src/heap_bitmap.cc - platform/art - Git at Google

 /*
  * Copyright (C) 2008 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *      http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */

 #include "heap_bitmap.h"

 #include "logging.h"
 #include "UniquePtr.h"
 #include "utils.h"

 namespace art {

 HeapBitmap* HeapBitmap::Create(const char* name, byte* heap_begin, size_t heap_capacity) {
   CHECK(heap_begin != NULL);
   // Round up since heap_capacity is not necessarily a multiple of kAlignment * kBitsPerWord.
   size_t bitmap_size = HB_OFFSET_TO_INDEX(RoundUp(heap_capacity, kAlignment * kBitsPerWord)) * kWordSize;
   UniquePtr<MemMap> mem_map(MemMap::MapAnonymous(name, NULL, bitmap_size, PROT_READ | PROT_WRITE));
   if (mem_map.get() == NULL) {
     LOG(ERROR) << "Failed to allocate bitmap " << name;
     return NULL;
   }
   word* bitmap_begin = reinterpret_cast<word*>(mem_map->Begin());
   return new HeapBitmap(name, mem_map.release(), bitmap_begin, bitmap_size, heap_begin);
 }

 // Clean up any resources associated with the bitmap.
 HeapBitmap::~HeapBitmap() {}

 // Fill the bitmap with zeroes.  Returns the bitmap's memory to the
 // system as a side-effect.
 void HeapBitmap::Clear() {
   if (bitmap_begin_ != NULL) {
     // This returns the memory to the system.  Successive page faults
     // will return zeroed memory.
     int result = madvise(bitmap_begin_, bitmap_size_, MADV_DONTNEED);
     if (result == -1) {
       PLOG(WARNING) << "madvise failed";
     }
     heap_end_ = heap_begin_ - 1;
   }
 }

 // Return true iff <obj> is within the range of pointers that this bitmap could potentially cover,
 // even if a bit has not been set for it.
 bool HeapBitmap::HasAddress(const void* obj) const {
   if (obj != NULL) {
     const uintptr_t offset = (uintptr_t)obj - heap_begin_;
     const size_t index = HB_OFFSET_TO_INDEX(offset);
     return index < bitmap_size_ / kWordSize;
   }
   return false;
 }

 void HeapBitmap::VisitRange(uintptr_t visit_begin, uintptr_t visit_end, Callback* visitor, void* arg) const {
   size_t start = HB_OFFSET_TO_INDEX(visit_begin - heap_begin_);
   size_t end = HB_OFFSET_TO_INDEX(visit_end - heap_begin_ - 1);
   for (size_t i = start; i <= end; i++) {
     word w = bitmap_begin_[i];
     if (w != 0) {
       word high_bit = 1 << (kBitsPerWord - 1);
       uintptr_t ptr_base = HB_INDEX_TO_OFFSET(i) + heap_begin_;
       while (w != 0) {
         const int shift = CLZ(w);
         Object* obj = reinterpret_cast<Object*>(ptr_base + shift * kAlignment);
         (*visitor)(obj, arg);
         w &= ~(high_bit >> shift);
       }
     }
   }
 }

 // Visits set bits in address order.  The callback is not permitted to
 // change the bitmap bits or max during the traversal.
 void HeapBitmap::Walk(HeapBitmap::Callback* callback, void* arg) {
   CHECK(bitmap_begin_ != NULL);
   CHECK(callback != NULL);
   if (heap_end_ < heap_begin_) {
     return;  // Bitmap is empty.
   }
   uintptr_t end = HB_OFFSET_TO_INDEX(heap_end_ - heap_begin_);
   for (uintptr_t i = 0; i <= end; ++i) {
     word w = bitmap_begin_[i];
     if (UNLIKELY(w != 0)) {
       word high_bit = 1 << (kBitsPerWord - 1);
       uintptr_t ptr_base = HB_INDEX_TO_OFFSET(i) + heap_begin_;
       while (w != 0) {
         const int shift = CLZ(w);
         Object* obj = reinterpret_cast<Object*>(ptr_base + shift * kAlignment);
         (*callback)(obj, arg);
         w &= ~(high_bit >> shift);
       }
     }
   }
 }

