summaryrefslogtreecommitdiffstats
path: root/layout/base/StackArena.cpp
blob: 1821b97215c4a56eb66fc755f7bff25ec82fc842 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this file,
 * You can obtain one at http://mozilla.org/MPL/2.0/. */

#include "StackArena.h"
#include "nsAlgorithm.h"
#include "nsDebug.h"

namespace mozilla {

// A block of memory that the stack will chop up and hand out.
struct StackBlock {
  // Subtract sizeof(StackBlock*) to give space for the |mNext| field.
  static const size_t MAX_USABLE_SIZE = 4096 - sizeof(StackBlock*);

  // A block of memory.
  char mBlock[MAX_USABLE_SIZE];

  // Another block of memory that would only be created if our stack
  // overflowed.
  StackBlock* mNext;

  StackBlock() : mNext(nullptr) { }
  ~StackBlock() { }
};

static_assert(sizeof(StackBlock) == 4096, "StackBlock must be 4096 bytes");

// We hold an array of marks. A push pushes a mark on the stack.
// A pop pops it off.
struct StackMark {
  // The block of memory from which we are currently handing out chunks.
  StackBlock* mBlock;

  // Our current position in the block.
  size_t mPos;
};

StackArena* AutoStackArena::gStackArena;

StackArena::StackArena()
{
  mMarkLength = 0;
  mMarks = nullptr;

  // Allocate our stack memory.
  mBlocks = new StackBlock();
  mCurBlock = mBlocks;

  mStackTop = 0;
  mPos = 0;
}

StackArena::~StackArena()
{
  // Free up our data.
  delete [] mMarks;
  while (mBlocks) {
    StackBlock* toDelete = mBlocks;
    mBlocks = mBlocks->mNext;
    delete toDelete;
  }
}

size_t
StackArena::SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const
{
  size_t n = 0;
  StackBlock *block = mBlocks;
  while (block) {
    n += aMallocSizeOf(block);
    block = block->mNext;
  }
  n += aMallocSizeOf(mMarks);
  return n;
}

static const int STACK_ARENA_MARK_INCREMENT = 50;

void
StackArena::Push()
{
  // Resize the mark array if we overrun it.  Failure to allocate the
  // mark array is not fatal; we just won't free to that mark.  This
  // allows callers not to worry about error checking.
  if (mStackTop >= mMarkLength) {
    uint32_t newLength = mStackTop + STACK_ARENA_MARK_INCREMENT;
    StackMark* newMarks = new StackMark[newLength];
    if (newMarks) {
      if (mMarkLength) {
        memcpy(newMarks, mMarks, sizeof(StackMark)*mMarkLength);
      }
      // Fill in any marks that we couldn't allocate during a prior call
      // to Push().
      for (; mMarkLength < mStackTop; ++mMarkLength) {
        NS_NOTREACHED("should only hit this on out-of-memory");
        newMarks[mMarkLength].mBlock = mCurBlock;
        newMarks[mMarkLength].mPos = mPos;
      }
      delete [] mMarks;
      mMarks = newMarks;
      mMarkLength = newLength;
    }
  }

  // Set a mark at the top (if we can).
  NS_ASSERTION(mStackTop < mMarkLength, "out of memory");
  if (mStackTop < mMarkLength) {
    mMarks[mStackTop].mBlock = mCurBlock;
    mMarks[mStackTop].mPos = mPos;
  }

  mStackTop++;
}

void*
StackArena::Allocate(size_t aSize)
{
  NS_ASSERTION(mStackTop > 0, "Allocate called without Push");

  // Align to a multiple of 8.
  aSize = NS_ROUNDUP<size_t>(aSize, 8);

  // On stack overflow, grab another block.
  if (mPos + aSize >= StackBlock::MAX_USABLE_SIZE) {
    NS_ASSERTION(aSize <= StackBlock::MAX_USABLE_SIZE,
                 "Requested memory is greater that our block size!!");
    if (mCurBlock->mNext == nullptr) {
      mCurBlock->mNext = new StackBlock();
    }

    mCurBlock = mCurBlock->mNext;
    mPos = 0;
  }

  // Return the chunk they need.
  void *result = mCurBlock->mBlock + mPos;
  mPos += aSize;

  return result;
}

void
StackArena::Pop()
{
  // Pop off the mark.
  NS_ASSERTION(mStackTop > 0, "unmatched pop");
  mStackTop--;

  if (mStackTop >= mMarkLength) {
    // We couldn't allocate the marks array at the time of the push, so
    // we don't know where we're freeing to.
    NS_NOTREACHED("out of memory");
    if (mStackTop == 0) {
      // But we do know if we've completely pushed the stack.
      mCurBlock = mBlocks;
      mPos = 0;
    }
    return;
  }

#ifdef DEBUG
  // Mark the "freed" memory with 0xdd to help with debugging of memory
  // allocation problems.
  {
    StackBlock *block = mMarks[mStackTop].mBlock, *block_end = mCurBlock;
    size_t pos = mMarks[mStackTop].mPos;
    for (; block != block_end; block = block->mNext, pos = 0) {
      memset(block->mBlock + pos, 0xdd, sizeof(block->mBlock) - pos);
    }
    memset(block->mBlock + pos, 0xdd, mPos - pos);
  }
#endif

  mCurBlock = mMarks[mStackTop].mBlock;
  mPos      = mMarks[mStackTop].mPos;
}

} // namespace mozilla