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v8/src/mark-compact.cc

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// Copyright 2006-2008 the V8 project authors. All rights reserved.
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
// * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
// * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following
// disclaimer in the documentation and/or other materials provided
// with the distribution.
// * Neither the name of Google Inc. nor the names of its
// contributors may be used to endorse or promote products derived
// from this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
#include "v8.h"
#include "execution.h"
#include "global-handles.h"
#include "ic-inl.h"
#include "mark-compact.h"
#include "stub-cache.h"
namespace v8 {
namespace internal {
// -------------------------------------------------------------------------
// MarkCompactCollector
bool MarkCompactCollector::force_compaction_ = false;
bool MarkCompactCollector::compacting_collection_ = false;
bool MarkCompactCollector::compact_on_next_gc_ = false;
int MarkCompactCollector::previous_marked_count_ = 0;
GCTracer* MarkCompactCollector::tracer_ = NULL;
#ifdef DEBUG
MarkCompactCollector::CollectorState MarkCompactCollector::state_ = IDLE;
// Counters used for debugging the marking phase of mark-compact or mark-sweep
// collection.
int MarkCompactCollector::live_bytes_ = 0;
int MarkCompactCollector::live_young_objects_ = 0;
int MarkCompactCollector::live_old_data_objects_ = 0;
int MarkCompactCollector::live_old_pointer_objects_ = 0;
int MarkCompactCollector::live_code_objects_ = 0;
int MarkCompactCollector::live_map_objects_ = 0;
int MarkCompactCollector::live_cell_objects_ = 0;
int MarkCompactCollector::live_lo_objects_ = 0;
#endif
void MarkCompactCollector::CollectGarbage() {
// Make sure that Prepare() has been called. The individual steps below will
// update the state as they proceed.
ASSERT(state_ == PREPARE_GC);
// Prepare has selected whether to compact the old generation or not.
// Tell the tracer.
if (IsCompacting()) tracer_->set_is_compacting();
MarkLiveObjects();
if (FLAG_collect_maps) ClearNonLiveTransitions();
SweepLargeObjectSpace();
if (IsCompacting()) {
EncodeForwardingAddresses();
UpdatePointers();
RelocateObjects();
RebuildRSets();
} else {
SweepSpaces();
}
Finish();
// Save the count of marked objects remaining after the collection and
// null out the GC tracer.
previous_marked_count_ = tracer_->marked_count();
ASSERT(previous_marked_count_ == 0);
tracer_ = NULL;
}
void MarkCompactCollector::Prepare(GCTracer* tracer) {
// Rather than passing the tracer around we stash it in a static member
// variable.
tracer_ = tracer;
#ifdef DEBUG
ASSERT(state_ == IDLE);
state_ = PREPARE_GC;
#endif
ASSERT(!FLAG_always_compact || !FLAG_never_compact);
compacting_collection_ =
FLAG_always_compact || force_compaction_ || compact_on_next_gc_;
compact_on_next_gc_ = false;
if (FLAG_never_compact) compacting_collection_ = false;
if (FLAG_collect_maps) CreateBackPointers();
#ifdef DEBUG
if (compacting_collection_) {
// We will write bookkeeping information to the remembered set area
// starting now.
Page::set_rset_state(Page::NOT_IN_USE);
}
#endif
PagedSpaces spaces;
while (PagedSpace* space = spaces.next()) {
space->PrepareForMarkCompact(compacting_collection_);
}
#ifdef DEBUG
live_bytes_ = 0;
live_young_objects_ = 0;
live_old_pointer_objects_ = 0;
live_old_data_objects_ = 0;
live_code_objects_ = 0;
live_map_objects_ = 0;
live_cell_objects_ = 0;
live_lo_objects_ = 0;
#endif
}
void MarkCompactCollector::Finish() {
#ifdef DEBUG
ASSERT(state_ == SWEEP_SPACES || state_ == REBUILD_RSETS);
state_ = IDLE;
#endif
// The stub cache is not traversed during GC; clear the cache to
// force lazy re-initialization of it. This must be done after the
// GC, because it relies on the new address of certain old space
// objects (empty string, illegal builtin).
StubCache::Clear();
// If we've just compacted old space there's no reason to check the
// fragmentation limit. Just return.
if (HasCompacted()) return;
// We compact the old generation on the next GC if it has gotten too
// fragmented (ie, we could recover an expected amount of space by
// reclaiming the waste and free list blocks).
static const int kFragmentationLimit = 15; // Percent.
static const int kFragmentationAllowed = 1 * MB; // Absolute.
int old_gen_recoverable = 0;
int old_gen_used = 0;
OldSpaces spaces;
while (OldSpace* space = spaces.next()) {
old_gen_recoverable += space->Waste() + space->AvailableFree();
old_gen_used += space->Size();
}
int old_gen_fragmentation =
static_cast<int>((old_gen_recoverable * 100.0) / old_gen_used);
if (old_gen_fragmentation > kFragmentationLimit &&
old_gen_recoverable > kFragmentationAllowed) {
compact_on_next_gc_ = true;
}
}
// -------------------------------------------------------------------------
// Phase 1: tracing and marking live objects.
// before: all objects are in normal state.
// after: a live object's map pointer is marked as '00'.
// Marking all live objects in the heap as part of mark-sweep or mark-compact
// collection. Before marking, all objects are in their normal state. After
// marking, live objects' map pointers are marked indicating that the object
// has been found reachable.
//
// The marking algorithm is a (mostly) depth-first (because of possible stack
// overflow) traversal of the graph of objects reachable from the roots. It
// uses an explicit stack of pointers rather than recursion. The young
// generation's inactive ('from') space is used as a marking stack. The
// objects in the marking stack are the ones that have been reached and marked
// but their children have not yet been visited.
//
// The marking stack can overflow during traversal. In that case, we set an
// overflow flag. When the overflow flag is set, we continue marking objects
// reachable from the objects on the marking stack, but no longer push them on
// the marking stack. Instead, we mark them as both marked and overflowed.
// When the stack is in the overflowed state, objects marked as overflowed
// have been reached and marked but their children have not been visited yet.
// After emptying the marking stack, we clear the overflow flag and traverse
// the heap looking for objects marked as overflowed, push them on the stack,
// and continue with marking. This process repeats until all reachable
// objects have been marked.
static MarkingStack marking_stack;
static inline HeapObject* ShortCircuitConsString(Object** p) {
// Optimization: If the heap object pointed to by p is a non-symbol
// cons string whose right substring is Heap::empty_string, update
// it in place to its left substring. Return the updated value.
//
// Here we assume that if we change *p, we replace it with a heap object
// (ie, the left substring of a cons string is always a heap object).
//
// The check performed is:
// object->IsConsString() && !object->IsSymbol() &&
// (ConsString::cast(object)->second() == Heap::empty_string())
// except the maps for the object and its possible substrings might be
// marked.
HeapObject* object = HeapObject::cast(*p);
MapWord map_word = object->map_word();
map_word.ClearMark();
InstanceType type = map_word.ToMap()->instance_type();
if ((type & kShortcutTypeMask) != kShortcutTypeTag) return object;
Object* second = reinterpret_cast<ConsString*>(object)->unchecked_second();
if (second != Heap::raw_unchecked_empty_string()) {
return object;
}
// Since we don't have the object's start, it is impossible to update the
// remembered set. Therefore, we only replace the string with its left
// substring when the remembered set does not change.
