src/common/PoolAlloc.h

//
// Copyright 2019 The ANGLE 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.
//
// PoolAlloc.h:
//    Defines the class interface for PoolAllocator.
//

#ifndef COMMON_POOLALLOC_H_
#define COMMON_POOLALLOC_H_

#if !defined(NDEBUG)
#    define ANGLE_POOL_ALLOC_GUARD_BLOCKS  // define to enable guard block checking
#endif

//
// This header defines an allocator that can be used to efficiently
// allocate a large number of small requests for heap memory, with the
// intention that they are not individually deallocated, but rather
// collectively deallocated at one time.
//
// This simultaneously
//
// * Makes each individual allocation much more efficient; the
//     typical allocation is trivial.
// * Completely avoids the cost of doing individual deallocation.
// * Saves the trouble of tracking down and plugging a large class of leaks.
//
// Individual classes can use this allocator by supplying their own
// new and delete methods.
//

#include "angleutils.h"
#include "common/debug.h"

namespace angle
{
class Allocation;
class PageHeader;

//
// There are several stacks.  One is to track the pushing and popping
// of the user, and not yet implemented.  The others are simply a
// repositories of free pages or used pages.
//
// Page stacks are linked together with a simple header at the beginning
// of each allocation obtained from the underlying OS.  Multi-page allocations
// are returned to the OS.  Individual page allocations are kept for future
// re-use.
//
// The "page size" used is not, nor must it match, the underlying OS
// page size.  But, having it be about that size or equal to a set of
// pages is likely most optimal.
//
class PoolAllocator : angle::NonCopyable
{
  public:
    enum class ReleaseStrategy : uint8_t
    {
        OnlyMultiPage,
        All,
    };

    static const int kDefaultAlignment = sizeof(void *);
    //
    // Create PoolAllocator. If alignment is set to 1 byte then fastAllocate()
    //  function can be used to make allocations with less overhead.
    //
    PoolAllocator(int growthIncrement = 8 * 1024, int allocationAlignment = kDefaultAlignment);

    //
    // Don't call the destructor just to free up the memory, call pop()
    //
    ~PoolAllocator();

    //
    // Initialize page size and alignment after construction
    //
    void initialize(int pageSize, int alignment);

    //
    // Call push() to establish a new place to pop memory to.  Does not
    // have to be called to get things started.
    //
    void push();

    //
    // Call pop() to free all memory allocated since the last call to push(),
    // or if no last call to push, frees all memory since first allocation.
    //
    void pop(ReleaseStrategy releaseStrategy = ReleaseStrategy::OnlyMultiPage);

    //
    // Call popAll() to free all memory allocated.
    //
    void popAll();

    //
    // Call allocate() to actually acquire memory.  Returns 0 if no memory
    // available, otherwise a properly aligned pointer to 'numBytes' of memory.
    //
    void *allocate(size_t numBytes);

    //
    // Call fastAllocate() for a faster allocate function that does minimal bookkeeping
    // preCondition: Allocator must have been created w/ alignment of 1
    ANGLE_INLINE uint8_t *fastAllocate(size_t numBytes)
    {
#if defined(ANGLE_DISABLE_POOL_ALLOC)
        return reinterpret_cast<uint8_t *>(allocate(numBytes));
#else
        ASSERT(mAlignment == 1);
        // No multi-page allocations
        ASSERT(numBytes <= (mPageSize - mPageHeaderSkip));
        //
        // Do the allocation, most likely case inline first, for efficiency.
        //
        if (numBytes <= mPageSize - mCurrentPageOffset)
        {
            //
            // Safe to allocate from mCurrentPageOffset.
            //
            uint8_t *memory = reinterpret_cast<uint8_t *>(mInUseList) + mCurrentPageOffset;
            mCurrentPageOffset += numBytes;
            return memory;
        }
        return allocateNewPage(numBytes);
#endif
    }

    // There is no deallocate.  The point of this class is that deallocation can be skipped by the
    // user of it, as the model of use is to simultaneously deallocate everything at once by calling
    // pop(), and to not have to solve memory leak problems.

    // Catch unwanted allocations.
    // TODO(jmadill): Remove this when we remove the global allocator.
    void lock();
    void unlock();

  private:
    size_t mAlignment;  // all returned allocations will be aligned at
                        // this granularity, which will be a power of 2
#if !defined(ANGLE_DISABLE_POOL_ALLOC)
    struct AllocState
    {
        size_t offset;
        PageHeader *page;
    };
    using AllocStack = std::vector<AllocState>;

    // Slow path of allocation when we have to get a new page.
    uint8_t *allocateNewPage(size_t numBytes);
    // Track allocations if and only if we're using guard blocks
    void *initializeAllocation(uint8_t *memory, size_t numBytes);

    // Granularity of allocation from the OS
    size_t mPageSize;
    // Amount of memory to skip to make room for the page header (which is the size of the page
    // header, or PageHeader in PoolAlloc.cpp)
    size_t mPageHeaderSkip;
    // Next offset in top of inUseList to allocate from.  This offset is not necessarily aligned to
    // anything.  When an allocation is made, the data is aligned to mAlignment, and the header (if
    // any) will align to pointer size by extension (since mAlignment is made aligned to at least
    // pointer size).
    size_t mCurrentPageOffset;
    // List of popped memory
    PageHeader *mFreeList;
    // List of all memory currently being used.  The head of this list is where allocations are
    // currently being made from.
    PageHeader *mInUseList;
    // Stack of where to allocate from, to partition pool
    AllocStack mStack;

    int mNumCalls;       // just an interesting statistic
    size_t mTotalBytes;  // just an interesting statistic

#else  // !defined(ANGLE_DISABLE_POOL_ALLOC)
    std::vector<std::vector<void *>> mStack;
#endif

    bool mLocked;
};

}  // namespace angle

#endif  // COMMON_POOLALLOC_H_