src/tint/utils/memory/block_allocator.h

// Copyright 2021 The Dawn & Tint Authors
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#ifndef SRC_TINT_UTILS_MEMORY_BLOCK_ALLOCATOR_H_
#define SRC_TINT_UTILS_MEMORY_BLOCK_ALLOCATOR_H_

#include <array>
#include <cstring>
#include <utility>

#include "src/tint/utils/macros/compiler.h"
#include "src/tint/utils/math/math.h"
#include "src/tint/utils/memory/bitcast.h"

// This file implements a custom allocator & iterator using C-style data access. It is not
// unexpected that -Wunsafe-buffer-usage triggers in this code, since the type of dynamic access
// being used cannot be guaranteed to be safe via static analysis. Attempting to change this code in
// simple ways to quiet these errors either a) negatively affects the performance by introducing
// unneeded copes, or b) uses typing shenanigans to work around the warning that other
// linters/analyses are unhappy with.
TINT_BEGIN_DISABLE_WARNING(UNSAFE_BUFFER_USAGE);

namespace tint {

/// A container and allocator of objects of (or deriving from) the template type `T`.
/// Objects are allocated by calling Create(), and are owned by the BlockAllocator.
/// When the BlockAllocator is destructed, all constructed objects are automatically destructed and
/// freed.
///
/// Objects held by the BlockAllocator can be iterated over using a View.
template <typename T, size_t BLOCK_SIZE = 64 * 1024, size_t BLOCK_ALIGNMENT = 16>
class BlockAllocator {
    /// Pointers is a chunk of T* pointers, forming a linked list.
    /// The list of Pointers are used to maintain the list of allocated objects.
    /// Pointers are allocated out of the block memory.
    struct Pointers {
        static constexpr size_t kMax = 32;
        std::array<T*, kMax> ptrs;
        Pointers* next;
        Pointers* prev;
        size_t count;
    };

    /// Block is linked list of memory blocks.
    /// Blocks are allocated out of heap memory.
    ///
    /// Note: We're not using std::aligned_storage here as this warns / errors on MSVC.
    struct alignas(BLOCK_ALIGNMENT) Block {
        uint8_t data[BLOCK_SIZE];
        Block* next = nullptr;
    };

    // Forward declaration
    template <bool IS_CONST>
    class TView;

    /// An iterator for the objects owned by the BlockAllocator.
    template <bool IS_CONST>
    class TIterator {
        using PointerTy = std::conditional_t<IS_CONST, const T*, T*>;

      public:
        /// Equality operator
        /// @param other the iterator to compare this iterator to
        /// @returns true if this iterator is equal to other
        bool operator==(const TIterator& other) const {
            return ptrs == other.ptrs && idx == other.idx;
        }

        /// Inequality operator
        /// @param other the iterator to compare this iterator to
        /// @returns true if this iterator is not equal to other
        bool operator!=(const TIterator& other) const { return !(*this == other); }

        /// Progress the iterator forward one element
        /// @returns this iterator
        TIterator& operator++() {
            if (ptrs != nullptr) {
                ++idx;
                if (idx >= ptrs->count) {
                    idx = 0;
                    ptrs = ptrs->next;
                }
            }
            return *this;
        }

        /// Progress the iterator backwards one element
        /// @returns this iterator
        TIterator& operator--() {
            if (ptrs != nullptr) {
                if (idx == 0) {
                    ptrs = ptrs->prev;
                    idx = ptrs->count - 1;
                }
                --idx;
            }
            return *this;
        }

        /// @returns the pointer to the object at the current iterator position
        PointerTy operator*() const { return ptrs->ptrs[idx]; }

      private:
        friend TView<IS_CONST>;  // Keep internal iterator impl private.
        explicit TIterator(const Pointers* p, size_t i) : ptrs(p), idx(i) {}

        /// The current Pointers
        const Pointers* ptrs = nullptr;
        /// The current index within #ptrs
        size_t idx = 0;
    };

    /// View provides begin() and end() methods for looping over the objects owned by the
    /// BlockAllocator.
    template <bool IS_CONST>
    class TView {
      public:
        /// The iterator type
        using iterator = TIterator<IS_CONST>;
        /// The const iterator type
        using const_iterator = TIterator<true>;

        /// @returns an iterator to the beginning of the view
        iterator begin() const { return iterator{allocator_->data.pointers.root, 0}; }

        /// @returns an iterator to the end of the view
        iterator end() const { return iterator{nullptr, 0}; }

      private:
        friend BlockAllocator;  // For BlockAllocator::operator View()
        explicit TView(BlockAllocator const* allocator) : allocator_(allocator) {}
        BlockAllocator const* const allocator_;
    };

  public:
    /// A forward-iterator type over the objects of the BlockAllocator
    using Iterator = TIterator</* const */ false>;

    /// An immutable forward-iterator type over the objects of the BlockAllocator
    using ConstIterator = TIterator</* const */ true>;

    /// View provides begin() and end() methods for looping over the objects owned by the
    /// BlockAllocator.
    using View = TView<false>;

    /// ConstView provides begin() and end() methods for looping over the objects owned by the
    /// BlockAllocator.
    using ConstView = TView<true>;