Object* first = reinterpret_cast<ConsString*>(object)->unchecked_first();
if (!Heap::InNewSpace(object) && Heap::InNewSpace(first)) return object;
*p = first;
return HeapObject::cast(first);
}
// Helper class for marking pointers in HeapObjects.
class MarkingVisitor : public ObjectVisitor {
public:
void VisitPointer(Object** p) {
MarkObjectByPointer(p);
}
void VisitPointers(Object** start, Object** end) {
// Mark all objects pointed to in [start, end).
const int kMinRangeForMarkingRecursion = 64;
if (end - start >= kMinRangeForMarkingRecursion) {
if (VisitUnmarkedObjects(start, end)) return;
// We are close to a stack overflow, so just mark the objects.
}
for (Object** p = start; p < end; p++) MarkObjectByPointer(p);
}
void VisitCodeTarget(RelocInfo* rinfo) {
ASSERT(RelocInfo::IsCodeTarget(rinfo->rmode()));
Code* code = Code::GetCodeFromTargetAddress(rinfo->target_address());
if (FLAG_cleanup_ics_at_gc && code->is_inline_cache_stub()) {
IC::Clear(rinfo->pc());
// Please note targets for cleared inline cached do not have to be
// marked since they are contained in Heap::non_monomorphic_cache().
} else {
MarkCompactCollector::MarkObject(code);
}
}
void VisitDebugTarget(RelocInfo* rinfo) {
ASSERT(RelocInfo::IsJSReturn(rinfo->rmode()) &&
rinfo->IsPatchedReturnSequence());
HeapObject* code = Code::GetCodeFromTargetAddress(rinfo->call_address());
MarkCompactCollector::MarkObject(code);
}
private:
// Mark object pointed to by p.
void MarkObjectByPointer(Object** p) {
if (!(*p)->IsHeapObject()) return;
HeapObject* object = ShortCircuitConsString(p);
MarkCompactCollector::MarkObject(object);
}
// Tells whether the mark sweep collection will perform compaction.
bool IsCompacting() { return MarkCompactCollector::IsCompacting(); }
// Visit an unmarked object.
void VisitUnmarkedObject(HeapObject* obj) {
#ifdef DEBUG
ASSERT(Heap::Contains(obj));
ASSERT(!obj->IsMarked());
#endif
Map* map = obj->map();
MarkCompactCollector::SetMark(obj);
// Mark the map pointer and the body.
MarkCompactCollector::MarkObject(map);
obj->IterateBody(map->instance_type(), obj->SizeFromMap(map), this);
}
// Visit all unmarked objects pointed to by [start, end).
// Returns false if the operation fails (lack of stack space).
inline bool VisitUnmarkedObjects(Object** start, Object** end) {
// Return false is we are close to the stack limit.
StackLimitCheck check;
if (check.HasOverflowed()) return false;
// Visit the unmarked objects.
for (Object** p = start; p < end; p++) {
if (!(*p)->IsHeapObject()) continue;
HeapObject* obj = HeapObject::cast(*p);
if (obj->IsMarked()) continue;
VisitUnmarkedObject(obj);
}
return true;
}
};
// Visitor class for marking heap roots.
class RootMarkingVisitor : public ObjectVisitor {
public:
void VisitPointer(Object** p) {
MarkObjectByPointer(p);
}
void VisitPointers(Object** start, Object** end) {
for (Object** p = start; p < end; p++) MarkObjectByPointer(p);
}
MarkingVisitor* stack_visitor() { return &stack_visitor_; }
private:
MarkingVisitor stack_visitor_;
void MarkObjectByPointer(Object** p) {
if (!(*p)->IsHeapObject()) return;
// Replace flat cons strings in place.
HeapObject* object = ShortCircuitConsString(p);
if (object->IsMarked()) return;
Map* map = object->map();
// Mark the object.
MarkCompactCollector::SetMark(object);
// Mark the map pointer and body, and push them on the marking stack.
MarkCompactCollector::MarkObject(map);
object->IterateBody(map->instance_type(), object->SizeFromMap(map),
&stack_visitor_);
// Mark all the objects reachable from the map and body. May leave
// overflowed objects in the heap.
MarkCompactCollector::EmptyMarkingStack(&stack_visitor_);
}
};
// Helper class for pruning the symbol table.
class SymbolTableCleaner : public ObjectVisitor {
public:
SymbolTableCleaner() : pointers_removed_(0) { }
void VisitPointers(Object** start, Object** end) {
// Visit all HeapObject pointers in [start, end).
for (Object** p = start; p < end; p++) {
if ((*p)->IsHeapObject() && !HeapObject::cast(*p)->IsMarked()) {
// Check if the symbol being pruned is an external symbol. We need to
// delete the associated external data as this symbol is going away.
// Since the object is not marked we can access its map word safely
// without having to worry about marking bits in the object header.
Map* map = HeapObject::cast(*p)->map();
// Since no objects have yet been moved we can safely access the map of
// the object.
uint32_t type = map->instance_type();
bool is_external = (type & kStringRepresentationMask) ==
kExternalStringTag;
if (is_external) {
bool is_two_byte = (type & kStringEncodingMask) == kTwoByteStringTag;
byte* resource_addr = reinterpret_cast<byte*>(*p) +
ExternalString::kResourceOffset -
kHeapObjectTag;
if (is_two_byte) {
v8::String::ExternalStringResource** resource =
reinterpret_cast<v8::String::ExternalStringResource**>
(resource_addr);
delete *resource;
// Clear the resource pointer in the symbol.
*resource = NULL;
} else {
v8::String::ExternalAsciiStringResource** resource =
reinterpret_cast<v8::String::ExternalAsciiStringResource**>
(resource_addr);
delete *resource;
// Clear the resource pointer in the symbol.
*resource = NULL;
}
}
// Set the entry to null_value (as deleted).
*p = Heap::raw_unchecked_null_value();
pointers_removed_++;
}
}
}
int PointersRemoved() {
return pointers_removed_;
}
private:
int pointers_removed_;
};
void MarkCompactCollector::MarkUnmarkedObject(HeapObject* object) {
ASSERT(!object->IsMarked());
ASSERT(Heap::Contains(object));
if (object->IsMap()) {
Map* map = Map::cast(object);
if (FLAG_cleanup_caches_in_maps_at_gc) {
map->ClearCodeCache();
}
SetMark(map);
if (FLAG_collect_maps &&
map->instance_type() >= FIRST_JS_OBJECT_TYPE &&
map->instance_type() <= JS_FUNCTION_TYPE) {
MarkMapContents(map);
} else {
marking_stack.Push(map);
}
} else {
SetMark(object);
marking_stack.Push(object);
}
}
void MarkCompactCollector::MarkMapContents(Map* map) {
MarkDescriptorArray(reinterpret_cast<DescriptorArray*>(
*HeapObject::RawField(map, Map::kInstanceDescriptorsOffset)));
// Mark the Object* fields of the Map.
// Since the descriptor array has been marked already, it is fine
// that one of these fields contains a pointer to it.
MarkingVisitor visitor; // Has no state or contents.
visitor.VisitPointers(HeapObject::RawField(map, Map::kPrototypeOffset),
HeapObject::RawField(map, Map::kSize));
}
void MarkCompactCollector::MarkDescriptorArray(
DescriptorArray* descriptors) {
if (descriptors->IsMarked()) return;
// Empty descriptor array is marked as a root before any maps are marked.
ASSERT(descriptors != Heap::raw_unchecked_empty_descriptor_array());
SetMark(descriptors);
FixedArray* contents = reinterpret_cast<FixedArray*>(
descriptors->get(DescriptorArray::kContentArrayIndex));
ASSERT(contents->IsHeapObject());
ASSERT(!contents->IsMarked());
ASSERT(contents->IsFixedArray());
ASSERT(contents->length() >= 2);
SetMark(contents);
// Contents contains (value, details) pairs. If the details say
// that the type of descriptor is MAP_TRANSITION, CONSTANT_TRANSITION,
// or NULL_DESCRIPTOR, we don't mark the value as live. Only for
// type MAP_TRANSITION is the value a Object* (a Map*).
for (int i = 0; i < contents->length(); i += 2) {
// If the pair (value, details) at index i, i+1 is not
// a transition or null descriptor, mark the value.
