src/common/StackContainer.h

// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// This file is a modified copy of Chromium's /src/base/containers/stack_container.h

#ifndef COMMON_STACKCONTAINER_H_
#define COMMON_STACKCONTAINER_H_

#include "common/Compiler.h"

#include <cstddef>
#include <vector>

// This allocator can be used with STL containers to provide a stack buffer
// from which to allocate memory and overflows onto the heap. This stack buffer
// would be allocated on the stack and allows us to avoid heap operations in
// some situations.
//
// STL likes to make copies of allocators, so the allocator itself can't hold
// the data. Instead, we make the creator responsible for creating a
// StackAllocator::Source which contains the data. Copying the allocator
// merely copies the pointer to this shared source, so all allocators created
// based on our allocator will share the same stack buffer.
//
// This stack buffer implementation is very simple. The first allocation that
// fits in the stack buffer will use the stack buffer. Any subsequent
// allocations will not use the stack buffer, even if there is unused room.
// This makes it appropriate for array-like containers, but the caller should
// be sure to reserve() in the container up to the stack buffer size. Otherwise
// the container will allocate a small array which will "use up" the stack
// buffer.
template <typename T, size_t stack_capacity>
class StackAllocator : public std::allocator<T> {
  public:
    typedef typename std::allocator<T>::pointer pointer;
    typedef typename std::allocator<T>::size_type size_type;

    // Backing store for the allocator. The container owner is responsible for
    // maintaining this for as long as any containers using this allocator are
    // live.
    struct Source {
        Source() : used_stack_buffer_(false) {
        }

        // Casts the buffer in its right type.
        T* stack_buffer() {
            return reinterpret_cast<T*>(stack_buffer_);
        }
        const T* stack_buffer() const {
            return reinterpret_cast<const T*>(&stack_buffer_);
        }

        // The buffer itself. It is not of type T because we don't want the
        // constructors and destructors to be automatically called. Define a POD
        // buffer of the right size instead.
        alignas(T) char stack_buffer_[sizeof(T[stack_capacity])];
#if defined(DAWN_COMPILER_GCC) && !defined(__x86_64__) && !defined(__i386__)
        static_assert(alignof(T) <= 16, "http://crbug.com/115612");
#endif

        // Set when the stack buffer is used for an allocation. We do not track
        // how much of the buffer is used, only that somebody is using it.
        bool used_stack_buffer_;
    };

    // Used by containers when they want to refer to an allocator of type U.
    template <typename U>
    struct rebind {
        typedef StackAllocator<U, stack_capacity> other;
    };

    // For the straight up copy c-tor, we can share storage.
    StackAllocator(const StackAllocator<T, stack_capacity>& rhs)
        : std::allocator<T>(), source_(rhs.source_) {
    }

    // ISO C++ requires the following constructor to be defined,
    // and std::vector in VC++2008SP1 Release fails with an error
    // in the class _Container_base_aux_alloc_real (from <xutility>)
    // if the constructor does not exist.
    // For this constructor, we cannot share storage; there's
    // no guarantee that the Source buffer of Ts is large enough
    // for Us.
    // TODO: If we were fancy pants, perhaps we could share storage
    // iff sizeof(T) == sizeof(U).
    template <typename U, size_t other_capacity>
    StackAllocator(const StackAllocator<U, other_capacity>& other) : source_(nullptr) {
    }

    // This constructor must exist. It creates a default allocator that doesn't
    // actually have a stack buffer. glibc's std::string() will compare the
    // current allocator against the default-constructed allocator, so this
    // should be fast.
    StackAllocator() : source_(nullptr) {
    }

    explicit StackAllocator(Source* source) : source_(source) {
    }

    // Actually do the allocation. Use the stack buffer if nobody has used it yet
    // and the size requested fits. Otherwise, fall through to the standard
    // allocator.
    pointer allocate(size_type n) {
        if (source_ && !source_->used_stack_buffer_ && n <= stack_capacity) {
            source_->used_stack_buffer_ = true;
            return source_->stack_buffer();
        } else {
            return std::allocator<T>::allocate(n);
        }
    }

    // Free: when trying to free the stack buffer, just mark it as free. For
    // non-stack-buffer pointers, just fall though to the standard allocator.
    void deallocate(pointer p, size_type n) {
        if (source_ && p == source_->stack_buffer())
            source_->used_stack_buffer_ = false;
        else
            std::allocator<T>::deallocate(p, n);
    }

  private:
    Source* source_;
};

