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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 SkArenaAlloc_DEFINED
#define SkArenaAlloc_DEFINED
#include "include/private/SkTFitsIn.h"
#include <array>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <cstdlib>
#include <cstring>
#include <limits>
#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 firstHeapAllocation bytes. If your data (plus a small overhead)
// fits in the user-provided block, SkArenaAlloc never uses the heap, and if it fits in
// firstHeapAllocation bytes, it'll use the heap only once. If 0 is specified for
// firstHeapAllocation, then blockSize is used unless that too is 0, then 1024 is used.
// Examples:
// char block[mostCasesSize];
// SkArenaAlloc arena(block, mostCasesSize);
// 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 pattern.
// 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, mostCasesSize);
// };
// 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 per 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 {
SkArenaAlloc(char* block, size_t blockSize, size_t firstHeapAllocation);
explicit SkArenaAlloc(size_t firstHeapAllocation)
: SkArenaAlloc(nullptr, 0, firstHeapAllocation)
template <typename T, typename... Args>
T* make(Args&&... args) {
uint32_t size = ToU32(sizeof(T));
uint32_t alignment = ToU32(alignof(T));
char* objStart;
if (std::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 = ToU32(objStart - fCursor);
// Advance to end of object to install footer.
fCursor = objStart + size;
FooterAction* releaser = [](char* objEnd) {
char* objStart = objEnd - (sizeof(T) + sizeof(Footer));
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>
T* makeArrayDefault(size_t count) {
T* array = this->allocUninitializedArray<T>(count);
for (size_t i = 0; i < count; i++) {
// Default initialization: if T is primitive then the value is left uninitialized.
new (&array[i]) T;
return array;
template <typename T>
T* makeArray(size_t count) {
T* array = this->allocUninitializedArray<T>(count);
for (size_t i = 0; i < count; i++) {
// Value initialization: if T is primitive then the value is zero-initialized.
new (&array[i]) T();
return array;
template <typename T, typename Initializer>
T* makeInitializedArray(size_t count, Initializer initializer) {
T* array = this->allocUninitializedArray<T>(count);
for (size_t i = 0; i < count; i++) {
new (&array[i]) T(initializer(i));
return array;
// Only use makeBytesAlignedTo if none of the typed variants are impractical to use.
void* makeBytesAlignedTo(size_t size, size_t align) {
auto objStart = this->allocObject(ToU32(size), ToU32(align));
fCursor = objStart + size;
return objStart;
// Destroy all allocated objects, free any heap allocations.
void reset();
static void AssertRelease(bool cond) { if (!cond) { ::abort(); } }
static uint32_t ToU32(size_t v) {
return (uint32_t)v;
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) {
uintptr_t mask = alignment - 1;
uintptr_t alignedOffset = (~reinterpret_cast<uintptr_t>(fCursor) + 1) & mask;
uintptr_t totalSize = size + alignedOffset;
AssertRelease(totalSize >= size);
if (totalSize > static_cast<uintptr_t>(fEnd - fCursor)) {
this->ensureSpace(size, alignment);
alignedOffset = (~reinterpret_cast<uintptr_t>(fCursor) + 1) & mask;
return fCursor + alignedOffset;
char* allocObjectWithFooter(uint32_t sizeIncludingFooter, uint32_t alignment);
template <typename T>
T* allocUninitializedArray(size_t countZ) {
uint32_t count = ToU32(countZ);
char* objStart;
AssertRelease(count <= std::numeric_limits<uint32_t>::max() / sizeof(T));
uint32_t arraySize = ToU32(count * sizeof(T));
uint32_t alignment = ToU32(alignof(T));
if (std::is_trivially_destructible<T>::value) {
objStart = this->allocObject(arraySize, alignment);
fCursor = objStart + arraySize;
} else {
constexpr uint32_t overhead = sizeof(Footer) + sizeof(uint32_t);
AssertRelease(arraySize <= std::numeric_limits<uint32_t>::max() - overhead);
uint32_t totalSize = arraySize + overhead;
objStart = this->allocObjectWithFooter(totalSize, alignment);
// Can never be UB because max value is alignof(T).
uint32_t padding = ToU32(objStart - fCursor);
// Advance to end of array to install footer.?
fCursor = objStart + arraySize;
[](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++) {
return objStart;
return (T*)objStart;
char* fDtorCursor;
char* fCursor;
char* fEnd;
char* const fFirstBlock;
const uint32_t fFirstSize;
const uint32_t fFirstHeapAllocationSize;
// Use the Fibonacci sequence as the growth factor for block size. The size of the block
// allocated is fFib0 * fFirstHeapAllocationSize. Using 2 ^ n * fFirstHeapAllocationSize
// had too much slop for Android.
uint32_t fFib0 {1}, fFib1 {1};
// Helper for defining allocators with inline/reserved storage.
// For argument declarations, stick to the base type (SkArenaAlloc).
// Note: Inheriting from the storage first means the storage will outlive the
// SkArenaAlloc, letting ~SkArenaAlloc read it as it calls destructors.
// (This is mostly only relevant for strict tools like MSAN.)
template <size_t InlineStorageSize>
class SkSTArenaAlloc : private std::array<char, InlineStorageSize>, public SkArenaAlloc {
explicit SkSTArenaAlloc(size_t firstHeapAllocation = InlineStorageSize)
: SkArenaAlloc{this->data(), this->size(), firstHeapAllocation} {}
#endif // SkArenaAlloc_DEFINED