PropertyDetails details(Smi::cast(contents->get(i + 1)));
if (details.type() < FIRST_PHANTOM_PROPERTY_TYPE) {
HeapObject* object = reinterpret_cast<HeapObject*>(contents->get(i));
if (object->IsHeapObject() && !object->IsMarked()) {
SetMark(object);
marking_stack.Push(object);
}
}
}
// The DescriptorArray descriptors contains a pointer to its contents array,
// but the contents array is already marked.
marking_stack.Push(descriptors);
}
void MarkCompactCollector::CreateBackPointers() {
HeapObjectIterator iterator(Heap::map_space());
while (iterator.has_next()) {
Object* next_object = iterator.next();
if (next_object->IsMap()) { // Could also be ByteArray on free list.
Map* map = Map::cast(next_object);
if (map->instance_type() >= FIRST_JS_OBJECT_TYPE &&
map->instance_type() <= JS_FUNCTION_TYPE) {
map->CreateBackPointers();
} else {
ASSERT(map->instance_descriptors() == Heap::empty_descriptor_array());
}
}
}
}
static int OverflowObjectSize(HeapObject* obj) {
// Recover the normal map pointer, it might be marked as live and
// overflowed.
MapWord map_word = obj->map_word();
map_word.ClearMark();
map_word.ClearOverflow();
return obj->SizeFromMap(map_word.ToMap());
}
// Fill the marking stack with overflowed objects returned by the given
// iterator. Stop when the marking stack is filled or the end of the space
// is reached, whichever comes first.
template<class T>
static void ScanOverflowedObjects(T* it) {
// The caller should ensure that the marking stack is initially not full,
// so that we don't waste effort pointlessly scanning for objects.
ASSERT(!marking_stack.is_full());
while (it->has_next()) {
HeapObject* object = it->next();
if (object->IsOverflowed()) {
object->ClearOverflow();
ASSERT(object->IsMarked());
ASSERT(Heap::Contains(object));
marking_stack.Push(object);
if (marking_stack.is_full()) return;
}
}
}
bool MarkCompactCollector::IsUnmarkedHeapObject(Object** p) {
return (*p)->IsHeapObject() && !HeapObject::cast(*p)->IsMarked();
}
class SymbolMarkingVisitor : public ObjectVisitor {
public:
void VisitPointers(Object** start, Object** end) {
MarkingVisitor marker;
for (Object** p = start; p < end; p++) {
if (!(*p)->IsHeapObject()) continue;
HeapObject* object = HeapObject::cast(*p);
// If the object is marked, we have marked or are in the process
// of marking subparts.
if (object->IsMarked()) continue;
// The object is unmarked, we do not need to unmark to use its
// map.
Map* map = object->map();
object->IterateBody(map->instance_type(),
object->SizeFromMap(map),
&marker);
}
}
};
void MarkCompactCollector::MarkSymbolTable() {
// Objects reachable from symbols are marked as live so as to ensure
// that if the symbol itself remains alive after GC for any reason,
// and if it is a sliced string or a cons string backed by an
// external string (even indirectly), then the external string does
// not receive a weak reference callback.
SymbolTable* symbol_table = Heap::raw_unchecked_symbol_table();
// Mark the symbol table itself.
SetMark(symbol_table);
// Explicitly mark the prefix.
MarkingVisitor marker;
symbol_table->IteratePrefix(&marker);
ProcessMarkingStack(&marker);
// Mark subparts of the symbols but not the symbols themselves
// (unless reachable from another symbol).
SymbolMarkingVisitor symbol_marker;
symbol_table->IterateElements(&symbol_marker);
ProcessMarkingStack(&marker);
}
void MarkCompactCollector::MarkRoots(RootMarkingVisitor* visitor) {
// Mark the heap roots including global variables, stack variables,
// etc., and all objects reachable from them.
Heap::IterateStrongRoots(visitor);
// Handle the symbol table specially.
MarkSymbolTable();
// There may be overflowed objects in the heap. Visit them now.
while (marking_stack.overflowed()) {
RefillMarkingStack();
EmptyMarkingStack(visitor->stack_visitor());
}
}
void MarkCompactCollector::MarkObjectGroups() {
List<ObjectGroup*>* object_groups = GlobalHandles::ObjectGroups();
for (int i = 0; i < object_groups->length(); i++) {
ObjectGroup* entry = object_groups->at(i);
if (entry == NULL) continue;
List<Object**>& objects = entry->objects_;
bool group_marked = false;
for (int j = 0; j < objects.length(); j++) {
Object* object = *objects[j];
if (object->IsHeapObject() && HeapObject::cast(object)->IsMarked()) {
group_marked = true;
break;
}
}
if (!group_marked) continue;
// An object in the group is marked, so mark as gray all white heap
// objects in the group.
for (int j = 0; j < objects.length(); ++j) {
if ((*objects[j])->IsHeapObject()) {
MarkObject(HeapObject::cast(*objects[j]));
}
}
// Once the entire group has been colored gray, set the object group
// to NULL so it won't be processed again.
delete object_groups->at(i);
object_groups->at(i) = NULL;
}
}
// Mark all objects reachable from the objects on the marking stack.
// Before: the marking stack contains zero or more heap object pointers.
// After: the marking stack is empty, and all objects reachable from the
// marking stack have been marked, or are overflowed in the heap.
void MarkCompactCollector::EmptyMarkingStack(MarkingVisitor* visitor) {
while (!marking_stack.is_empty()) {
HeapObject* object = marking_stack.Pop();
ASSERT(object->IsHeapObject());
ASSERT(Heap::Contains(object));
ASSERT(object->IsMarked());
ASSERT(!object->IsOverflowed());
// Because the object is marked, we have to recover the original map
// pointer and use it to mark the object's body.
MapWord map_word = object->map_word();
map_word.ClearMark();
Map* map = map_word.ToMap();
MarkObject(map);
object->IterateBody(map->instance_type(), object->SizeFromMap(map),
visitor);
}
}
// Sweep the heap for overflowed objects, clear their overflow bits, and
// push them on the marking stack. Stop early if the marking stack fills
// before sweeping completes. If sweeping completes, there are no remaining
// overflowed objects in the heap so the overflow flag on the markings stack
// is cleared.
void MarkCompactCollector::RefillMarkingStack() {
ASSERT(marking_stack.overflowed());
SemiSpaceIterator new_it(Heap::new_space(), &OverflowObjectSize);
ScanOverflowedObjects(&new_it);
if (marking_stack.is_full()) return;
HeapObjectIterator old_pointer_it(Heap::old_pointer_space(),
&OverflowObjectSize);
ScanOverflowedObjects(&old_pointer_it);
if (marking_stack.is_full()) return;
HeapObjectIterator old_data_it(Heap::old_data_space(), &OverflowObjectSize);
ScanOverflowedObjects(&old_data_it);
if (marking_stack.is_full()) return;
HeapObjectIterator code_it(Heap::code_space(), &OverflowObjectSize);
ScanOverflowedObjects(&code_it);
if (marking_stack.is_full()) return;
HeapObjectIterator map_it(Heap::map_space(), &OverflowObjectSize);
ScanOverflowedObjects(&map_it);
if (marking_stack.is_full()) return;
HeapObjectIterator cell_it(Heap::cell_space(), &OverflowObjectSize);
ScanOverflowedObjects(&cell_it);
if (marking_stack.is_full()) return;
LargeObjectIterator lo_it(Heap::lo_space(), &OverflowObjectSize);
ScanOverflowedObjects(&lo_it);
if (marking_stack.is_full()) return;
marking_stack.clear_overflowed();
}
// Mark all objects reachable (transitively) from objects on the marking
// stack. Before: the marking stack contains zero or more heap object
// pointers. After: the marking stack is empty and there are no overflowed
// objects in the heap.
