ctemplate/src/base/arena.cc

// Copyright (c) 2006, Google Inc.
// All rights reserved.
//
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// modification, are permitted provided that the following conditions are
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//
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// notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
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//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

// ---
// Author: Daniel Dulitz
// Reorganized by Craig Silverstein
//
// This approach to arenas overcomes many of the limitations described
// in the "Specialized allocators" section of
//     http://www.pdos.lcs.mit.edu/~dm/c++-new.html
//
// A somewhat similar approach to Gladiator, but for heap-detection, was
// suggested by Ron van der Wal and Scott Meyers at
//     http://www.aristeia.com/BookErrata/M27Comments_frames.html

#include "config.h"
#include <sys/types.h>         // one place uintptr_t might be
#ifdef HAVE_INTTYPES_H
# include <inttypes.h>         // another place uintptr_t might be
#endif
#ifdef HAVE_UNISTD_H           // last place uintptr_t might be
# include <unistd.h>           // also, for getpagesize()
#endif
#include "base/arena.h"

_START_GOOGLE_NAMESPACE_

using std::vector;

// We used to only keep track of how much space has been allocated in
// debug mode. Now we track this for optimized builds, as well. If you
// want to play with the old scheme to see if this helps performance,
// change this ARENASET() macro to a NOP. However, NOTE: some
// applications of arenas depend on this space information (exported
// via bytes_allocated()).
#define ARENASET(x) (x)

// ----------------------------------------------------------------------
// BaseArena::BaseArena()
// BaseArena::~BaseArena()
//    Destroying the arena automatically calls Reset()
// ----------------------------------------------------------------------

BaseArena::BaseArena(char* first, const size_t block_size)
  : first_block_we_own_(first ? 1 : 0),
    block_size_(block_size),
    freestart_(NULL),                   // set for real in Reset()
    last_alloc_(NULL),
    remaining_(0),
    blocks_alloced_(1),
    overflow_blocks_(NULL) {
  assert(block_size > kDefaultAlignment);
  if (first)
    first_blocks_[0] = first;
  else
    first_blocks_[0] = reinterpret_cast<char*>(::operator new(block_size_));
  Reset();
}

BaseArena::~BaseArena() {
  FreeBlocks();
  assert(overflow_blocks_ == NULL);    // FreeBlocks() should do that
  // The first X blocks stay allocated always by default.  Delete them now.
  for ( int i = first_block_we_own_; i < blocks_alloced_; ++i )
    ::operator delete(first_blocks_[i]);
}

// ----------------------------------------------------------------------
// BaseArena::block_count()
//    Only reason this is in .cc file is because it involves STL.
// ----------------------------------------------------------------------

int BaseArena::block_count() const {
  return blocks_alloced_ + (overflow_blocks_ ? overflow_blocks_->size() : 0);
}

// ----------------------------------------------------------------------
// BaseArena::Reset()
//    Clears all the memory an arena is using.
// ----------------------------------------------------------------------

void BaseArena::Reset() {
  FreeBlocks();
  freestart_ = first_blocks_[0];
  remaining_ = block_size_;
  last_alloc_ = NULL;

  ARENASET(status_.bytes_allocated_ = block_size_);

  // We do not know for sure whether or not the first block is aligned,
  // so we fix that right now.
  const int overage = reinterpret_cast<uintptr_t>(freestart_) &
                      (kDefaultAlignment-1);
  if (overage > 0) {
    const int waste = kDefaultAlignment - overage;
    freestart_ += waste;
    remaining_ -= waste;
  }
  freestart_when_empty_ = freestart_;
  assert(!(reinterpret_cast<uintptr_t>(freestart_)&(kDefaultAlignment-1)));
}

// ----------------------------------------------------------------------
// BaseArena::MakeNewBlock()
//    Our sbrk() equivalent.  We always make blocks of the same size
//    (though GetMemory() can also make a new block for really big
//    data.
// ----------------------------------------------------------------------

void BaseArena::MakeNewBlock() {
  freestart_ = reinterpret_cast<char*>(::operator new(block_size_));
  ARENASET(status_.bytes_allocated_ += block_size_);
  remaining_ = block_size_;
  if ( blocks_alloced_ < sizeof(first_blocks_)/sizeof(*first_blocks_) ) {
    first_blocks_[blocks_alloced_++] = freestart_;
  } else {                   // oops, out of space, move to the vector
    if (overflow_blocks_ == NULL) overflow_blocks_ = new vector<char*>;
    overflow_blocks_->push_back(freestart_);
  }
}

