smallobj.cpp

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////////////////////////////////////////////////////////////////////////////////
// The Loki Library
// Copyright (c) 2001 by Andrei Alexandrescu
// This code accompanies the book:
// Alexandrescu, Andrei. "Modern C++ Design: Generic Programming and Design 
//     Patterns Applied". Copyright (c) 2001. Addison-Wesley.
// Permission to use, copy, modify, distribute and sell this software for any 
//     purpose is hereby granted without fee, provided that the above copyright 
//     notice appear in all copies and that both that copyright notice and this 
//     permission notice appear in supporting documentation.
// The author or Addison-Welsey Longman make no representations about the 
//     suitability of this software for any purpose. It is provided "as is" 
//     without express or implied warranty.
////////////////////////////////////////////////////////////////////////////////

// Last update: March 20, 2001

#include "SmallObj.h"
#include <cassert>
#include <algorithm>

using namespace Loki;

////////////////////////////////////////////////////////////////////////////////
// FixedAllocator::Chunk::Init
// Initializes a chunk object
////////////////////////////////////////////////////////////////////////////////

void FixedAllocator::Chunk::Init(std::size_t blockSize, unsigned char blocks)
{
    assert(blockSize > 0);
    assert(blocks > 0);
    // Overflow check
    assert((blockSize * blocks) / blockSize == blocks);
    
    pData_ = new unsigned char[blockSize * blocks];
    Reset(blockSize, blocks);
}

////////////////////////////////////////////////////////////////////////////////
// FixedAllocator::Chunk::Reset
// Clears an already allocated chunk
////////////////////////////////////////////////////////////////////////////////

void FixedAllocator::Chunk::Reset(std::size_t blockSize, unsigned char blocks)
{
    assert(blockSize > 0);
    assert(blocks > 0);
    // Overflow check
    assert((blockSize * blocks) / blockSize == blocks);

    firstAvailableBlock_ = 0;
    blocksAvailable_ = blocks;

    unsigned char i = 0;
    unsigned char* p = pData_;
    for (; i != blocks; p += blockSize)
    {
        *p = ++i;
    }
}

////////////////////////////////////////////////////////////////////////////////
// FixedAllocator::Chunk::Release
// Releases the data managed by a chunk
////////////////////////////////////////////////////////////////////////////////

void FixedAllocator::Chunk::Release()
{
    delete[] pData_;
}

////////////////////////////////////////////////////////////////////////////////
// FixedAllocator::Chunk::Allocate
// Allocates a block from a chunk
////////////////////////////////////////////////////////////////////////////////

void* FixedAllocator::Chunk::Allocate(std::size_t blockSize)
{
    if (!blocksAvailable_) return 0;
    
    assert((firstAvailableBlock_ * blockSize) / blockSize == 
        firstAvailableBlock_);

    unsigned char* pResult =
        pData_ + (firstAvailableBlock_ * blockSize);
    firstAvailableBlock_ = *pResult;
    --blocksAvailable_;
    
    return pResult;
}

////////////////////////////////////////////////////////////////////////////////
// FixedAllocator::Chunk::Deallocate
// Dellocates a block from a chunk
////////////////////////////////////////////////////////////////////////////////

void FixedAllocator::Chunk::Deallocate(void* p, std::size_t blockSize)
{
    assert(p >= pData_);

    unsigned char* toRelease = static_cast<unsigned char*>(p);
    // Alignment check
    assert((toRelease - pData_) % blockSize == 0);

    *toRelease = firstAvailableBlock_;
    firstAvailableBlock_ = static_cast<unsigned char>(
        (toRelease - pData_) / blockSize);
    // Truncation check
    assert(firstAvailableBlock_ == (toRelease - pData_) / blockSize);

    ++blocksAvailable_;
}

////////////////////////////////////////////////////////////////////////////////
// FixedAllocator::FixedAllocator
// Creates a FixedAllocator object of a fixed block size
////////////////////////////////////////////////////////////////////////////////

FixedAllocator::FixedAllocator(std::size_t blockSize)
    : blockSize_(blockSize)
    , allocChunk_(0)
    , deallocChunk_(0)
{
    assert(blockSize_ > 0);
    
    prev_ = next_ = this;

    std::size_t numBlocks = DEFAULT_CHUNK_SIZE / blockSize;
    if (numBlocks > UCHAR_MAX) numBlocks = UCHAR_MAX;
    else if (numBlocks == 0) numBlocks = 8 * blockSize;
    
    numBlocks_ = static_cast<unsigned char>(numBlocks);
    assert(numBlocks_ == numBlocks);
}