void MarkCompactCollector::ProcessMarkingStack(MarkingVisitor* visitor) {
EmptyMarkingStack(visitor);
while (marking_stack.overflowed()) {
RefillMarkingStack();
EmptyMarkingStack(visitor);
}
}
void MarkCompactCollector::ProcessObjectGroups(MarkingVisitor* visitor) {
bool work_to_do = true;
ASSERT(marking_stack.is_empty());
while (work_to_do) {
MarkObjectGroups();
work_to_do = !marking_stack.is_empty();
ProcessMarkingStack(visitor);
}
}
void MarkCompactCollector::MarkLiveObjects() {
#ifdef DEBUG
ASSERT(state_ == PREPARE_GC);
state_ = MARK_LIVE_OBJECTS;
#endif
// The to space contains live objects, the from space is used as a marking
// stack.
marking_stack.Initialize(Heap::new_space()->FromSpaceLow(),
Heap::new_space()->FromSpaceHigh());
ASSERT(!marking_stack.overflowed());
RootMarkingVisitor root_visitor;
MarkRoots(&root_visitor);
// The objects reachable from the roots are marked, yet unreachable
// objects are unmarked. Mark objects reachable from object groups
// containing at least one marked object, and continue until no new
// objects are reachable from the object groups.
ProcessObjectGroups(root_visitor.stack_visitor());
// The objects reachable from the roots or object groups are marked,
// yet unreachable objects are unmarked. Mark objects reachable
// only from weak global handles.
//
// First we identify nonlive weak handles and mark them as pending
// destruction.
GlobalHandles::IdentifyWeakHandles(&IsUnmarkedHeapObject);
// Then we mark the objects and process the transitive closure.
GlobalHandles::IterateWeakRoots(&root_visitor);
while (marking_stack.overflowed()) {
RefillMarkingStack();
EmptyMarkingStack(root_visitor.stack_visitor());
}
// Repeat the object groups to mark unmarked groups reachable from the
// weak roots.
ProcessObjectGroups(root_visitor.stack_visitor());
// Prune the symbol table removing all symbols only pointed to by the
// symbol table. Cannot use symbol_table() here because the symbol
// table is marked.
SymbolTable* symbol_table = Heap::raw_unchecked_symbol_table();
SymbolTableCleaner v;
symbol_table->IterateElements(&v);
symbol_table->ElementsRemoved(v.PointersRemoved());
// Remove object groups after marking phase.
GlobalHandles::RemoveObjectGroups();
}
static int CountMarkedCallback(HeapObject* obj) {
MapWord map_word = obj->map_word();
map_word.ClearMark();
return obj->SizeFromMap(map_word.ToMap());
}
#ifdef DEBUG
void MarkCompactCollector::UpdateLiveObjectCount(HeapObject* obj) {
live_bytes_ += obj->Size();
if (Heap::new_space()->Contains(obj)) {
live_young_objects_++;
} else if (Heap::map_space()->Contains(obj)) {
ASSERT(obj->IsMap());
live_map_objects_++;
} else if (Heap::cell_space()->Contains(obj)) {
ASSERT(obj->IsJSGlobalPropertyCell());
live_cell_objects_++;
} else if (Heap::old_pointer_space()->Contains(obj)) {
live_old_pointer_objects_++;
} else if (Heap::old_data_space()->Contains(obj)) {
live_old_data_objects_++;
} else if (Heap::code_space()->Contains(obj)) {
live_code_objects_++;
} else if (Heap::lo_space()->Contains(obj)) {
live_lo_objects_++;
} else {
UNREACHABLE();
}
}
#endif // DEBUG
void MarkCompactCollector::SweepLargeObjectSpace() {
#ifdef DEBUG
ASSERT(state_ == MARK_LIVE_OBJECTS);
state_ =
compacting_collection_ ? ENCODE_FORWARDING_ADDRESSES : SWEEP_SPACES;
#endif
// Deallocate unmarked objects and clear marked bits for marked objects.
Heap::lo_space()->FreeUnmarkedObjects();
}
// Safe to use during marking phase only.
bool MarkCompactCollector::SafeIsMap(HeapObject* object) {
MapWord metamap = object->map_word();
metamap.ClearMark();
return metamap.ToMap()->instance_type() == MAP_TYPE;
}
void MarkCompactCollector::ClearNonLiveTransitions() {
HeapObjectIterator map_iterator(Heap::map_space(), &CountMarkedCallback);
// Iterate over the map space, setting map transitions that go from
// a marked map to an unmarked map to null transitions. At the same time,
// set all the prototype fields of maps back to their original value,
// dropping the back pointers temporarily stored in the prototype field.
// Setting the prototype field requires following the linked list of
// back pointers, reversing them all at once. This allows us to find
// those maps with map transitions that need to be nulled, and only
// scan the descriptor arrays of those maps, not all maps.
// All of these actions are carried out only on maps of JSObects
// and related subtypes.
while (map_iterator.has_next()) {
Map* map = reinterpret_cast<Map*>(map_iterator.next());
if (!map->IsMarked() && map->IsByteArray()) continue;
ASSERT(SafeIsMap(map));
// Only JSObject and subtypes have map transitions and back pointers.
if (map->instance_type() < FIRST_JS_OBJECT_TYPE) continue;
if (map->instance_type() > JS_FUNCTION_TYPE) continue;
// Follow the chain of back pointers to find the prototype.
Map* current = map;
while (SafeIsMap(current)) {
current = reinterpret_cast<Map*>(current->prototype());
ASSERT(current->IsHeapObject());
}
Object* real_prototype = current;
// Follow back pointers, setting them to prototype,
// clearing map transitions when necessary.
current = map;
bool on_dead_path = !current->IsMarked();
Object* next;
while (SafeIsMap(current)) {
next = current->prototype();
// There should never be a dead map above a live map.
ASSERT(on_dead_path || current->IsMarked());
// A live map above a dead map indicates a dead transition.
// This test will always be false on the first iteration.
if (on_dead_path && current->IsMarked()) {
on_dead_path = false;
current->ClearNonLiveTransitions(real_prototype);
}
*HeapObject::RawField(current, Map::kPrototypeOffset) =
real_prototype;
current = reinterpret_cast<Map*>(next);
}
}
}
// -------------------------------------------------------------------------
// Phase 2: Encode forwarding addresses.
// When compacting, forwarding addresses for objects in old space and map
// space are encoded in their map pointer word (along with an encoding of
// their map pointers).
//
// 31 21 20 10 9 0
// +-----------------+------------------+-----------------+
// |forwarding offset|page offset of map|page index of map|
// +-----------------+------------------+-----------------+
// 11 bits 11 bits 10 bits
//
// An address range [start, end) can have both live and non-live objects.
// Maximal non-live regions are marked so they can be skipped on subsequent
// sweeps of the heap. A distinguished map-pointer encoding is used to mark
// free regions of one-word size (in which case the next word is the start
// of a live object). A second distinguished map-pointer encoding is used
// to mark free regions larger than one word, and the size of the free
// region (including the first word) is written to the second word of the
// region.