 // Similar to Walk but the callback routine is permitted to change the bitmap bits and end during
 // traversal.  Used by the the root marking scan exclusively.
 //
 // The callback is invoked with a finger argument.  The finger is a pointer to an address not yet
 // visited by the traversal.  If the callback sets a bit for an address at or above the finger, this
 // address will be visited by the traversal.  If the callback sets a bit for an address below the
 // finger, this address will not be visited (typiscally such an address would be placed on the
 // marking stack).
 void HeapBitmap::ScanWalk(uintptr_t scan_begin, uintptr_t scan_end, ScanCallback* callback, void* arg) {
   CHECK(bitmap_begin_ != NULL);
   CHECK(callback != NULL);
   CHECK_LE(scan_begin, scan_end);
   CHECK_GE(scan_begin, heap_begin_);
   size_t start = HB_OFFSET_TO_INDEX(scan_begin - heap_begin_);
   if (scan_end < heap_end_) {
     // The end of the space we're looking at is before the current maximum bitmap PC, scan to that
     // and don't recompute end on each iteration
     size_t end = HB_OFFSET_TO_INDEX(scan_end - heap_begin_ - 1);
     for (size_t i = start; i <= end; i++) {
       word w = bitmap_begin_[i];
       if (UNLIKELY(w != 0)) {
         word high_bit = 1 << (kBitsPerWord - 1);
         uintptr_t ptr_base = HB_INDEX_TO_OFFSET(i) + heap_begin_;
         void* finger = reinterpret_cast<void*>(HB_INDEX_TO_OFFSET(i + 1) + heap_begin_);
         while (w != 0) {
           const int shift = CLZ(w);
           Object* obj = reinterpret_cast<Object*>(ptr_base + shift * kAlignment);
           (*callback)(obj, finger, arg);
           w &= ~(high_bit >> shift);
         }
       }
     }
   } else {
     size_t end = HB_OFFSET_TO_INDEX(heap_end_ - heap_begin_);
     for (size_t i = start; i <= end; i++) {
       word w = bitmap_begin_[i];
       if (UNLIKELY(w != 0)) {
         word high_bit = 1 << (kBitsPerWord - 1);
         uintptr_t ptr_base = HB_INDEX_TO_OFFSET(i) + heap_begin_;
         void* finger = reinterpret_cast<void*>(HB_INDEX_TO_OFFSET(i + 1) + heap_begin_);
         while (w != 0) {
           const int shift = CLZ(w);
           Object* obj = reinterpret_cast<Object*>(ptr_base + shift * kAlignment);
           (*callback)(obj, finger, arg);
           w &= ~(high_bit >> shift);
         }
       }
       // update 'end' in case callback modified bitmap
       end = HB_OFFSET_TO_INDEX(heap_end_ - heap_begin_);
     }
   }
 }