    /// Constructor
    BlockAllocator() = default;

    /// Move constructor
    /// @param rhs the BlockAllocator to move
    BlockAllocator(BlockAllocator&& rhs) { std::swap(data, rhs.data); }

    /// Move assignment operator
    /// @param rhs the BlockAllocator to move
    /// @return this BlockAllocator
    BlockAllocator& operator=(BlockAllocator&& rhs) {
        if (this != &rhs) {
            Reset();
            std::swap(data, rhs.data);
        }
        return *this;
    }

    /// Destructor
    ~BlockAllocator() { Reset(); }

    /// @return a View of all objects owned by this BlockAllocator
    View Objects() { return View(this); }

    /// @return a ConstView of all objects owned by this BlockAllocator
    ConstView Objects() const { return ConstView(this); }

    /// Creates a new `TYPE` owned by the BlockAllocator.
    /// When the BlockAllocator is destructed the object will be destructed and freed.
    /// @param args the arguments to pass to the constructor
    /// @returns the pointer to the constructed object
    template <typename TYPE = T, typename... ARGS>
    TYPE* Create(ARGS&&... args) {
        static_assert(std::is_same<T, TYPE>::value || std::is_base_of<T, TYPE>::value,
                      "TYPE does not derive from T");
        static_assert(std::is_same<T, TYPE>::value || std::has_virtual_destructor<T>::value,
                      "TYPE requires a virtual destructor when calling Create() for a type "
                      "that is not T");

        auto* ptr = Allocate<TYPE>();
        new (ptr) TYPE(std::forward<ARGS>(args)...);
        AddObjectPointer(ptr);
        data.count++;

        return ptr;
    }

    /// Frees all allocations from the allocator.
    void Reset() {
        for (auto ptr : Objects()) {
            ptr->~T();
        }
        auto* block = data.block.root;
        while (block != nullptr) {
            auto* next = block->next;
            delete block;
            block = next;
        }
        data = {};
    }

    /// @returns the total number of allocated objects.
    size_t Count() const { return data.count; }

  private:
    BlockAllocator(const BlockAllocator&) = delete;
    BlockAllocator& operator=(const BlockAllocator&) = delete;

    /// Allocates an instance of TYPE from the current block, or from a newly allocated block if the
    /// current block is full.
    template <typename TYPE>
    TYPE* Allocate() {
        static_assert(sizeof(TYPE) <= BLOCK_SIZE,
                      "Cannot construct TYPE with size greater than BLOCK_SIZE");
        static_assert(alignof(TYPE) <= BLOCK_ALIGNMENT, "alignof(TYPE) is greater than ALIGNMENT");

        auto& block = data.block;

        block.current_offset = tint::RoundUp(alignof(TYPE), block.current_offset);
        if (block.current_offset + sizeof(TYPE) > BLOCK_SIZE) {
            // Allocate a new block from the heap
            auto* prev_block = block.current;
            block.current = new Block;
            if (!block.current) {
                return nullptr;  // out of memory
            }
            block.current->next = nullptr;
            block.current_offset = 0;
            if (prev_block) {
                prev_block->next = block.current;
            } else {
                block.root = block.current;
            }
        }

        auto* base = &block.current->data[0];
        auto* ptr = tint::Bitcast<TYPE*>(base + block.current_offset);
        block.current_offset += sizeof(TYPE);
        return ptr;
    }

    /// Adds `ptr` to the linked list of objects owned by this BlockAllocator.
    /// Once added, `ptr` will be tracked for destruction when the BlockAllocator is destructed.
    void AddObjectPointer(T* ptr) {
        auto& pointers = data.pointers;

        if (!pointers.current || pointers.current->count == Pointers::kMax) {
            auto* prev_pointers = pointers.current;
            pointers.current = Allocate<Pointers>();
            if (!pointers.current) {
                return;  // out of memory
            }
            pointers.current->next = nullptr;
            pointers.current->prev = prev_pointers;
            pointers.current->count = 0;

            if (prev_pointers) {
                prev_pointers->next = pointers.current;
            } else {
                pointers.root = pointers.current;
            }
        }

        pointers.current->ptrs[pointers.current->count++] = ptr;
    }

    struct {
        struct {
            /// The root block of the block linked list
            Block* root = nullptr;
            /// The current (end) block of the blocked linked list.
            /// New allocations come from this block
            Block* current = nullptr;
            /// The byte offset in #current for the next allocation.
            /// Initialized with BLOCK_SIZE so that the first allocation triggers a block
            /// allocation.
            size_t current_offset = BLOCK_SIZE;
        } block;

        struct {
            /// The root Pointers structure of the pointers linked list
            Pointers* root = nullptr;
            /// The current (end) Pointers structure of the pointers linked list.
            /// AddObjectPointer() adds to this structure.
            Pointers* current = nullptr;
        } pointers;

        size_t count = 0;
    } data;
};

}  // namespace tint

TINT_END_DISABLE_WARNING(UNSAFE_BUFFER_USAGE);

#endif  // SRC_TINT_UTILS_MEMORY_BLOCK_ALLOCATOR_H_