//
// Any valid map page offset must lie in the object area of the page, so map
// page offsets less than Page::kObjectStartOffset are invalid. We use a
// pair of distinguished invalid map encodings (for single word and multiple
// words) to indicate free regions in the page found during computation of
// forwarding addresses and skipped over in subsequent sweeps.
static const uint32_t kSingleFreeEncoding = 0;
static const uint32_t kMultiFreeEncoding = 1;
// Encode a free region, defined by the given start address and size, in the
// first word or two of the region.
void EncodeFreeRegion(Address free_start, int free_size) {
ASSERT(free_size >= kIntSize);
if (free_size == kIntSize) {
Memory::uint32_at(free_start) = kSingleFreeEncoding;
} else {
ASSERT(free_size >= 2 * kIntSize);
Memory::uint32_at(free_start) = kMultiFreeEncoding;
Memory::int_at(free_start + kIntSize) = free_size;
}
#ifdef DEBUG
// Zap the body of the free region.
if (FLAG_enable_slow_asserts) {
for (int offset = 2 * kIntSize;
offset < free_size;
offset += kPointerSize) {
Memory::Address_at(free_start + offset) = kZapValue;
}
}
#endif
}
// Try to promote all objects in new space. Heap numbers and sequential
// strings are promoted to the code space, large objects to large object space,
// and all others to the old space.
inline Object* MCAllocateFromNewSpace(HeapObject* object, int object_size) {
Object* forwarded;
if (object_size > Heap::MaxObjectSizeInPagedSpace()) {
forwarded = Failure::Exception();
} else {
OldSpace* target_space = Heap::TargetSpace(object);
ASSERT(target_space == Heap::old_pointer_space() ||
target_space == Heap::old_data_space());
forwarded = target_space->MCAllocateRaw(object_size);
}
if (forwarded->IsFailure()) {
forwarded = Heap::new_space()->MCAllocateRaw(object_size);
}
return forwarded;
}
// Allocation functions for the paged spaces call the space's MCAllocateRaw.
inline Object* MCAllocateFromOldPointerSpace(HeapObject* ignore,
int object_size) {
return Heap::old_pointer_space()->MCAllocateRaw(object_size);
}
inline Object* MCAllocateFromOldDataSpace(HeapObject* ignore, int object_size) {
return Heap::old_data_space()->MCAllocateRaw(object_size);
}
inline Object* MCAllocateFromCodeSpace(HeapObject* ignore, int object_size) {
return Heap::code_space()->MCAllocateRaw(object_size);
}
inline Object* MCAllocateFromMapSpace(HeapObject* ignore, int object_size) {
return Heap::map_space()->MCAllocateRaw(object_size);
}
inline Object* MCAllocateFromCellSpace(HeapObject* ignore, int object_size) {
return Heap::cell_space()->MCAllocateRaw(object_size);
}
// The forwarding address is encoded at the same offset as the current
// to-space object, but in from space.
inline void EncodeForwardingAddressInNewSpace(HeapObject* old_object,
int object_size,
Object* new_object,
int* ignored) {
int offset =
Heap::new_space()->ToSpaceOffsetForAddress(old_object->address());
Memory::Address_at(Heap::new_space()->FromSpaceLow() + offset) =
HeapObject::cast(new_object)->address();
}
// The forwarding address is encoded in the map pointer of the object as an
// offset (in terms of live bytes) from the address of the first live object
// in the page.
inline void EncodeForwardingAddressInPagedSpace(HeapObject* old_object,
int object_size,
Object* new_object,
int* offset) {
// Record the forwarding address of the first live object if necessary.
if (*offset == 0) {
Page::FromAddress(old_object->address())->mc_first_forwarded =
HeapObject::cast(new_object)->address();
}
MapWord encoding =
MapWord::EncodeAddress(old_object->map()->address(), *offset);
old_object->set_map_word(encoding);
*offset += object_size;
ASSERT(*offset <= Page::kObjectAreaSize);
}
// Most non-live objects are ignored.
inline void IgnoreNonLiveObject(HeapObject* object) {}
// A code deletion event is logged for non-live code objects.
inline void LogNonLiveCodeObject(HeapObject* object) {
if (object->IsCode()) LOG(CodeDeleteEvent(object->address()));
}
// Function template that, given a range of addresses (eg, a semispace or a
// paged space page), iterates through the objects in the range to clear
// mark bits and compute and encode forwarding addresses. As a side effect,
// maximal free chunks are marked so that they can be skipped on subsequent
// sweeps.
//
// The template parameters are an allocation function, a forwarding address
// encoding function, and a function to process non-live objects.
template<MarkCompactCollector::AllocationFunction Alloc,
MarkCompactCollector::EncodingFunction Encode,
MarkCompactCollector::ProcessNonLiveFunction ProcessNonLive>
inline void EncodeForwardingAddressesInRange(Address start,
Address end,
int* offset) {
// The start address of the current free region while sweeping the space.
// This address is set when a transition from live to non-live objects is
// encountered. A value (an encoding of the 'next free region' pointer)
// is written to memory at this address when a transition from non-live to
// live objects is encountered.
Address free_start = NULL;
// A flag giving the state of the previously swept object. Initially true
// to ensure that free_start is initialized to a proper address before
// trying to write to it.
bool is_prev_alive = true;
int object_size; // Will be set on each iteration of the loop.
for (Address current = start; current < end; current += object_size) {
HeapObject* object = HeapObject::FromAddress(current);
if (object->IsMarked()) {
object->ClearMark();
MarkCompactCollector::tracer()->decrement_marked_count();
object_size = object->Size();
Object* forwarded = Alloc(object, object_size);
// Allocation cannot fail, because we are compacting the space.
ASSERT(!forwarded->IsFailure());
Encode(object, object_size, forwarded, offset);
#ifdef DEBUG
if (FLAG_gc_verbose) {
PrintF("forward %p -> %p.\n", object->address(),
HeapObject::cast(forwarded)->address());
}
#endif
if (!is_prev_alive) { // Transition from non-live to live.
EncodeFreeRegion(free_start, current - free_start);
is_prev_alive = true;
}
} else { // Non-live object.
object_size = object->Size();
ProcessNonLive(object);
if (is_prev_alive) { // Transition from live to non-live.
free_start = current;
is_prev_alive = false;
}
}
}
// If we ended on a free region, mark it.
if (!is_prev_alive) EncodeFreeRegion(free_start, end - free_start);
}
// Functions to encode the forwarding pointers in each compactable space.
void MarkCompactCollector::EncodeForwardingAddressesInNewSpace() {
int ignored;
EncodeForwardingAddressesInRange<MCAllocateFromNewSpace,
EncodeForwardingAddressInNewSpace,
IgnoreNonLiveObject>(
Heap::new_space()->bottom(),
Heap::new_space()->top(),
&ignored);
}
template<MarkCompactCollector::AllocationFunction Alloc,
MarkCompactCollector::ProcessNonLiveFunction ProcessNonLive>
void MarkCompactCollector::EncodeForwardingAddressesInPagedSpace(
PagedSpace* space) {
PageIterator it(space, PageIterator::PAGES_IN_USE);
while (it.has_next()) {
Page* p = it.next();
// The offset of each live object in the page from the first live object
// in the page.
int offset = 0;
EncodeForwardingAddressesInRange<Alloc,
EncodeForwardingAddressInPagedSpace,
ProcessNonLive>(
p->ObjectAreaStart(),
p->AllocationTop(),
&offset);
}
}
static void SweepSpace(NewSpace* space) {
HeapObject* object;
for (Address current = space->bottom();
current < space->top();
current += object->Size()) {
object = HeapObject::FromAddress(current);
if (object->IsMarked()) {
object->ClearMark();
MarkCompactCollector::tracer()->decrement_marked_count();
} else {
// We give non-live objects a map that will correctly give their size,
// since their existing map might not be live after the collection.
int size = object->Size();
if (size >= ByteArray::kHeaderSize) {
object->set_map(Heap::raw_unchecked_byte_array_map());
ByteArray::cast(object)->set_length(ByteArray::LengthFor(size));
} else {
ASSERT(size == kPointerSize);
object->set_map(Heap::raw_unchecked_one_pointer_filler_map());
}
ASSERT(object->Size() == size);
}
// The object is now unmarked for the call to Size() at the top of the
// loop.