 // Walk through the bitmaps in increasing address order, and find the
 // object pointers that correspond to garbage objects.  Call
 // <callback> zero or more times with lists of these object pointers.
 //
 // The callback is not permitted to increase the max of either bitmap.
 void HeapBitmap::SweepWalk(const HeapBitmap& live_bitmap,
                            const HeapBitmap& mark_bitmap,
                            uintptr_t sweep_begin, uintptr_t sweep_end,
                            HeapBitmap::SweepCallback* callback, void* arg) {
   CHECK(live_bitmap.bitmap_begin_ != NULL);
   CHECK(mark_bitmap.bitmap_begin_ != NULL);
   CHECK_EQ(live_bitmap.heap_begin_, mark_bitmap.heap_begin_);
   CHECK_EQ(live_bitmap.bitmap_size_, mark_bitmap.bitmap_size_);
   CHECK(callback != NULL);
   CHECK_LE(sweep_begin, sweep_end);
   CHECK_GE(sweep_begin, live_bitmap.heap_begin_);
   sweep_end = std::min(sweep_end - 1, live_bitmap.heap_end_);
   if (live_bitmap.heap_end_ < live_bitmap.heap_begin_) {
     // Easy case; both are obviously empty.
     // TODO: this should never happen
     return;
   }
   // TODO: rewrite the callbacks to accept a std::vector<Object*> rather than a Object**?
   std::vector<Object*> pointer_buf(4 * kBitsPerWord);
   Object** pb = &pointer_buf[0];
   size_t start = HB_OFFSET_TO_INDEX(sweep_begin - live_bitmap.heap_begin_);
   size_t end = HB_OFFSET_TO_INDEX(sweep_end - live_bitmap.heap_begin_);
   word* live = live_bitmap.bitmap_begin_;
   word* mark = mark_bitmap.bitmap_begin_;
   for (size_t i = start; i <= end; i++) {
     word garbage = live[i] & ~mark[i];
     if (UNLIKELY(garbage != 0)) {
       word high_bit = 1 << (kBitsPerWord - 1);
       uintptr_t ptr_base = HB_INDEX_TO_OFFSET(i) + live_bitmap.heap_begin_;
       while (garbage != 0) {
         int shift = CLZ(garbage);
         garbage &= ~(high_bit >> shift);
         *pb++ = reinterpret_cast<Object*>(ptr_base + shift * kAlignment);
       }
       // Make sure that there are always enough slots available for an
       // entire word of one bits.
       if (pb >= &pointer_buf[pointer_buf.size() - kBitsPerWord]) {
         (*callback)(pb - &pointer_buf[0], &pointer_buf[0], arg);
         pb = &pointer_buf[0];
       }
     }
   }
   if (pb > &pointer_buf[0]) {
     (*callback)(pb - &pointer_buf[0], &pointer_buf[0], arg);
   }
 }

 }  // namespace art

 // Support needed for in order traversal
 #include "object.h"
 #include "object_utils.h"

 namespace art {

 static void WalkFieldsInOrder(HeapBitmap* visited, HeapBitmap::Callback* callback, Object* obj,
                               void* arg);

 // Walk instance fields of the given Class. Separate function to allow recursion on the super
 // class.
 static void WalkInstanceFields(HeapBitmap* visited, HeapBitmap::Callback* callback, Object* obj,
                                Class* klass, void* arg) {
   // Visit fields of parent classes first.
   Class* super = klass->GetSuperClass();
   if (super != NULL) {
     WalkInstanceFields(visited, callback, obj, super, arg);
   }
   // Walk instance fields
   ObjectArray<Field>* fields = klass->GetIFields();
   if (fields != NULL) {
     for (int32_t i = 0; i < fields->GetLength(); i++) {
       Field* field = fields->Get(i);
       FieldHelper fh(field);
       if (!fh.GetType()->IsPrimitive()) {
         Object* value = field->GetObj(obj);
         if (value != NULL) {
           WalkFieldsInOrder(visited, callback, value,  arg);
         }
       }
     }
   }
 }

 // For an unvisited object, visit it then all its children found via fields.
 static void WalkFieldsInOrder(HeapBitmap* visited, HeapBitmap::Callback* callback, Object* obj,
                               void* arg) {
   if (visited->Test(obj)) {
     return;
   }
   // visit the object itself
   (*callback)(obj, arg);
   visited->Set(obj);
   // Walk instance fields of all objects
   Class* klass = obj->GetClass();
   WalkInstanceFields(visited, callback, obj, klass, arg);
   // Walk static fields of a Class
   if (obj->IsClass()) {
     ObjectArray<Field>* fields = klass->GetSFields();
     if (fields != NULL) {
       for (int32_t i = 0; i < fields->GetLength(); i++) {
         Field* field = fields->Get(i);
         FieldHelper fh(field);
         if (!fh.GetType()->IsPrimitive()) {
           Object* value = field->GetObj(NULL);
           if (value != NULL) {
             WalkFieldsInOrder(visited, callback, value, arg);
           }
         }
       }
     }
   } else if (obj->IsObjectArray()) {
     // Walk elements of an object array
     ObjectArray<Object>* obj_array = obj->AsObjectArray<Object>();
     int32_t length = obj_array->GetLength();
     for (int32_t i = 0; i < length; i++) {
       Object* value = obj_array->Get(i);
       if (value != NULL) {
         WalkFieldsInOrder(visited, callback, value, arg);
       }
     }
   }
 }