// A wrapper around STL containers that maintains a stack-sized buffer that the
// initial capacity of the vector is based on. Growing the container beyond the
// stack capacity will transparently overflow onto the heap. The container must
// support reserve().
//
// This will not work with std::string since some implementations allocate
// more bytes than requested in calls to reserve(), forcing the allocation onto
// the heap.  http://crbug.com/709273
//
// WATCH OUT: the ContainerType MUST use the proper StackAllocator for this
// type. This object is really intended to be used only internally. You'll want
// to use the wrappers below for different types.
template <typename TContainerType, size_t stack_capacity>
class StackContainer {
  public:
    typedef TContainerType ContainerType;
    typedef typename ContainerType::value_type ContainedType;
    typedef StackAllocator<ContainedType, stack_capacity> Allocator;

    // Allocator must be constructed before the container!
    StackContainer() : allocator_(&stack_data_), container_(allocator_) {
        // Make the container use the stack allocation by reserving our buffer size
        // before doing anything else.
        container_.reserve(stack_capacity);
    }

    // Getters for the actual container.
    //
    // Danger: any copies of this made using the copy constructor must have
    // shorter lifetimes than the source. The copy will share the same allocator
    // and therefore the same stack buffer as the original. Use std::copy to
    // copy into a "real" container for longer-lived objects.
    ContainerType& container() {
        return container_;
    }
    const ContainerType& container() const {
        return container_;
    }

    // Support operator-> to get to the container. This allows nicer syntax like:
    //   StackContainer<...> foo;
    //   std::sort(foo->begin(), foo->end());
    ContainerType* operator->() {
        return &container_;
    }
    const ContainerType* operator->() const {
        return &container_;
    }

    // Retrieves the stack source so that that unit tests can verify that the
    // buffer is being used properly.
    const typename Allocator::Source& stack_data() const {
        return stack_data_;
    }

  protected:
    typename Allocator::Source stack_data_;
    Allocator allocator_;
    ContainerType container_;

  private:
    StackContainer(const StackContainer& rhs) = delete;
    StackContainer& operator=(const StackContainer& rhs) = delete;
    StackContainer(StackContainer&& rhs) = delete;
    StackContainer& operator=(StackContainer&& rhs) = delete;
};

// Range-based iteration support for StackContainer.
template <typename TContainerType, size_t stack_capacity>
auto begin(const StackContainer<TContainerType, stack_capacity>& stack_container)
    -> decltype(begin(stack_container.container())) {
    return begin(stack_container.container());
}

template <typename TContainerType, size_t stack_capacity>
auto begin(StackContainer<TContainerType, stack_capacity>& stack_container)
    -> decltype(begin(stack_container.container())) {
    return begin(stack_container.container());
}

template <typename TContainerType, size_t stack_capacity>
auto end(StackContainer<TContainerType, stack_capacity>& stack_container)
    -> decltype(end(stack_container.container())) {
    return end(stack_container.container());
}

template <typename TContainerType, size_t stack_capacity>
auto end(const StackContainer<TContainerType, stack_capacity>& stack_container)
    -> decltype(end(stack_container.container())) {
    return end(stack_container.container());
}

// StackVector -----------------------------------------------------------------

// Example:
//   StackVector<int, 16> foo;
//   foo->push_back(22);  // we have overloaded operator->
//   foo[0] = 10;         // as well as operator[]
template <typename T, size_t stack_capacity>
class StackVector
    : public StackContainer<std::vector<T, StackAllocator<T, stack_capacity>>, stack_capacity> {
  public:
    StackVector()
        : StackContainer<std::vector<T, StackAllocator<T, stack_capacity>>, stack_capacity>() {
    }

    // We need to put this in STL containers sometimes, which requires a copy
    // constructor. We can't call the regular copy constructor because that will
    // take the stack buffer from the original. Here, we create an empty object
    // and make a stack buffer of its own.
    StackVector(const StackVector<T, stack_capacity>& other)
        : StackContainer<std::vector<T, StackAllocator<T, stack_capacity>>, stack_capacity>() {
        this->container().assign(other->begin(), other->end());
    }

    StackVector<T, stack_capacity>& operator=(const StackVector<T, stack_capacity>& other) {
        this->container().assign(other->begin(), other->end());
        return *this;
    }

    // Vectors are commonly indexed, which isn't very convenient even with
    // operator-> (using "->at()" does exception stuff we don't want).
    T& operator[](size_t i) {
        return this->container().operator[](i);
    }
    const T& operator[](size_t i) const {
        return this->container().operator[](i);
    }

  private:
    // StackVector(const StackVector& rhs) = delete;
    // StackVector& operator=(const StackVector& rhs) = delete;
    StackVector(StackVector&& rhs) = delete;
    StackVector& operator=(StackVector&& rhs) = delete;
};

#endif  // COMMON_STACKCONTAINER_H_