}
}
static void SweepSpace(PagedSpace* space, DeallocateFunction dealloc) {
PageIterator it(space, PageIterator::PAGES_IN_USE);
while (it.has_next()) {
Page* p = it.next();
bool is_previous_alive = true;
Address free_start = NULL;
HeapObject* object;
for (Address current = p->ObjectAreaStart();
current < p->AllocationTop();
current += object->Size()) {
object = HeapObject::FromAddress(current);
if (object->IsMarked()) {
object->ClearMark();
MarkCompactCollector::tracer()->decrement_marked_count();
if (!is_previous_alive) { // Transition from free to live.
dealloc(free_start, current - free_start);
is_previous_alive = true;
}
} else {
if (object->IsCode()) {
// Notify the logger that compiled code has been collected.
LOG(CodeDeleteEvent(Code::cast(object)->address()));
}
if (is_previous_alive) { // Transition from live to free.
free_start = current;
is_previous_alive = false;
}
}
// The object is now unmarked for the call to Size() at the top of the
// loop.
}
// If the last region was not live we need to deallocate from
// free_start to the allocation top in the page.
if (!is_previous_alive) {
int free_size = p->AllocationTop() - free_start;
if (free_size > 0) {
dealloc(free_start, free_size);
}
}
}
}
void MarkCompactCollector::DeallocateOldPointerBlock(Address start,
int size_in_bytes) {
Heap::ClearRSetRange(start, size_in_bytes);
Heap::old_pointer_space()->Free(start, size_in_bytes);
}
void MarkCompactCollector::DeallocateOldDataBlock(Address start,
int size_in_bytes) {
Heap::old_data_space()->Free(start, size_in_bytes);
}
void MarkCompactCollector::DeallocateCodeBlock(Address start,
int size_in_bytes) {
Heap::code_space()->Free(start, size_in_bytes);
}
void MarkCompactCollector::DeallocateMapBlock(Address start,
int size_in_bytes) {
// Objects in map space are frequently assumed to have size Map::kSize and a
// valid map in their first word. Thus, we break the free block up into
// chunks and free them separately.
ASSERT(size_in_bytes % Map::kSize == 0);
Heap::ClearRSetRange(start, size_in_bytes);
Address end = start + size_in_bytes;
for (Address a = start; a < end; a += Map::kSize) {
Heap::map_space()->Free(a);
}
}
void MarkCompactCollector::DeallocateCellBlock(Address start,
int size_in_bytes) {
// Free-list elements in cell space are assumed to have a fixed size.
// We break the free block into chunks and add them to the free list
// individually.
int size = Heap::cell_space()->object_size_in_bytes();
ASSERT(size_in_bytes % size == 0);
Heap::ClearRSetRange(start, size_in_bytes);
Address end = start + size_in_bytes;
for (Address a = start; a < end; a += size) {
Heap::cell_space()->Free(a);
}
}
void MarkCompactCollector::EncodeForwardingAddresses() {
ASSERT(state_ == ENCODE_FORWARDING_ADDRESSES);
// Objects in the active semispace of the young generation may be
// relocated to the inactive semispace (if not promoted). Set the
// relocation info to the beginning of the inactive semispace.
Heap::new_space()->MCResetRelocationInfo();
// Compute the forwarding pointers in each space.
EncodeForwardingAddressesInPagedSpace<MCAllocateFromOldPointerSpace,
IgnoreNonLiveObject>(
Heap::old_pointer_space());
EncodeForwardingAddressesInPagedSpace<MCAllocateFromOldDataSpace,
IgnoreNonLiveObject>(
Heap::old_data_space());
EncodeForwardingAddressesInPagedSpace<MCAllocateFromCodeSpace,
LogNonLiveCodeObject>(
Heap::code_space());
EncodeForwardingAddressesInPagedSpace<MCAllocateFromCellSpace,
IgnoreNonLiveObject>(
Heap::cell_space());
// Compute new space next to last after the old and code spaces have been
// compacted. Objects in new space can be promoted to old or code space.
EncodeForwardingAddressesInNewSpace();
// Compute map space last because computing forwarding addresses
// overwrites non-live objects. Objects in the other spaces rely on
// non-live map pointers to get the sizes of non-live objects.
EncodeForwardingAddressesInPagedSpace<MCAllocateFromMapSpace,
IgnoreNonLiveObject>(
Heap::map_space());
// Write relocation info to the top page, so we can use it later. This is
// done after promoting objects from the new space so we get the correct
// allocation top.
Heap::old_pointer_space()->MCWriteRelocationInfoToPage();
Heap::old_data_space()->MCWriteRelocationInfoToPage();
Heap::code_space()->MCWriteRelocationInfoToPage();
Heap::map_space()->MCWriteRelocationInfoToPage();
Heap::cell_space()->MCWriteRelocationInfoToPage();
}
void MarkCompactCollector::SweepSpaces() {
ASSERT(state_ == SWEEP_SPACES);
ASSERT(!IsCompacting());
// Noncompacting collections simply sweep the spaces to clear the mark
// bits and free the nonlive blocks (for old and map spaces). We sweep
// the map space last because freeing non-live maps overwrites them and
// the other spaces rely on possibly non-live maps to get the sizes for
// non-live objects.
SweepSpace(Heap::old_pointer_space(), &DeallocateOldPointerBlock);
SweepSpace(Heap::old_data_space(), &DeallocateOldDataBlock);
SweepSpace(Heap::code_space(), &DeallocateCodeBlock);
SweepSpace(Heap::cell_space(), &DeallocateCellBlock);
SweepSpace(Heap::new_space());
SweepSpace(Heap::map_space(), &DeallocateMapBlock);
}
// Iterate the live objects in a range of addresses (eg, a page or a
// semispace). The live regions of the range have been linked into a list.
// The first live region is [first_live_start, first_live_end), and the last
// address in the range is top. The callback function is used to get the
// size of each live object.
int MarkCompactCollector::IterateLiveObjectsInRange(
Address start,
Address end,
HeapObjectCallback size_func) {
int live_objects = 0;
Address current = start;
while (current < end) {
uint32_t encoded_map = Memory::uint32_at(current);
if (encoded_map == kSingleFreeEncoding) {
current += kPointerSize;
} else if (encoded_map == kMultiFreeEncoding) {
current += Memory::int_at(current + kIntSize);
} else {
live_objects++;
current += size_func(HeapObject::FromAddress(current));
}
}
return live_objects;
}
int MarkCompactCollector::IterateLiveObjects(NewSpace* space,
HeapObjectCallback size_f) {
ASSERT(MARK_LIVE_OBJECTS < state_ && state_ <= RELOCATE_OBJECTS);
return IterateLiveObjectsInRange(space->bottom(), space->top(), size_f);
}
int MarkCompactCollector::IterateLiveObjects(PagedSpace* space,
HeapObjectCallback size_f) {
ASSERT(MARK_LIVE_OBJECTS < state_ && state_ <= RELOCATE_OBJECTS);
int total = 0;
PageIterator it(space, PageIterator::PAGES_IN_USE);
while (it.has_next()) {
Page* p = it.next();
total += IterateLiveObjectsInRange(p->ObjectAreaStart(),
p->AllocationTop(),
size_f);
}
return total;
}
// -------------------------------------------------------------------------
// Phase 3: Update pointers
// Helper class for updating pointers in HeapObjects.