 // Visits set bits with an in order traversal.  The callback is not permitted to change the bitmap
 // bits or max during the traversal.
 void HeapBitmap::InOrderWalk(HeapBitmap::Callback* callback, void* arg) {
   UniquePtr<HeapBitmap> visited(Create("bitmap for in-order walk",
                                        reinterpret_cast<byte*>(heap_begin_),
                                        HB_INDEX_TO_OFFSET(bitmap_size_ / kWordSize)));
   CHECK(bitmap_begin_ != NULL);
   CHECK(callback != NULL);
   uintptr_t end = HB_OFFSET_TO_INDEX(heap_end_ - heap_begin_);
   for (uintptr_t i = 0; i <= end; ++i) {
     word w = bitmap_begin_[i];
     if (UNLIKELY(w != 0)) {
       word high_bit = 1 << (kBitsPerWord - 1);
       uintptr_t ptr_base = HB_INDEX_TO_OFFSET(i) + heap_begin_;
       while (w != 0) {
         const int shift = CLZ(w);
         Object* obj = reinterpret_cast<Object*>(ptr_base + shift * kAlignment);
         WalkFieldsInOrder(visited.get(), callback, obj, arg);
         w &= ~(high_bit >> shift);
       }
     }
   }
 }

 }  // namespace art
	/*
	* Copyright (C) 2008 The Android Open Source Project
	*
	* Licensed under the Apache License, Version 2.0 (the "License");
	* you may not use this file except in compliance with the License.
	* You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/

	#include "heap_bitmap.h"

	#include "logging.h"
	#include "UniquePtr.h"
	#include "utils.h"

	namespace art {

	HeapBitmap* HeapBitmap::Create(const char* name, byte* heap_begin, size_t heap_capacity) {
	CHECK(heap_begin != NULL);
	// Round up since heap_capacity is not necessarily a multiple of kAlignment * kBitsPerWord.
	size_t bitmap_size = HB_OFFSET_TO_INDEX(RoundUp(heap_capacity, kAlignment * kBitsPerWord)) * kWordSize;
	UniquePtr<MemMap> mem_map(MemMap::MapAnonymous(name, NULL, bitmap_size, PROT_READ \| PROT_WRITE));
	if (mem_map.get() == NULL) {
	LOG(ERROR) << "Failed to allocate bitmap " << name;
	return NULL;
	}
	word* bitmap_begin = reinterpret_cast<word*>(mem_map->Begin());
	return new HeapBitmap(name, mem_map.release(), bitmap_begin, bitmap_size, heap_begin);
	}

	// Clean up any resources associated with the bitmap.
	HeapBitmap::~HeapBitmap() {}

	// Fill the bitmap with zeroes. Returns the bitmap's memory to the
	// system as a side-effect.
	void HeapBitmap::Clear() {
	if (bitmap_begin_ != NULL) {
	// This returns the memory to the system. Successive page faults
	// will return zeroed memory.
	int result = madvise(bitmap_begin_, bitmap_size_, MADV_DONTNEED);
	if (result == -1) {
	PLOG(WARNING) << "madvise failed";
	}
	heap_end_ = heap_begin_ - 1;
	}
	}

