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1 // Copyright 2012 the V8 project authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
4 
5 #include <string.h>
6 
7 #include "src/v8.h"
8 #include "src/zone-inl.h"
9 
10 namespace v8 {
11 namespace internal {
12 
13 
14 // Segments represent chunks of memory: They have starting address
15 // (encoded in the this pointer) and a size in bytes. Segments are
16 // chained together forming a LIFO structure with the newest segment
17 // available as segment_head_. Segments are allocated using malloc()
18 // and de-allocated using free().
19 
20 class Segment {
21  public:
Initialize(Segment * next,int size)22   void Initialize(Segment* next, int size) {
23     next_ = next;
24     size_ = size;
25   }
26 
next() const27   Segment* next() const { return next_; }
clear_next()28   void clear_next() { next_ = NULL; }
29 
size() const30   int size() const { return size_; }
capacity() const31   int capacity() const { return size_ - sizeof(Segment); }
32 
start() const33   Address start() const { return address(sizeof(Segment)); }
end() const34   Address end() const { return address(size_); }
35 
36  private:
37   // Computes the address of the nth byte in this segment.
address(int n) const38   Address address(int n) const {
39     return Address(this) + n;
40   }
41 
42   Segment* next_;
43   int size_;
44 };
45 
46 
Zone(Isolate * isolate)47 Zone::Zone(Isolate* isolate)
48     : allocation_size_(0),
49       segment_bytes_allocated_(0),
50       position_(0),
51       limit_(0),
52       segment_head_(NULL),
53       isolate_(isolate) {
54 }
55 
56 
~Zone()57 Zone::~Zone() {
58   DeleteAll();
59   DeleteKeptSegment();
60 
61   DCHECK(segment_bytes_allocated_ == 0);
62 }
63 
64 
New(int size)65 void* Zone::New(int size) {
66   // Round up the requested size to fit the alignment.
67   size = RoundUp(size, kAlignment);
68 
69   // If the allocation size is divisible by 8 then we return an 8-byte aligned
70   // address.
71   if (kPointerSize == 4 && kAlignment == 4) {
72     position_ += ((~size) & 4) & (reinterpret_cast<intptr_t>(position_) & 4);
73   } else {
74     DCHECK(kAlignment >= kPointerSize);
75   }
76 
77   // Check if the requested size is available without expanding.
78   Address result = position_;
79 
80   int size_with_redzone =
81 #ifdef V8_USE_ADDRESS_SANITIZER
82       size + kASanRedzoneBytes;
83 #else
84       size;
85 #endif
86 
87   if (size_with_redzone > limit_ - position_) {
88      result = NewExpand(size_with_redzone);
89   } else {
90      position_ += size_with_redzone;
91   }
92 
93 #ifdef V8_USE_ADDRESS_SANITIZER
94   Address redzone_position = result + size;
95   DCHECK(redzone_position + kASanRedzoneBytes == position_);
96   ASAN_POISON_MEMORY_REGION(redzone_position, kASanRedzoneBytes);
97 #endif
98 
99   // Check that the result has the proper alignment and return it.
100   DCHECK(IsAddressAligned(result, kAlignment, 0));
101   allocation_size_ += size;
102   return reinterpret_cast<void*>(result);
103 }
104 
105 
DeleteAll()106 void Zone::DeleteAll() {
107 #ifdef DEBUG
108   // Constant byte value used for zapping dead memory in debug mode.
109   static const unsigned char kZapDeadByte = 0xcd;
110 #endif
111 
112   // Find a segment with a suitable size to keep around.
113   Segment* keep = NULL;
114   // Traverse the chained list of segments, zapping (in debug mode)
115   // and freeing every segment except the one we wish to keep.
116   for (Segment* current = segment_head_; current != NULL; ) {
117     Segment* next = current->next();
118     if (keep == NULL && current->size() <= kMaximumKeptSegmentSize) {
119       // Unlink the segment we wish to keep from the list.
120       keep = current;
121       keep->clear_next();
122     } else {
123       int size = current->size();
124 #ifdef DEBUG
125       // Un-poison first so the zapping doesn't trigger ASan complaints.
126       ASAN_UNPOISON_MEMORY_REGION(current, size);
127       // Zap the entire current segment (including the header).
128       memset(current, kZapDeadByte, size);
129 #endif
130       DeleteSegment(current, size);
131     }
132     current = next;
133   }
134 
135   // If we have found a segment we want to keep, we must recompute the
136   // variables 'position' and 'limit' to prepare for future allocate
137   // attempts. Otherwise, we must clear the position and limit to
138   // force a new segment to be allocated on demand.
139   if (keep != NULL) {
140     Address start = keep->start();
141     position_ = RoundUp(start, kAlignment);
142     limit_ = keep->end();
143     // Un-poison so we can re-use the segment later.