class UpdatingVisitor: public ObjectVisitor {
public:
void VisitPointer(Object** p) {
UpdatePointer(p);
}
void VisitPointers(Object** start, Object** end) {
// Mark all HeapObject pointers in [start, end)
for (Object** p = start; p < end; p++) UpdatePointer(p);
}
void VisitCodeTarget(RelocInfo* rinfo) {
ASSERT(RelocInfo::IsCodeTarget(rinfo->rmode()));
Object* target = Code::GetCodeFromTargetAddress(rinfo->target_address());
VisitPointer(&target);
rinfo->set_target_address(
reinterpret_cast<Code*>(target)->instruction_start());
}
void VisitDebugTarget(RelocInfo* rinfo) {
ASSERT(RelocInfo::IsJSReturn(rinfo->rmode()) &&
rinfo->IsPatchedReturnSequence());
Object* target = Code::GetCodeFromTargetAddress(rinfo->call_address());
VisitPointer(&target);
rinfo->set_call_address(
reinterpret_cast<Code*>(target)->instruction_start());
}
private:
void UpdatePointer(Object** p) {
if (!(*p)->IsHeapObject()) return;
HeapObject* obj = HeapObject::cast(*p);
Address old_addr = obj->address();
Address new_addr;
ASSERT(!Heap::InFromSpace(obj));
if (Heap::new_space()->Contains(obj)) {
Address forwarding_pointer_addr =
Heap::new_space()->FromSpaceLow() +
Heap::new_space()->ToSpaceOffsetForAddress(old_addr);
new_addr = Memory::Address_at(forwarding_pointer_addr);
#ifdef DEBUG
ASSERT(Heap::old_pointer_space()->Contains(new_addr) ||
Heap::old_data_space()->Contains(new_addr) ||
Heap::new_space()->FromSpaceContains(new_addr) ||
Heap::lo_space()->Contains(HeapObject::FromAddress(new_addr)));
if (Heap::new_space()->FromSpaceContains(new_addr)) {
ASSERT(Heap::new_space()->FromSpaceOffsetForAddress(new_addr) <=
Heap::new_space()->ToSpaceOffsetForAddress(old_addr));
}
#endif
} else if (Heap::lo_space()->Contains(obj)) {
// Don't move objects in the large object space.
return;
} else {
#ifdef DEBUG
PagedSpaces spaces;
PagedSpace* original_space = spaces.next();
while (original_space != NULL) {
if (original_space->Contains(obj)) break;
original_space = spaces.next();
}
ASSERT(original_space != NULL);
#endif
new_addr = MarkCompactCollector::GetForwardingAddressInOldSpace(obj);
ASSERT(original_space->Contains(new_addr));
ASSERT(original_space->MCSpaceOffsetForAddress(new_addr) <=
original_space->MCSpaceOffsetForAddress(old_addr));
}
*p = HeapObject::FromAddress(new_addr);
#ifdef DEBUG
if (FLAG_gc_verbose) {
PrintF("update %p : %p -> %p\n",
reinterpret_cast<Address>(p), old_addr, new_addr);
}
#endif
}
};
void MarkCompactCollector::UpdatePointers() {
#ifdef DEBUG
ASSERT(state_ == ENCODE_FORWARDING_ADDRESSES);
state_ = UPDATE_POINTERS;
#endif
UpdatingVisitor updating_visitor;
Heap::IterateRoots(&updating_visitor);
GlobalHandles::IterateWeakRoots(&updating_visitor);
int live_maps = IterateLiveObjects(Heap::map_space(),
&UpdatePointersInOldObject);
int live_pointer_olds = IterateLiveObjects(Heap::old_pointer_space(),
&UpdatePointersInOldObject);
int live_data_olds = IterateLiveObjects(Heap::old_data_space(),
&UpdatePointersInOldObject);
int live_codes = IterateLiveObjects(Heap::code_space(),
&UpdatePointersInOldObject);
int live_cells = IterateLiveObjects(Heap::cell_space(),
&UpdatePointersInOldObject);
int live_news = IterateLiveObjects(Heap::new_space(),
&UpdatePointersInNewObject);
// Large objects do not move, the map word can be updated directly.
LargeObjectIterator it(Heap::lo_space());
while (it.has_next()) UpdatePointersInNewObject(it.next());
USE(live_maps);
USE(live_pointer_olds);
USE(live_data_olds);
USE(live_codes);
USE(live_cells);
USE(live_news);
ASSERT(live_maps == live_map_objects_);
ASSERT(live_data_olds == live_old_data_objects_);
ASSERT(live_pointer_olds == live_old_pointer_objects_);
ASSERT(live_codes == live_code_objects_);
ASSERT(live_cells == live_cell_objects_);
ASSERT(live_news == live_young_objects_);
}
int MarkCompactCollector::UpdatePointersInNewObject(HeapObject* obj) {
// Keep old map pointers
Map* old_map = obj->map();
ASSERT(old_map->IsHeapObject());
Address forwarded = GetForwardingAddressInOldSpace(old_map);
ASSERT(Heap::map_space()->Contains(old_map));
ASSERT(Heap::map_space()->Contains(forwarded));
#ifdef DEBUG
if (FLAG_gc_verbose) {
PrintF("update %p : %p -> %p\n", obj->address(), old_map->address(),
forwarded);
}
#endif
// Update the map pointer.
obj->set_map(reinterpret_cast<Map*>(HeapObject::FromAddress(forwarded)));
// We have to compute the object size relying on the old map because
// map objects are not relocated yet.
int obj_size = obj->SizeFromMap(old_map);
// Update pointers in the object body.
UpdatingVisitor updating_visitor;
obj->IterateBody(old_map->instance_type(), obj_size, &updating_visitor);
return obj_size;
}
int MarkCompactCollector::UpdatePointersInOldObject(HeapObject* obj) {
// Decode the map pointer.
MapWord encoding = obj->map_word();
Address map_addr = encoding.DecodeMapAddress(Heap::map_space());
ASSERT(Heap::map_space()->Contains(HeapObject::FromAddress(map_addr)));
// At this point, the first word of map_addr is also encoded, cannot
// cast it to Map* using Map::cast.
Map* map = reinterpret_cast<Map*>(HeapObject::FromAddress(map_addr));
int obj_size = obj->SizeFromMap(map);
InstanceType type = map->instance_type();
// Update map pointer.
Address new_map_addr = GetForwardingAddressInOldSpace(map);
int offset = encoding.DecodeOffset();
obj->set_map_word(MapWord::EncodeAddress(new_map_addr, offset));
#ifdef DEBUG
if (FLAG_gc_verbose) {
PrintF("update %p : %p -> %p\n", obj->address(),
map_addr, new_map_addr);
}
#endif
// Update pointers in the object body.
UpdatingVisitor updating_visitor;
obj->IterateBody(type, obj_size, &updating_visitor);
return obj_size;
}
Address MarkCompactCollector::GetForwardingAddressInOldSpace(HeapObject* obj) {
// Object should either in old or map space.
MapWord encoding = obj->map_word();
// Offset to the first live object's forwarding address.
int offset = encoding.DecodeOffset();
Address obj_addr = obj->address();
// Find the first live object's forwarding address.
Page* p = Page::FromAddress(obj_addr);
Address first_forwarded = p->mc_first_forwarded;
// Page start address of forwarded address.
Page* forwarded_page = Page::FromAddress(first_forwarded);
int forwarded_offset = forwarded_page->Offset(first_forwarded);
// Find end of allocation of in the page of first_forwarded.
Address mc_top = forwarded_page->mc_relocation_top;
int mc_top_offset = forwarded_page->Offset(mc_top);
// Check if current object's forward pointer is in the same page
// as the first live object's forwarding pointer
if (forwarded_offset + offset < mc_top_offset) {
// In the same page.
return first_forwarded + offset;
}
// Must be in the next page, NOTE: this may cross chunks.