	// Return true iff <obj> is within the range of pointers that this bitmap could potentially cover,
	// even if a bit has not been set for it.
	bool HeapBitmap::HasAddress(const void* obj) const {
	if (obj != NULL) {
	const uintptr_t offset = (uintptr_t)obj - heap_begin_;
	const size_t index = HB_OFFSET_TO_INDEX(offset);
	return index < bitmap_size_ / kWordSize;
	}
	return false;
	}

	void HeapBitmap::VisitRange(uintptr_t visit_begin, uintptr_t visit_end, Callback* visitor, void* arg) const {
	size_t start = HB_OFFSET_TO_INDEX(visit_begin - heap_begin_);
	size_t end = HB_OFFSET_TO_INDEX(visit_end - heap_begin_ - 1);
	for (size_t i = start; i <= end; i++) {
	word w = bitmap_begin_[i];
	if (w != 0) {
	word high_bit = 1 << (kBitsPerWord - 1);
	uintptr_t ptr_base = HB_INDEX_TO_OFFSET(i) + heap_begin_;
	while (w != 0) {
	const int shift = CLZ(w);
	Object* obj = reinterpret_cast<Object>(ptr_base + shift kAlignment);
	(*visitor)(obj, arg);
	w &= ~(high_bit >> shift);
	}
	}
	}
	}

	// Visits set bits in address order. The callback is not permitted to
	// change the bitmap bits or max during the traversal.
	void HeapBitmap::Walk(HeapBitmap::Callback* callback, void* arg) {
	CHECK(bitmap_begin_ != NULL);
	CHECK(callback != NULL);
	if (heap_end_ < heap_begin_) {
	return; // Bitmap is empty.
	}
	uintptr_t end = HB_OFFSET_TO_INDEX(heap_end_ - heap_begin_);
	for (uintptr_t i = 0; i <= end; ++i) {
	word w = bitmap_begin_[i];
	if (UNLIKELY(w != 0)) {
	word high_bit = 1 << (kBitsPerWord - 1);
	uintptr_t ptr_base = HB_INDEX_TO_OFFSET(i) + heap_begin_;
	while (w != 0) {
	const int shift = CLZ(w);
	Object* obj = reinterpret_cast<Object>(ptr_base + shift kAlignment);
	(*callback)(obj, arg);
	w &= ~(high_bit >> shift);
	}
	}
	}
	}

	// Similar to Walk but the callback routine is permitted to change the bitmap bits and end during
	// traversal. Used by the the root marking scan exclusively.
	//
	// The callback is invoked with a finger argument. The finger is a pointer to an address not yet
	// visited by the traversal. If the callback sets a bit for an address at or above the finger, this
	// address will be visited by the traversal. If the callback sets a bit for an address below the
	// finger, this address will not be visited (typiscally such an address would be placed on the
	// marking stack).
	void HeapBitmap::ScanWalk(uintptr_t scan_begin, uintptr_t scan_end, ScanCallback* callback, void* arg) {
	CHECK(bitmap_begin_ != NULL);
	CHECK(callback != NULL);
	CHECK_LE(scan_begin, scan_end);
	CHECK_GE(scan_begin, heap_begin_);
	size_t start = HB_OFFSET_TO_INDEX(scan_begin - heap_begin_);
	if (scan_end < heap_end_) {
	// The end of the space we're looking at is before the current maximum bitmap PC, scan to that
	// and don't recompute end on each iteration
	size_t end = HB_OFFSET_TO_INDEX(scan_end - heap_begin_ - 1);
	for (size_t i = start; i <= end; i++) {
	word w = bitmap_begin_[i];
	if (UNLIKELY(w != 0)) {
	word high_bit = 1 << (kBitsPerWord - 1);
	uintptr_t ptr_base = HB_INDEX_TO_OFFSET(i) + heap_begin_;
	void* finger = reinterpret_cast<void*>(HB_INDEX_TO_OFFSET(i + 1) + heap_begin_);
	while (w != 0) {
	const int shift = CLZ(w);
	Object* obj = reinterpret_cast<Object>(ptr_base + shift kAlignment);
	(*callback)(obj, finger, arg);
	w &= ~(high_bit >> shift);
	}
	}
	}
	} else {
	size_t end = HB_OFFSET_TO_INDEX(heap_end_ - heap_begin_);
	for (size_t i = start; i <= end; i++) {
	word w = bitmap_begin_[i];
	if (UNLIKELY(w != 0)) {
	word high_bit = 1 << (kBitsPerWord - 1);
	uintptr_t ptr_base = HB_INDEX_TO_OFFSET(i) + heap_begin_;
	void* finger = reinterpret_cast<void*>(HB_INDEX_TO_OFFSET(i + 1) + heap_begin_);
	while (w != 0) {
	const int shift = CLZ(w);
	Object* obj = reinterpret_cast<Object>(ptr_base + shift kAlignment);
	(*callback)(obj, finger, arg);
	w &= ~(high_bit >> shift);
	}
	}
	// update 'end' in case callback modified bitmap
	end = HB_OFFSET_TO_INDEX(heap_end_ - heap_begin_);
	}
	}
	}