////////////////////////////////////////////////////////////////////////////////
// FixedAllocator::FixedAllocator(const FixedAllocator&)
// Creates a FixedAllocator object of a fixed block size
////////////////////////////////////////////////////////////////////////////////

FixedAllocator::FixedAllocator(const FixedAllocator& rhs)
    : blockSize_(rhs.blockSize_)
    , numBlocks_(rhs.numBlocks_)
    , chunks_(rhs.chunks_)
{
    prev_ = &rhs;
    next_ = rhs.next_;
    rhs.next_->prev_ = this;
    rhs.next_ = this;
    
    allocChunk_ = rhs.allocChunk_
        ? &chunks_.front() + (rhs.allocChunk_ - &rhs.chunks_.front())
        : 0;

    deallocChunk_ = rhs.deallocChunk_
        ? &chunks_.front() + (rhs.deallocChunk_ - &rhs.chunks_.front())
        : 0;
}

FixedAllocator& FixedAllocator::operator=(const FixedAllocator& rhs)
{
    FixedAllocator copy(rhs);
    copy.Swap(*this);
    return *this;
}

////////////////////////////////////////////////////////////////////////////////
// FixedAllocator::~FixedAllocator
////////////////////////////////////////////////////////////////////////////////

FixedAllocator::~FixedAllocator()
{
    if (prev_ != this)
    {
        prev_->next_ = next_;
        next_->prev_ = prev_;
        return;
    }
    
    assert(prev_ == next_);
    Chunks::iterator i = chunks_.begin();
    for (; i != chunks_.end(); ++i)
    {
       assert(i->blocksAvailable_ == numBlocks_);
       i->Release();
    }
}

////////////////////////////////////////////////////////////////////////////////
// FixedAllocator::Swap
////////////////////////////////////////////////////////////////////////////////

void FixedAllocator::Swap(FixedAllocator& rhs)
{
    using namespace std;
    
    swap(blockSize_, rhs.blockSize_);
    swap(numBlocks_, rhs.numBlocks_);
    chunks_.swap(rhs.chunks_);
    swap(allocChunk_, rhs.allocChunk_);
    swap(deallocChunk_, rhs.deallocChunk_);
}

////////////////////////////////////////////////////////////////////////////////
// FixedAllocator::Allocate
// Allocates a block of fixed size
////////////////////////////////////////////////////////////////////////////////

void* FixedAllocator::Allocate()
{
    if (allocChunk_ == 0 || allocChunk_->blocksAvailable_ == 0)
    {
        Chunks::iterator i = chunks_.begin();
        for (;; ++i)
        {
            if (i == chunks_.end())
            {
                // Initialize
                chunks_.reserve(chunks_.size() + 1);
                Chunk newChunk;
                newChunk.Init(blockSize_, numBlocks_);
                chunks_.push_back(newChunk);
                allocChunk_ = &chunks_.back();
                deallocChunk_ = &chunks_.front();
                break;
            }
            if (i->blocksAvailable_ > 0)
            {
                allocChunk_ = &*i;
                break;
            }
        }
    }
    assert(allocChunk_ != 0);
    assert(allocChunk_->blocksAvailable_ > 0);
    
    return allocChunk_->Allocate(blockSize_);
}

////////////////////////////////////////////////////////////////////////////////
// FixedAllocator::Deallocate
// Deallocates a block previously allocated with Allocate
// (undefined behavior if called with the wrong pointer)
////////////////////////////////////////////////////////////////////////////////

void FixedAllocator::Deallocate(void* p)
{
    assert(!chunks_.empty());
    assert(&chunks_.front() <= deallocChunk_);
    assert(&chunks_.back() >= deallocChunk_);
    
    deallocChunk_  = VicinityFind(p);
    assert(deallocChunk_);

    DoDeallocate(p);
}

////////////////////////////////////////////////////////////////////////////////
// FixedAllocator::VicinityFind (internal)
// Finds the chunk corresponding to a pointer, using an efficient search
////////////////////////////////////////////////////////////////////////////////

FixedAllocator::Chunk* FixedAllocator::VicinityFind(void* p)
{
    assert(!chunks_.empty());
    assert(deallocChunk_);

    const std::size_t chunkLength = numBlocks_ * blockSize_;

