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// Copyright 2020 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef INCLUDE_CPPGC_ALLOCATION_H_
#define INCLUDE_CPPGC_ALLOCATION_H_
#include <atomic>
#include <cstddef>
#include <cstdint>
#include <new>
#include <type_traits>
#include <utility>
#include "cppgc/custom-space.h"
#include "cppgc/internal/api-constants.h"
#include "cppgc/internal/gc-info.h"
#include "cppgc/type-traits.h"
#include "v8config.h" // NOLINT(build/include_directory)
namespace cppgc {
/**
* AllocationHandle is used to allocate garbage-collected objects.
*/
class AllocationHandle;
namespace internal {
class V8_EXPORT MakeGarbageCollectedTraitInternal {
protected:
static inline void MarkObjectAsFullyConstructed(const void* payload) {
// See api_constants for an explanation of the constants.
std::atomic<uint16_t>* atomic_mutable_bitfield =
reinterpret_cast<std::atomic<uint16_t>*>(
const_cast<uint16_t*>(reinterpret_cast<const uint16_t*>(
reinterpret_cast<const uint8_t*>(payload) -
api_constants::kFullyConstructedBitFieldOffsetFromPayload)));
// It's safe to split use load+store here (instead of a read-modify-write
// operation), since it's guaranteed that this 16-bit bitfield is only
// modified by a single thread. This is cheaper in terms of code bloat (on
// ARM) and performance.
uint16_t value = atomic_mutable_bitfield->load(std::memory_order_relaxed);
value |= api_constants::kFullyConstructedBitMask;
atomic_mutable_bitfield->store(value, std::memory_order_release);
}
template <typename U, typename CustomSpace>
struct SpacePolicy {
static void* Allocate(AllocationHandle& handle, size_t size) {
// Custom space.
static_assert(std::is_base_of<CustomSpaceBase, CustomSpace>::value,
"Custom space must inherit from CustomSpaceBase.");
return MakeGarbageCollectedTraitInternal::Allocate(
handle, size, internal::GCInfoTrait<U>::Index(),
CustomSpace::kSpaceIndex);
}
};
template <typename U>
struct SpacePolicy<U, void> {
static void* Allocate(AllocationHandle& handle, size_t size) {
// Default space.
return MakeGarbageCollectedTraitInternal::Allocate(
handle, size, internal::GCInfoTrait<U>::Index());
}
};
private:
static void* Allocate(cppgc::AllocationHandle& handle, size_t size,
GCInfoIndex index);
static void* Allocate(cppgc::AllocationHandle& handle, size_t size,
GCInfoIndex index, CustomSpaceIndex space_index);
friend class HeapObjectHeader;
};
} // namespace internal
/**
* Base trait that provides utilities for advancers users that have custom
* allocation needs (e.g., overriding size). It's expected that users override
* MakeGarbageCollectedTrait (see below) and inherit from
* MakeGarbageCollectedTraitBase and make use of the low-level primitives
* offered to allocate and construct an object.
*/
template <typename T>
class MakeGarbageCollectedTraitBase
: private internal::MakeGarbageCollectedTraitInternal {
private:
static_assert(internal::IsGarbageCollectedType<T>::value,
"T needs to be a garbage collected object");
static_assert(!IsGarbageCollectedWithMixinTypeV<T> ||
sizeof(T) <=
internal::api_constants::kLargeObjectSizeThreshold,
"GarbageCollectedMixin may not be a large object");
protected:
/**
* Allocates memory for an object of type T.
*
* \param handle AllocationHandle identifying the heap to allocate the object
* on.
* \param size The size that should be reserved for the object.
* \returns the memory to construct an object of type T on.
*/
V8_INLINE static void* Allocate(AllocationHandle& handle, size_t size) {
static_assert(
std::is_base_of<typename T::ParentMostGarbageCollectedType, T>::value,
"U of GarbageCollected<U> must be a base of T. Check "
"GarbageCollected<T> base class inheritance.");
return SpacePolicy<
typename internal::GCInfoFolding<
T, typename T::ParentMostGarbageCollectedType>::ResultType,
typename SpaceTrait<T>::Space>::Allocate(handle, size);
}
/**
* Marks an object as fully constructed, resulting in precise handling by the
* garbage collector.