Page* next_page = forwarded_page->next_page();
ASSERT(next_page->is_valid());
offset -= (mc_top_offset - forwarded_offset);
offset += Page::kObjectStartOffset;
ASSERT_PAGE_OFFSET(offset);
ASSERT(next_page->OffsetToAddress(offset) < next_page->mc_relocation_top);
return next_page->OffsetToAddress(offset);
}
// -------------------------------------------------------------------------
// Phase 4: Relocate objects
void MarkCompactCollector::RelocateObjects() {
#ifdef DEBUG
ASSERT(state_ == UPDATE_POINTERS);
state_ = RELOCATE_OBJECTS;
#endif
// Relocates objects, always relocate map objects first. Relocating
// objects in other space relies on map objects to get object size.
int live_maps = IterateLiveObjects(Heap::map_space(), &RelocateMapObject);
int live_pointer_olds = IterateLiveObjects(Heap::old_pointer_space(),
&RelocateOldPointerObject);
int live_data_olds = IterateLiveObjects(Heap::old_data_space(),
&RelocateOldDataObject);
int live_codes = IterateLiveObjects(Heap::code_space(), &RelocateCodeObject);
int live_cells = IterateLiveObjects(Heap::cell_space(), &RelocateCellObject);
int live_news = IterateLiveObjects(Heap::new_space(), &RelocateNewObject);
USE(live_maps);
USE(live_data_olds);
USE(live_pointer_olds);
USE(live_codes);
USE(live_cells);
USE(live_news);
ASSERT(live_maps == live_map_objects_);
ASSERT(live_data_olds == live_old_data_objects_);
ASSERT(live_pointer_olds == live_old_pointer_objects_);
ASSERT(live_codes == live_code_objects_);
ASSERT(live_cells == live_cell_objects_);
ASSERT(live_news == live_young_objects_);
// Flip from and to spaces
Heap::new_space()->Flip();
// Set age_mark to bottom in to space
Address mark = Heap::new_space()->bottom();
Heap::new_space()->set_age_mark(mark);
Heap::new_space()->MCCommitRelocationInfo();
#ifdef DEBUG
// It is safe to write to the remembered sets as remembered sets on a
// page-by-page basis after committing the m-c forwarding pointer.
Page::set_rset_state(Page::IN_USE);
#endif
PagedSpaces spaces;
while (PagedSpace* space = spaces.next()) space->MCCommitRelocationInfo();
}
int MarkCompactCollector::RelocateMapObject(HeapObject* obj) {
// Recover map pointer.
MapWord encoding = obj->map_word();
Address map_addr = encoding.DecodeMapAddress(Heap::map_space());
ASSERT(Heap::map_space()->Contains(HeapObject::FromAddress(map_addr)));
// Get forwarding address before resetting map pointer
Address new_addr = GetForwardingAddressInOldSpace(obj);
// Reset map pointer. The meta map object may not be copied yet so
// Map::cast does not yet work.
obj->set_map(reinterpret_cast<Map*>(HeapObject::FromAddress(map_addr)));
Address old_addr = obj->address();
if (new_addr != old_addr) {
memmove(new_addr, old_addr, Map::kSize); // copy contents
}
#ifdef DEBUG
if (FLAG_gc_verbose) {
PrintF("relocate %p -> %p\n", old_addr, new_addr);
}
#endif
return Map::kSize;
}
static inline int RestoreMap(HeapObject* obj,
PagedSpace* space,
Address new_addr,
Address map_addr) {
// This must be a non-map object, and the function relies on the
// assumption that the Map space is compacted before the other paged
// spaces (see RelocateObjects).
// Reset map pointer.
obj->set_map(Map::cast(HeapObject::FromAddress(map_addr)));
int obj_size = obj->Size();
ASSERT_OBJECT_SIZE(obj_size);
ASSERT(space->MCSpaceOffsetForAddress(new_addr) <=
space->MCSpaceOffsetForAddress(obj->address()));
#ifdef DEBUG
if (FLAG_gc_verbose) {
PrintF("relocate %p -> %p\n", obj->address(), new_addr);
}
#endif
return obj_size;
}
int MarkCompactCollector::RelocateOldNonCodeObject(HeapObject* obj,
PagedSpace* space) {
// Recover map pointer.
MapWord encoding = obj->map_word();
Address map_addr = encoding.DecodeMapAddress(Heap::map_space());
ASSERT(Heap::map_space()->Contains(map_addr));
// Get forwarding address before resetting map pointer.
Address new_addr = GetForwardingAddressInOldSpace(obj);
// Reset the map pointer.
int obj_size = RestoreMap(obj, space, new_addr, map_addr);
Address old_addr = obj->address();
if (new_addr != old_addr) {
memmove(new_addr, old_addr, obj_size); // Copy contents
}
ASSERT(!HeapObject::FromAddress(new_addr)->IsCode());
return obj_size;
}
int MarkCompactCollector::RelocateOldPointerObject(HeapObject* obj) {
return RelocateOldNonCodeObject(obj, Heap::old_pointer_space());
}
int MarkCompactCollector::RelocateOldDataObject(HeapObject* obj) {
return RelocateOldNonCodeObject(obj, Heap::old_data_space());
}
int MarkCompactCollector::RelocateCellObject(HeapObject* obj) {
return RelocateOldNonCodeObject(obj, Heap::cell_space());
}
int MarkCompactCollector::RelocateCodeObject(HeapObject* obj) {
// Recover map pointer.
MapWord encoding = obj->map_word();
Address map_addr = encoding.DecodeMapAddress(Heap::map_space());
ASSERT(Heap::map_space()->Contains(HeapObject::FromAddress(map_addr)));
// Get forwarding address before resetting map pointer
Address new_addr = GetForwardingAddressInOldSpace(obj);
// Reset the map pointer.
int obj_size = RestoreMap(obj, Heap::code_space(), new_addr, map_addr);
Address old_addr = obj->address();
if (new_addr != old_addr) {
memmove(new_addr, old_addr, obj_size); // Copy contents.
}
HeapObject* copied_to = HeapObject::FromAddress(new_addr);
if (copied_to->IsCode()) {
// May also update inline cache target.
Code::cast(copied_to)->Relocate(new_addr - old_addr);
// Notify the logger that compiled code has moved.
LOG(CodeMoveEvent(old_addr, new_addr));
}
return obj_size;
}
int MarkCompactCollector::RelocateNewObject(HeapObject* obj) {
int obj_size = obj->Size();
// Get forwarding address
Address old_addr = obj->address();
int offset = Heap::new_space()->ToSpaceOffsetForAddress(old_addr);
Address new_addr =
Memory::Address_at(Heap::new_space()->FromSpaceLow() + offset);
#ifdef DEBUG
if (Heap::new_space()->FromSpaceContains(new_addr)) {
ASSERT(Heap::new_space()->FromSpaceOffsetForAddress(new_addr) <=
Heap::new_space()->ToSpaceOffsetForAddress(old_addr));
} else {
ASSERT(Heap::TargetSpace(obj) == Heap::old_pointer_space() ||
Heap::TargetSpace(obj) == Heap::old_data_space());
}
#endif
// New and old addresses cannot overlap.
memcpy(reinterpret_cast<void*>(new_addr),
reinterpret_cast<void*>(old_addr),
obj_size);
#ifdef DEBUG
if (FLAG_gc_verbose) {
PrintF("relocate %p -> %p\n", old_addr, new_addr);
}
#endif
return obj_size;
}
// -------------------------------------------------------------------------
// Phase 5: rebuild remembered sets
void MarkCompactCollector::RebuildRSets() {
#ifdef DEBUG
ASSERT(state_ == RELOCATE_OBJECTS);
state_ = REBUILD_RSETS;
#endif
Heap::RebuildRSets();
}
} } // namespace v8::internal
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