCount);
// If T is primitive then no initialization takes place.
for (size_t i = 0; i < safeCount; i++) {
new (&array[i]) T;
}
return array;
}
template <typename T>
T* makeArray(size_t count) {
uint32_t safeCount = SkTo<uint32_t>(count);
T* array = (T*)this->commonArrayAlloc<T>(safeCount);
// If T is primitive then the memory is initialized. For example, an array of chars will
// be zeroed.
for (size_t i = 0; i < safeCount; i++) {
new (&array[i]) T();
}
return array;
}
// Destroy all allocated objects, free any heap allocations.
void reset();
private:
using Footer = int64_t;
using FooterAction = char* (char*);
static char* SkipPod(char* footerEnd);
static void RunDtorsOnBlock(char* footerEnd);
static char* NextBlock(char* footerEnd);
void installFooter(FooterAction* releaser, uint32_t padding);
void installUint32Footer(FooterAction* action, uint32_t value, uint32_t padding);
void installPtrFooter(FooterAction* action, char* ptr, uint32_t padding);
void ensureSpace(uint32_t size, uint32_t alignment);
char* allocObject(uint32_t size, uint32_t alignment);
char* allocObjectWithFooter(uint32_t sizeIncludingFooter, uint32_t alignment);
template <typename T>
char* commonArrayAlloc(uint32_t count) {
char* objStart;
uint32_t arraySize = SkTo<uint32_t>(count * sizeof(T));
uint32_t alignment = SkTo<uint32_t>(alignof(T));
if (skstd::is_trivially_destructible<T>::value) {
objStart = this->allocObject(arraySize, alignment);
fCursor = objStart + arraySize;
} else {
uint32_t totalSize = arraySize + sizeof(Footer) + sizeof(uint32_t);
objStart = this->allocObjectWithFooter(totalSize, alignment);
// Can never be UB because max value is alignof(T).
uint32_t padding = SkTo<uint32_t>(objStart - fCursor);
// Advance to end of array to install footer.?
fCursor = objStart + arraySize;
this->installUint32Footer(
[](char* footerEnd) {
char* objEnd = footerEnd - (sizeof(Footer) + sizeof(uint32_t));
uint32_t count;
memmove(&count, objEnd, sizeof(uint32_t));
char* objStart = objEnd - count * sizeof(T);
T* array = (T*) objStart;
for (uint32_t i = 0; i < count; i++) {
array[i].~T();
}
return objStart;
},
SkTo<uint32_t>(count),
padding);
}
return objStart;
}
char* fDtorCursor;
char* fCursor;
char* fEnd;
char* const fFirstBlock;
const uint32_t fFirstSize;
const uint32_t fExtraSize;
// Use the Fibonacci sequence as the growth factor for block size. The size of the block
// allocated is fFib0 * fExtraSize. Using 2 ^ n * fExtraSize had too much slop for Android.
uint32_t fFib0 {1}, fFib1 {1};
};
#endif//SkFixedAlloc_DEFINED
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