	// Walk through the bitmaps in increasing address order, and find the
	// object pointers that correspond to garbage objects. Call
	// <callback> zero or more times with lists of these object pointers.
	//
	// The callback is not permitted to increase the max of either bitmap.
	void HeapBitmap::SweepWalk(const HeapBitmap& live_bitmap,
	const HeapBitmap& mark_bitmap,
	uintptr_t sweep_begin, uintptr_t sweep_end,
	HeapBitmap::SweepCallback* callback, void* arg) {
	CHECK(live_bitmap.bitmap_begin_ != NULL);
	CHECK(mark_bitmap.bitmap_begin_ != NULL);
	CHECK_EQ(live_bitmap.heap_begin_, mark_bitmap.heap_begin_);
	CHECK_EQ(live_bitmap.bitmap_size_, mark_bitmap.bitmap_size_);
	CHECK(callback != NULL);
	CHECK_LE(sweep_begin, sweep_end);
	CHECK_GE(sweep_begin, live_bitmap.heap_begin_);
	sweep_end = std::min(sweep_end - 1, live_bitmap.heap_end_);
	if (live_bitmap.heap_end_ < live_bitmap.heap_begin_) {
	// Easy case; both are obviously empty.
	// TODO: this should never happen
	return;
	}
	// TODO: rewrite the callbacks to accept a std::vector<Object> rather than a Object*?
	std::vector<Object> pointer_buf(4 kBitsPerWord);
	Object** pb = &pointer_buf[0];
	size_t start = HB_OFFSET_TO_INDEX(sweep_begin - live_bitmap.heap_begin_);
	size_t end = HB_OFFSET_TO_INDEX(sweep_end - live_bitmap.heap_begin_);
	word* live = live_bitmap.bitmap_begin_;
	word* mark = mark_bitmap.bitmap_begin_;
	for (size_t i = start; i <= end; i++) {
	word garbage = live[i] & ~mark[i];
	if (UNLIKELY(garbage != 0)) {
	word high_bit = 1 << (kBitsPerWord - 1);
	uintptr_t ptr_base = HB_INDEX_TO_OFFSET(i) + live_bitmap.heap_begin_;
	while (garbage != 0) {
	int shift = CLZ(garbage);
	garbage &= ~(high_bit >> shift);
	pb++ = reinterpret_cast<Object>(ptr_base + shift * kAlignment);
	}
	// Make sure that there are always enough slots available for an
	// entire word of one bits.
	if (pb >= &pointer_buf[pointer_buf.size() - kBitsPerWord]) {
	(*callback)(pb - &pointer_buf[0], &pointer_buf[0], arg);
	pb = &pointer_buf[0];
	}
	}
	}
	if (pb > &pointer_buf[0]) {
	(*callback)(pb - &pointer_buf[0], &pointer_buf[0], arg);
	}
	}

