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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 <string.h>
#include "v8.h"
#include "zone-inl.h"
namespace v8 {
namespace internal {
// 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, int size) {
next_ = next;
size_ = size;
}
Segment* next() const { return next_; }
void clear_next() { next_ = NULL; }
int size() const { return size_; }
int 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(int n) const {
return Address(this) + n;
}
Segment* next_;
int size_;
};
Zone::Zone(Isolate* isolate)
: allocation_size_(0),
segment_bytes_allocated_(0),
position_(0),
limit_(0),
segment_head_(NULL),
isolate_(isolate) {
}
Zone::~Zone() {
DeleteAll();
DeleteKeptSegment();
ASSERT(segment_bytes_allocated_ == 0);
}
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 = NULL;
// 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 != NULL; ) {
Segment* next = current->next();
if (keep == NULL && current->size() <= kMaximumKeptSegmentSize) {
// Unlink the segment we wish to keep from the list.
keep = current;
keep->clear_next();
} else {
int 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 != NULL) {
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;
}
// 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
ASSERT(segment_head_ == NULL || segment_head_->next() == NULL);
if (segment_head_ != NULL) {
int 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_ = NULL;
}
ASSERT(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(int size) {
Segment* result = reinterpret_cast<Segment*>(Malloced::New(size));
adjust_segment_bytes_allocated(size);
if (result != NULL) {
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, int size) {
adjust_segment_bytes_allocated(-size);
Malloced::Delete(segment);
}
Address Zone::NewExpand(int size) {
// Make sure the requested size is already properly aligned and that
// there isn't enough room in the Zone to satisfy the request.
ASSERT(size == RoundDown(size, kAlignment));
ASSERT(size > limit_ - position_);
// 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 == NULL) ? 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 + static_cast<size_t>(size);
// Guard against integer overflow.
if (new_size_no_overhead < static_cast<size_t>(size) ||
new_size < static_cast<size_t>(kSegmentOverhead)) {
V8::FatalProcessOutOfMemory("Zone");
return NULL;
}
if (new_size < static_cast<size_t>(kMinimumSegmentSize)) {
new_size = kMinimumSegmentSize;
} else if (new_size > static_cast<size_t>(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, static_cast<size_t>(kMaximumSegmentSize));
}
if (new_size > INT_MAX) {
V8::FatalProcessOutOfMemory("Zone");
return NULL;
}
Segment* segment = NewSegment(static_cast<int>(new_size));
if (segment == NULL) {
V8::FatalProcessOutOfMemory("Zone");
return NULL;
}
// 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)
if (reinterpret_cast<uintptr_t>(position_)
< reinterpret_cast<uintptr_t>(result)) {
V8::FatalProcessOutOfMemory("Zone");
return NULL;
}
limit_ = segment->end();
ASSERT(position_ <= limit_);
return result;
}
} } // namespace v8::internal
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