// ----------------------------------------------------------------------
// BaseArena::GetMemoryFallback()
//    We take memory out of our pool, aligned on the byte boundary
//    requested.  If we don't have space in our current pool, we
//    allocate a new block (wasting the remaining space in the
//    current block) and give you that.  If your memory needs are
//    too big for a single block, we make a special your-memory-only
//    allocation -- this is equivalent to not using the arena at all.
// ----------------------------------------------------------------------

void* BaseArena::GetMemoryFallback(const size_t size, const int align) {
  if (0 == size) {
    return NULL;             // stl/stl_alloc.h says this is okay
  }

  assert (align > 0 && 0 == (align & (align - 1))); // must be power of 2

  // If the object is more than a quarter of the block size, allocate
  // it separately to avoid wasting too much space in leftover bytes
  if (block_size_ == 0 || size > block_size_/4) {
    // then it gets its own block in the arena
    assert(align <= kDefaultAlignment);   // because that's what new gives us
    void* ret = ::operator new (size);
    ARENASET(status_.bytes_allocated_ += size);
    // This block stays separate from the rest of the world; in particular
    // we don't update last_alloc_ so you can't reclaim space on this block.
    if ( blocks_alloced_ < sizeof(first_blocks_)/sizeof(*first_blocks_) ) {
      first_blocks_[blocks_alloced_++] = reinterpret_cast<char*>(ret);
    } else {                   // oops, out of space, move to the vector
      if (overflow_blocks_ == NULL) overflow_blocks_ = new vector<char*>;
      overflow_blocks_->push_back(reinterpret_cast<char*>(ret));
    }
    return ret;
  }

  const int overage =
    (reinterpret_cast<uintptr_t>(freestart_) & (align-1));
  if (overage) {
    const int waste = align - overage;
    freestart_ += waste;
    if (waste < remaining_) {
      remaining_ -= waste;
    } else {
      remaining_ = 0;
    }
  }
  if (size > remaining_) {
    MakeNewBlock();
  }
  remaining_ -= size;
  last_alloc_ = freestart_;
  freestart_ += size;
  assert(0 == (reinterpret_cast<uintptr_t>(last_alloc_) & (align-1)));
  return reinterpret_cast<void*>(last_alloc_);
}

// ----------------------------------------------------------------------
// BaseArena::ReturnMemoryFallback()
// BaseArena::FreeBlocks()
//    Unlike GetMemory(), which does actual work, ReturnMemory() is a
//    no-op: we don't "free" memory until Reset() is called.  We do
//    update some stats, though.  Note we do no checking that the
//    pointer you pass in was actually allocated by us, or that it
//    was allocated for the size you say, so be careful here!
//       FreeBlocks() does the work for Reset(), actually freeing all
//    memory allocated in one fell swoop.
// ----------------------------------------------------------------------

void BaseArena::FreeBlocks() {
  for ( int i = 1; i < blocks_alloced_; ++i ) {  // keep first block alloced
    ::operator delete(first_blocks_[i]);
    first_blocks_[i] = NULL;
  }
  blocks_alloced_ = 1;
  if (overflow_blocks_ != NULL) {
    vector<char *>::iterator it;
    for (it = overflow_blocks_->begin(); it != overflow_blocks_->end(); ++it) {
      ::operator delete(*it);
    }
    delete overflow_blocks_;             // These should be used very rarely
    overflow_blocks_ = NULL;
  }
}

// ----------------------------------------------------------------------
// BaseArena::AdjustLastAlloc()
//    If you realize you didn't want your last alloc to be for
//    the size you asked, after all, you can fix it by calling
//    this.  We'll grow or shrink the last-alloc region if we
//    can (we can always shrink, but we might not be able to
//    grow if you want to grow too big.
//      RETURNS true if we successfully modified the last-alloc
//    region, false if the pointer you passed in wasn't actually
//    the last alloc or if you tried to grow bigger than we could.
// ----------------------------------------------------------------------

bool BaseArena::AdjustLastAlloc(void *last_alloc, const int newsize) {
  // It's only legal to call this on the last thing you alloced.
  if (last_alloc == NULL || last_alloc != last_alloc_)  return false;
  // last_alloc_ should never point into a "big" block, w/ size >= block_size_
  assert(freestart_ >= last_alloc_ && freestart_ <= last_alloc_ + block_size_);
  const int delta = newsize - (freestart_-last_alloc_); // newsize - oldsize
  if (delta > 0 && delta > remaining_)  return false;   // not enough room

  assert(freestart_ && last_alloc_ && last_alloc_ <= freestart_);
  freestart_ += delta;
  remaining_ -= delta;
  return true;
}

// ----------------------------------------------------------------------
// UnsafeArena::Realloc()
//    If you decide you want to grow -- or shrink -- a memory region,
//    we'll do it for you here.  Typically this will involve copying
//    the existing memory to somewhere else on the arena that has
//    more space reserved.  But if you're reallocing the last-allocated
//    block, we may be able to accomodate you just by updating a
//    pointer.  In any case, we return a pointer to the new memory
//    location, which may be the same as the pointer you passed in.
//       Here's an example of how you might use Realloc():
//
//    compr_buf = arena->Alloc(uncompr_size);  // get too-much space
//    int compr_size;
//    zlib.Compress(uncompr_buf, uncompr_size, compr_buf, &compr_size);
//    compr_buf = arena->Realloc(compr_buf, uncompr_size, compr_size);
// ----------------------------------------------------------------------

char* UnsafeArena::Realloc(char* s, int oldsize, int newsize) {
  assert(oldsize >= 0 && newsize >= 0);
  if ( AdjustLastAlloc(s, newsize) )             // in case s was last alloc
    return s;
  if ( newsize <= oldsize ) {
    return s;  // no need to do anything; we're ain't reclaiming any memory!
  }
  char * newstr = Alloc(newsize);
  memcpy(newstr, s, oldsize < newsize ? oldsize : newsize);
  return newstr;
}

_END_GOOGLE_NAMESPACE_