	} // namespace art

	// Support needed for in order traversal
	#include "object.h"
	#include "object_utils.h"

	namespace art {

	static void WalkFieldsInOrder(HeapBitmap* visited, HeapBitmap::Callback* callback, Object* obj,
	void* arg);

	// Walk instance fields of the given Class. Separate function to allow recursion on the super
	// class.
	static void WalkInstanceFields(HeapBitmap* visited, HeapBitmap::Callback* callback, Object* obj,
	Class* klass, void* arg) {
	// Visit fields of parent classes first.
	Class* super = klass->GetSuperClass();
	if (super != NULL) {
	WalkInstanceFields(visited, callback, obj, super, arg);
	}
	// Walk instance fields
	ObjectArray<Field>* fields = klass->GetIFields();
	if (fields != NULL) {
	for (int32_t i = 0; i < fields->GetLength(); i++) {
	Field* field = fields->Get(i);
	FieldHelper fh(field);
	if (!fh.GetType()->IsPrimitive()) {
	Object* value = field->GetObj(obj);
	if (value != NULL) {
	WalkFieldsInOrder(visited, callback, value, arg);
	}
	}
	}
	}
	}

	// For an unvisited object, visit it then all its children found via fields.
	static void WalkFieldsInOrder(HeapBitmap* visited, HeapBitmap::Callback* callback, Object* obj,
	void* arg) {
	if (visited->Test(obj)) {
	return;
	}
	// visit the object itself
	(*callback)(obj, arg);
	visited->Set(obj);
	// Walk instance fields of all objects
	Class* klass = obj->GetClass();
	WalkInstanceFields(visited, callback, obj, klass, arg);
	// Walk static fields of a Class
	if (obj->IsClass()) {
	ObjectArray<Field>* fields = klass->GetSFields();
	if (fields != NULL) {
	for (int32_t i = 0; i < fields->GetLength(); i++) {
	Field* field = fields->Get(i);
	FieldHelper fh(field);
	if (!fh.GetType()->IsPrimitive()) {
	Object* value = field->GetObj(NULL);
	if (value != NULL) {
	WalkFieldsInOrder(visited, callback, value, arg);
	}
	}
	}
	}
	} else if (obj->IsObjectArray()) {
	// Walk elements of an object array
	ObjectArray<Object>* obj_array = obj->AsObjectArray<Object>();
	int32_t length = obj_array->GetLength();
	for (int32_t i = 0; i < length; i++) {
	Object* value = obj_array->Get(i);
	if (value != NULL) {
	WalkFieldsInOrder(visited, callback, value, arg);
	}
	}
	}
	}

	// Visits set bits with an in order traversal. The callback is not permitted to change the bitmap
	// bits or max during the traversal.
	void HeapBitmap::InOrderWalk(HeapBitmap::Callback* callback, void* arg) {
	UniquePtr<HeapBitmap> visited(Create("bitmap for in-order walk",
	reinterpret_cast<byte*>(heap_begin_),
	HB_INDEX_TO_OFFSET(bitmap_size_ / kWordSize)));
	CHECK(bitmap_begin_ != NULL);
	CHECK(callback != NULL);
	uintptr_t end = HB_OFFSET_TO_INDEX(heap_end_ - heap_begin_);
	for (uintptr_t i = 0; i <= end; ++i) {
	word w = bitmap_begin_[i];
	if (UNLIKELY(w != 0)) {
	word high_bit = 1 << (kBitsPerWord - 1);
	uintptr_t ptr_base = HB_INDEX_TO_OFFSET(i) + heap_begin_;
	while (w != 0) {
	const int shift = CLZ(w);
	Object* obj = reinterpret_cast<Object>(ptr_base + shift kAlignment);
	WalkFieldsInOrder(visited.get(), callback, obj, arg);
	w &= ~(high_bit >> shift);
	}
	}
	}
	}

	} // namespace art