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// Copyright 2012 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "src/zone.h"
#include <cstring>
#include "src/v8.h"
#ifdef V8_USE_ADDRESS_SANITIZER
#include <sanitizer/asan_interface.h>
#endif // V8_USE_ADDRESS_SANITIZER
namespace v8 {
namespace internal {
namespace {
#if V8_USE_ADDRESS_SANITIZER
const size_t kASanRedzoneBytes = 24; // Must be a multiple of 8.
#else
#define ASAN_POISON_MEMORY_REGION(start, size) \
do { \
USE(start); \
USE(size); \
} while (false)
#define ASAN_UNPOISON_MEMORY_REGION(start, size) \
do { \
USE(start); \
USE(size); \
} while (false)
const size_t kASanRedzoneBytes = 0;
#endif // V8_USE_ADDRESS_SANITIZER
} // namespace
// Segments represent chunks of memory: They have starting address
// (encoded in the this pointer) and a size in bytes. Segments are
// chained together forming a LIFO structure with the newest segment
// available as segment_head_. Segments are allocated using malloc()
// and de-allocated using free().
class Segment {
public:
void Initialize(Segment* next, size_t size) {
next_ = next;
size_ = size;
}
Segment* next() const { return next_; }
void clear_next() { next_ = nullptr; }
size_t size() const { return size_; }
size_t capacity() const { return size_ - sizeof(Segment); }
Address start() const { return address(sizeof(Segment)); }
Address end() const { return address(size_); }
private:
// Computes the address of the nth byte in this segment.
Address address(size_t n) const { return Address(this) + n; }
Segment* next_;
size_t size_;
};
Zone::Zone(base::AccountingAllocator* allocator)
: allocation_size_(0),
segment_bytes_allocated_(0),
position_(0),
limit_(0),
allocator_(allocator),
segment_head_(nullptr) {}
Zone::~Zone() {
DeleteAll();
DeleteKeptSegment();
DCHECK(segment_bytes_allocated_ == 0);
}
void* Zone::New(size_t size) {
// Round up the requested size to fit the alignment.
size = RoundUp(size, kAlignment);
// If the allocation size is divisible by 8 then we return an 8-byte aligned
// address.
if (kPointerSize == 4 && kAlignment == 4) {
position_ += ((~size) & 4) & (reinterpret_cast<intptr_t>(position_) & 4);
} else {
DCHECK(kAlignment >= kPointerSize);
}
// Check if the requested size is available without expanding.
Address result = position_;
const size_t size_with_redzone = size + kASanRedzoneBytes;
if (limit_ < position_ + size_with_redzone) {
result = NewExpand(size_with_redzone);
} else {
position_ += size_with_redzone;
}
Address redzone_position = result + size;
DCHECK(redzone_position + kASanRedzoneBytes == position_);
ASAN_POISON_MEMORY_REGION(redzone_position, kASanRedzoneBytes);
// Check that the result has the proper alignment and return it.
DCHECK(IsAddressAligned(result, kAlignment, 0));
allocation_size_ += size;
return reinterpret_cast<void*>(result);
}
void Zone::DeleteAll() {
#ifdef DEBUG
// Constant byte value used for zapping dead memory in debug mode.
static const unsigned char kZapDeadByte = 0xcd;
#endif
// Find a segment with a suitable size to keep around.
Segment* keep = nullptr;
// Traverse the chained list of segments, zapping (in debug mode)
// and freeing every segment except the one we wish to keep.
for (Segment* current = segment_head_; current;) {
Segment* next = current->next();
if (!keep && current->size() <= kMaximumKeptSegmentSize) {
// Unlink the segment we wish to keep from the list.
keep = current;
keep->clear_next();
} else {
size_t size = current->size();
#ifdef DEBUG
// Un-poison first so the zapping doesn't trigger ASan complaints.
ASAN_UNPOISON_MEMORY_REGION(current, size);
// Zap the entire current segment (including the header).
memset(current, kZapDeadByte, size);
#endif
DeleteSegment(current, size);
}
current = next;
}
// If we have found a segment we want to keep, we must recompute the
// variables 'position' and 'limit' to prepare for future allocate
// attempts. Otherwise, we must clear the position and limit to
// force a new segment to be allocated on demand.
if (keep) {
Address start = keep->start();
position_ = RoundUp(start, kAlignment);
limit_ = keep->end();
// Un-poison so we can re-use the segment later.