144     ASAN_UNPOISON_MEMORY_REGION(start, keep->capacity());
145 #ifdef DEBUG
146     // Zap the contents of the kept segment (but not the header).
147     memset(start, kZapDeadByte, keep->capacity());
148 #endif
149   } else {
150     position_ = limit_ = 0;
151   }
152 
153   // Update the head segment to be the kept segment (if any).
154   segment_head_ = keep;
155 }
156 
157 
DeleteKeptSegment()158 void Zone::DeleteKeptSegment() {
159 #ifdef DEBUG
160   // Constant byte value used for zapping dead memory in debug mode.
161   static const unsigned char kZapDeadByte = 0xcd;
162 #endif
163 
164   DCHECK(segment_head_ == NULL || segment_head_->next() == NULL);
165   if (segment_head_ != NULL) {
166     int size = segment_head_->size();
167 #ifdef DEBUG
168     // Un-poison first so the zapping doesn't trigger ASan complaints.
169     ASAN_UNPOISON_MEMORY_REGION(segment_head_, size);
170     // Zap the entire kept segment (including the header).
171     memset(segment_head_, kZapDeadByte, size);
172 #endif
173     DeleteSegment(segment_head_, size);
174     segment_head_ = NULL;
175   }
176 
177   DCHECK(segment_bytes_allocated_ == 0);
178 }
179 
180 
181 // Creates a new segment, sets it size, and pushes it to the front
182 // of the segment chain. Returns the new segment.
NewSegment(int size)183 Segment* Zone::NewSegment(int size) {
184   Segment* result = reinterpret_cast<Segment*>(Malloced::New(size));
185   adjust_segment_bytes_allocated(size);
186   if (result != NULL) {
187     result->Initialize(segment_head_, size);
188     segment_head_ = result;
189   }
190   return result;
191 }
192 
193 
194 // Deletes the given segment. Does not touch the segment chain.
DeleteSegment(Segment * segment,int size)195 void Zone::DeleteSegment(Segment* segment, int size) {
196   adjust_segment_bytes_allocated(-size);
197   Malloced::Delete(segment);
198 }
199 
200 
NewExpand(int size)201 Address Zone::NewExpand(int size) {
202   // Make sure the requested size is already properly aligned and that
203   // there isn't enough room in the Zone to satisfy the request.
204   DCHECK(size == RoundDown(size, kAlignment));
205   DCHECK(size > limit_ - position_);
206 
207   // Compute the new segment size. We use a 'high water mark'
208   // strategy, where we increase the segment size every time we expand
209   // except that we employ a maximum segment size when we delete. This
210   // is to avoid excessive malloc() and free() overhead.
211   Segment* head = segment_head_;
212   const size_t old_size = (head == NULL) ? 0 : head->size();
213   static const size_t kSegmentOverhead = sizeof(Segment) + kAlignment;
214   const size_t new_size_no_overhead = size + (old_size << 1);
215   size_t new_size = kSegmentOverhead + new_size_no_overhead;
216   const size_t min_new_size = kSegmentOverhead + static_cast<size_t>(size);
217   // Guard against integer overflow.
218   if (new_size_no_overhead < static_cast<size_t>(size) ||
219       new_size < static_cast<size_t>(kSegmentOverhead)) {
220     V8::FatalProcessOutOfMemory("Zone");
221     return NULL;
222   }
223   if (new_size < static_cast<size_t>(kMinimumSegmentSize)) {
224     new_size = kMinimumSegmentSize;
225   } else if (new_size > static_cast<size_t>(kMaximumSegmentSize)) {
226     // Limit the size of new segments to avoid growing the segment size
227     // exponentially, thus putting pressure on contiguous virtual address space.
228     // All the while making sure to allocate a segment large enough to hold the
229     // requested size.
230     new_size = Max(min_new_size, static_cast<size_t>(kMaximumSegmentSize));
231   }
232   if (new_size > INT_MAX) {
233     V8::FatalProcessOutOfMemory("Zone");
234     return NULL;
235   }
236   Segment* segment = NewSegment(static_cast<int>(new_size));
237   if (segment == NULL) {
238     V8::FatalProcessOutOfMemory("Zone");
239     return NULL;
240   }
241 
242   // Recompute 'top' and 'limit' based on the new segment.
243   Address result = RoundUp(segment->start(), kAlignment);
244   position_ = result + size;
245   // Check for address overflow.
246   // (Should not happen since the segment is guaranteed to accomodate
247   // size bytes + header and alignment padding)
248   if (reinterpret_cast<uintptr_t>(position_)
249       < reinterpret_cast<uintptr_t>(result)) {
250     V8::FatalProcessOutOfMemory("Zone");
251     return NULL;
252   }
253   limit_ = segment->end();
254   DCHECK(position_ <= limit_);
255   return result;
256 }
257 
258 
259 } }  // namespace v8::internal
260