third_party/re2/util/arena.cc

   1 // Copyright 2000 The RE2 Authors.  All Rights Reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 #include "util/util.h"
   6
   7 namespace re2 {
   8
   9 // ----------------------------------------------------------------------
  10 // UnsafeArena::UnsafeArena()
  11 // UnsafeArena::~UnsafeArena()
  12 //    Destroying the arena automatically calls Reset()
  13 // ----------------------------------------------------------------------
  14
  15
  16 UnsafeArena::UnsafeArena(const size_t block_size)
  17   : block_size_(block_size),
  18     freestart_(NULL),                   // set for real in Reset()
  19     last_alloc_(NULL),
  20     remaining_(0),
  21     blocks_alloced_(1),
  22     overflow_blocks_(NULL) {
  23   assert(block_size > kDefaultAlignment);
  24
  25   first_blocks_[0].mem = reinterpret_cast<char*>(malloc(block_size_));
  26   first_blocks_[0].size = block_size_;
  27
  28   Reset();
  29 }
  30
  31 UnsafeArena::~UnsafeArena() {
  32   FreeBlocks();
  33   assert(overflow_blocks_ == NULL);    // FreeBlocks() should do that
  34   // The first X blocks stay allocated always by default.  Delete them now.
  35   for (int i = 0; i < blocks_alloced_; i++)
  36     free(first_blocks_[i].mem);
  37 }
  38
  39 // ----------------------------------------------------------------------
  40 // UnsafeArena::Reset()
  41 //    Clears all the memory an arena is using.
  42 // ----------------------------------------------------------------------
  43
  44 void UnsafeArena::Reset() {
  45   FreeBlocks();
  46   freestart_ = first_blocks_[0].mem;
  47   remaining_ = first_blocks_[0].size;
  48   last_alloc_ = NULL;
  49
  50   // We do not know for sure whether or not the first block is aligned,
  51   // so we fix that right now.
  52   const int overage = reinterpret_cast<uintptr_t>(freestart_) &
  53                       (kDefaultAlignment-1);
  54   if (overage > 0) {
  55     const int waste = kDefaultAlignment - overage;
  56     freestart_ += waste;
  57     remaining_ -= waste;
  58   }
  59   freestart_when_empty_ = freestart_;
  60   assert(!(reinterpret_cast<uintptr_t>(freestart_)&(kDefaultAlignment-1)));
  61 }
  62
  63 // -------------------------------------------------------------
  64 // UnsafeArena::AllocNewBlock()
  65 //    Adds and returns an AllocatedBlock.
  66 //    The returned AllocatedBlock* is valid until the next call
  67 //    to AllocNewBlock or Reset.  (i.e. anything that might
  68 //    affect overflow_blocks_).
  69 // -------------------------------------------------------------
  70
  71 UnsafeArena::AllocatedBlock* UnsafeArena::AllocNewBlock(const size_t block_size) {
  72   AllocatedBlock *block;
  73   // Find the next block.
  74   if ( blocks_alloced_ < arraysize(first_blocks_) ) {
  75     // Use one of the pre-allocated blocks
  76     block = &first_blocks_[blocks_alloced_++];
  77   } else {                   // oops, out of space, move to the vector
  78     if (overflow_blocks_ == NULL) overflow_blocks_ = new vector<AllocatedBlock>;
  79     // Adds another block to the vector.
  80     overflow_blocks_->resize(overflow_blocks_->size()+1);
  81     // block points to the last block of the vector.
  82     block = &overflow_blocks_->back();
  83   }
  84
  85   block->mem = reinterpret_cast<char*>(malloc(block_size));
  86   block->size = block_size;
  87
  88   return block;
  89 }
  90
  91 // ----------------------------------------------------------------------
  92 // UnsafeArena::GetMemoryFallback()
  93 //    We take memory out of our pool, aligned on the byte boundary
  94 //    requested.  If we don't have space in our current pool, we
  95 //    allocate a new block (wasting the remaining space in the
  96 //    current block) and give you that.  If your memory needs are
  97 //    too big for a single block, we make a special your-memory-only
  98 //    allocation -- this is equivalent to not using the arena at all.
  99 // ----------------------------------------------------------------------
 100
 101 void* UnsafeArena::GetMemoryFallback(const size_t size, const int align) {
 102   if (size == 0)
 103     return NULL;             // stl/stl_alloc.h says this is okay
 104
 105   assert(align > 0 && 0 == (align & (align - 1)));  // must be power of 2
 106
 107   // If the object is more than a quarter of the block size, allocate
 108   // it separately to avoid wasting too much space in leftover bytes
 109   if (block_size_ == 0 || size > block_size_/4) {
 110     // then it gets its own block in the arena
 111     assert(align <= kDefaultAlignment);   // because that's what new gives us
 112     // This block stays separate from the rest of the world; in particular
 113     // we don't update last_alloc_ so you can't reclaim space on this block.
 114     return AllocNewBlock(size)->mem;
 115   }
 116
 117   const int overage =
 118     (reinterpret_cast<uintptr_t>(freestart_) & (align-1));
 119   if (overage) {
 120     const int waste = align - overage;
 121     freestart_ += waste;
 122     if (waste < remaining_) {
 123       remaining_ -= waste;
 124     } else {
 125       remaining_ = 0;
 126     }
 127   }
 128   if (size > remaining_) {
 129     AllocatedBlock *block = AllocNewBlock(block_size_);
 130     freestart_ = block->mem;
 131     remaining_ = block->size;
 132   }
 133   remaining_ -= size;
 134   last_alloc_ = freestart_;
 135   freestart_ += size;
 136   assert((reinterpret_cast<uintptr_t>(last_alloc_) & (align-1)) == 0);
 137   return reinterpret_cast<void*>(last_alloc_);
 138 }
 139
 140 // ----------------------------------------------------------------------
 141 // UnsafeArena::FreeBlocks()
 142 //    Unlike GetMemory(), which does actual work, ReturnMemory() is a
 143 //    no-op: we don't "free" memory until Reset() is called.  We do
 144 //    update some stats, though.  Note we do no checking that the
 145 //    pointer you pass in was actually allocated by us, or that it
 146 //    was allocated for the size you say, so be careful here!
 147 //       FreeBlocks() does the work for Reset(), actually freeing all
 148 //    memory allocated in one fell swoop.
 149 // ----------------------------------------------------------------------
 150
 151 void UnsafeArena::FreeBlocks() {
 152   for ( int i = 1; i < blocks_alloced_; ++i ) {  // keep first block alloced
 153     free(first_blocks_[i].mem);
 154     first_blocks_[i].mem = NULL;
 155     first_blocks_[i].size = 0;
 156   }
 157   blocks_alloced_ = 1;
 158   if (overflow_blocks_ != NULL) {
 159     vector<AllocatedBlock>::iterator it;
 160     for (it = overflow_blocks_->begin(); it != overflow_blocks_->end(); ++it) {
 161       free(it->mem);
 162     }
 163     delete overflow_blocks_;             // These should be used very rarely
 164     overflow_blocks_ = NULL;
 165   }
 166 }
 167
 168 }  // namespace re2