ASAN_UNPOISON_MEMORY_REGION(start, keep->capacity());
#ifdef DEBUG
// Zap the contents of the kept segment (but not the header).
memset(start, kZapDeadByte, keep->capacity());
#endif
} else {
position_ = limit_ = 0;
}
allocation_size_ = 0;
// Update the head segment to be the kept segment (if any).
segment_head_ = keep;
}
void Zone::DeleteKeptSegment() {
#ifdef DEBUG
// Constant byte value used for zapping dead memory in debug mode.
static const unsigned char kZapDeadByte = 0xcd;
#endif
DCHECK(segment_head_ == nullptr || segment_head_->next() == nullptr);
if (segment_head_ != nullptr) {
size_t size = segment_head_->size();
#ifdef DEBUG
// Un-poison first so the zapping doesn't trigger ASan complaints.
ASAN_UNPOISON_MEMORY_REGION(segment_head_, size);
// Zap the entire kept segment (including the header).
memset(segment_head_, kZapDeadByte, size);
#endif
DeleteSegment(segment_head_, size);
segment_head_ = nullptr;
}
DCHECK(segment_bytes_allocated_ == 0);
}
// Creates a new segment, sets it size, and pushes it to the front
// of the segment chain. Returns the new segment.
Segment* Zone::NewSegment(size_t size) {
Segment* result = reinterpret_cast<Segment*>(allocator_->Allocate(size));
segment_bytes_allocated_ += size;
if (result != nullptr) {
result->Initialize(segment_head_, size);
segment_head_ = result;
}
return result;
}
// Deletes the given segment. Does not touch the segment chain.
void Zone::DeleteSegment(Segment* segment, size_t size) {
segment_bytes_allocated_ -= size;
allocator_->Free(segment, size);
}
Address Zone::NewExpand(size_t size) {
// Make sure the requested size is already properly aligned and that
// there isn't enough room in the Zone to satisfy the request.
DCHECK_EQ(size, RoundDown(size, kAlignment));
DCHECK_LT(limit_, position_ + size);
// Compute the new segment size. We use a 'high water mark'
// strategy, where we increase the segment size every time we expand
// except that we employ a maximum segment size when we delete. This
// is to avoid excessive malloc() and free() overhead.
Segment* head = segment_head_;
const size_t old_size = (head == nullptr) ? 0 : head->size();
static const size_t kSegmentOverhead = sizeof(Segment) + kAlignment;
const size_t new_size_no_overhead = size + (old_size << 1);
size_t new_size = kSegmentOverhead + new_size_no_overhead;
const size_t min_new_size = kSegmentOverhead + size;
// Guard against integer overflow.
if (new_size_no_overhead < size || new_size < kSegmentOverhead) {
V8::FatalProcessOutOfMemory("Zone");
return nullptr;
}
if (new_size < kMinimumSegmentSize) {
new_size = kMinimumSegmentSize;
} else if (new_size > kMaximumSegmentSize) {
// Limit the size of new segments to avoid growing the segment size
// exponentially, thus putting pressure on contiguous virtual address space.
// All the while making sure to allocate a segment large enough to hold the
// requested size.
new_size = Max(min_new_size, kMaximumSegmentSize);
}
if (new_size > INT_MAX) {
V8::FatalProcessOutOfMemory("Zone");
return nullptr;
}
Segment* segment = NewSegment(new_size);
if (segment == nullptr) {
V8::FatalProcessOutOfMemory("Zone");
return nullptr;
}
// Recompute 'top' and 'limit' based on the new segment.
Address result = RoundUp(segment->start(), kAlignment);
position_ = result + size;
// Check for address overflow.
// (Should not happen since the segment is guaranteed to accomodate
// size bytes + header and alignment padding)
DCHECK(reinterpret_cast<uintptr_t>(position_) >=
reinterpret_cast<uintptr_t>(result));
limit_ = segment->end();
DCHECK(position_ <= limit_);
return result;
}
} // namespace internal
} // namespace v8