    Chunk* lo = deallocChunk_;
    Chunk* hi = deallocChunk_ + 1;
    Chunk* loBound = &chunks_.front();
    Chunk* hiBound = &chunks_.back() + 1;

    for (;;)
    {
        if (lo)
        {
            if (p >= lo->pData_ && p < lo->pData_ + chunkLength)
            {
                return lo;
            }
            if (lo == loBound) lo = 0;
            else --lo;
        }
        
        if (hi)
        {
            if (p >= hi->pData_ && p < hi->pData_ + chunkLength)
            {
                return hi;
            }
            if (++hi == hiBound) hi = 0;
        }
    }
    assert(false);
    return 0;
}

////////////////////////////////////////////////////////////////////////////////
// FixedAllocator::DoDeallocate (internal)
// Performs deallocation. Assumes deallocChunk_ points to the correct chunk
////////////////////////////////////////////////////////////////////////////////

void FixedAllocator::DoDeallocate(void* p)
{
    assert(deallocChunk_->pData_ <= p);
    assert(deallocChunk_->pData_ + numBlocks_ * blockSize_ > p);

    // call into the chunk, will adjust the inner list but won't release memory
    deallocChunk_->Deallocate(p, blockSize_);

    if (deallocChunk_->blocksAvailable_ == numBlocks_)
    {
        // deallocChunk_ is completely free, should we release it? 
        
        Chunk& lastChunk = chunks_.back();
        
        if (&lastChunk == deallocChunk_)
        {
            // check if we have two last chunks empty
            
            if (chunks_.size() > 1 && 
                deallocChunk_[-1].blocksAvailable_ == numBlocks_)
            {
                // Two free chunks, discard the last one
                lastChunk.Release();
                chunks_.pop_back();
                allocChunk_ = deallocChunk_ = &chunks_.front();
            }
            return;
        }
        
        if (lastChunk.blocksAvailable_ == numBlocks_)
        {
            // Two free blocks, discard one
            lastChunk.Release();
            chunks_.pop_back();
            allocChunk_ = deallocChunk_;
        }
        else
        {
            // move the empty chunk to the end
            std::swap(*deallocChunk_, lastChunk);
            allocChunk_ = &chunks_.back();
        }
    }
}

////////////////////////////////////////////////////////////////////////////////
// SmallObjAllocator::SmallObjAllocator
// Creates an allocator for small objects given chunk size and maximum 'small'
//     object size
////////////////////////////////////////////////////////////////////////////////

SmallObjAllocator::SmallObjAllocator(
        std::size_t chunkSize, 
        std::size_t maxObjectSize)
    : pLastAlloc_(0), pLastDealloc_(0)
    , chunkSize_(chunkSize), maxObjectSize_(maxObjectSize) 
{   
}

////////////////////////////////////////////////////////////////////////////////
// SmallObjAllocator::Allocate
// Allocates 'numBytes' memory
// Uses an internal pool of FixedAllocator objects for small objects  
////////////////////////////////////////////////////////////////////////////////

void* SmallObjAllocator::Allocate(std::size_t numBytes)
{
    if (numBytes > maxObjectSize_) return operator new(numBytes);
    
    if (pLastAlloc_ && pLastAlloc_->BlockSize() == numBytes)
    {
        return pLastAlloc_->Allocate();
    }
    Pool::iterator i = std::lower_bound(pool_.begin(), pool_.end(), numBytes);
    if (i == pool_.end() || i->BlockSize() != numBytes)
    {
        i = pool_.insert(i, FixedAllocator(numBytes));
        pLastDealloc_ = &*pool_.begin();
    }
    pLastAlloc_ = &*i;
    return pLastAlloc_->Allocate();
}

////////////////////////////////////////////////////////////////////////////////
// SmallObjAllocator::Deallocate
// Deallocates memory previously allocated with Allocate
// (undefined behavior if you pass any other pointer)
////////////////////////////////////////////////////////////////////////////////

void SmallObjAllocator::Deallocate(void* p, std::size_t numBytes)
{
    if (numBytes > maxObjectSize_) return operator delete(p);

    if (pLastDealloc_ && pLastDealloc_->BlockSize() == numBytes)
    {
        pLastDealloc_->Deallocate(p);
        return;
    }
    Pool::iterator i = std::lower_bound(pool_.begin(), pool_.end(), numBytes);
    assert(i != pool_.end());
    assert(i->BlockSize() == numBytes);
    pLastDealloc_ = &*i;
    pLastDealloc_->Deallocate(p);
}

////////////////////////////////////////////////////////////////////////////////
// Change log:
// March 20: fix exception safety issue in FixedAllocator::Allocate 
//     (thanks to Chris Udazvinis for pointing that out)
// June 20, 2001: ported by Nick Thurn to gcc 2.95.3. Kudos, Nick!!!
////////////////////////////////////////////////////////////////////////////////