*
* \param payload The base pointer the object is allocated at.
*/
V8_INLINE static void MarkObjectAsFullyConstructed(const void* payload) {
internal::MakeGarbageCollectedTraitInternal::MarkObjectAsFullyConstructed(
payload);
}
};
/**
* Passed to MakeGarbageCollected to specify how many bytes should be appended
* to the allocated object.
*
* Example:
* \code
* class InlinedArray final : public GarbageCollected<InlinedArray> {
* public:
* explicit InlinedArray(size_t bytes) : size(bytes), byte_array(this + 1) {}
* void Trace(Visitor*) const {}
* size_t size;
* char* byte_array;
* };
*
* auto* inlined_array = MakeGarbageCollected<InlinedArray(
* GetAllocationHandle(), AdditionalBytes(4), 4);
* for (size_t i = 0; i < 4; i++) {
* Process(inlined_array->byte_array[i]);
* }
* \endcode
*/
struct AdditionalBytes {
constexpr explicit AdditionalBytes(size_t bytes) : value(bytes) {}
const size_t value;
};
/**
* Default trait class that specifies how to construct an object of type T.
* Advanced users may override how an object is constructed using the utilities
* that are provided through MakeGarbageCollectedTraitBase.
*
* Any trait overriding construction must
* - allocate through `MakeGarbageCollectedTraitBase<T>::Allocate`;
* - mark the object as fully constructed using
* `MakeGarbageCollectedTraitBase<T>::MarkObjectAsFullyConstructed`;
*/
template <typename T>
class MakeGarbageCollectedTrait : public MakeGarbageCollectedTraitBase<T> {
public:
template <typename... Args>
static T* Call(AllocationHandle& handle, Args&&... args) {
void* memory =
MakeGarbageCollectedTraitBase<T>::Allocate(handle, sizeof(T));
T* object = ::new (memory) T(std::forward<Args>(args)...);
MakeGarbageCollectedTraitBase<T>::MarkObjectAsFullyConstructed(object);
return object;
}
template <typename... Args>
static T* Call(AllocationHandle& handle, AdditionalBytes additional_bytes,
Args&&... args) {
void* memory = MakeGarbageCollectedTraitBase<T>::Allocate(
handle, sizeof(T) + additional_bytes.value);
T* object = ::new (memory) T(std::forward<Args>(args)...);
MakeGarbageCollectedTraitBase<T>::MarkObjectAsFullyConstructed(object);
return object;
}
};
/**
* Allows users to specify a post-construction callback for specific types. The
* callback is invoked on the instance of type T right after it has been
* constructed. This can be useful when the callback requires a
* fully-constructed object to be able to dispatch to virtual methods.
*/
template <typename T, typename = void>
struct PostConstructionCallbackTrait {
static void Call(T*) {}
};
/**
* Constructs a managed object of type T where T transitively inherits from
* GarbageCollected.
*
* \param args List of arguments with which an instance of T will be
* constructed.
* \returns an instance of type T.
*/
template <typename T, typename... Args>
V8_INLINE T* MakeGarbageCollected(AllocationHandle& handle, Args&&... args) {
T* object =
MakeGarbageCollectedTrait<T>::Call(handle, std::forward<Args>(args)...);
PostConstructionCallbackTrait<T>::Call(object);
return object;
}
/**
* Constructs a managed object of type T where T transitively inherits from
* GarbageCollected. Created objects will have additional bytes appended to
* it. Allocated memory would suffice for `sizeof(T) + additional_bytes`.
*
* \param additional_bytes Denotes how many bytes to append to T.
* \param args List of arguments with which an instance of T will be
* constructed.
* \returns an instance of type T.
*/
template <typename T, typename... Args>
V8_INLINE T* MakeGarbageCollected(AllocationHandle& handle,
AdditionalBytes additional_bytes,
Args&&... args) {
T* object = MakeGarbageCollectedTrait<T>::Call(handle, additional_bytes,
std::forward<Args>(args)...);
PostConstructionCallbackTrait<T>::Call(object);
return object;
}
} // namespace cppgc
#endif // INCLUDE_CPPGC_ALLOCATION_H_
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