HotSpot Stop-and-Copy GC

rednaxelafx的Cheney算法的伪代码。如果不用forwarding的话，维护一个旧地址到新地址的映射也可以。

其中重点部分：

 void Heap::collect() {
   // The from-space contains objects, and the to-space is empty now.
 
   address scanned = _to_space->bottom();
 
   // scavenge objects directly referenced by the root set
   foreach (Object** slot in ROOTS) {
     process_reference(slot);
   }
 
   // breadth-first scanning of object graph
   while (scanned < _to_space->top()) {
     Object* parent_obj = (Object*) scanned;
     foreach (Object** slot in parent_obj->obejct_fields()) {
       process_reference(slot);
       // note: _to_space->top() moves if any object is newly copied
       //       into to-space.
     }
     scanned += parent_obj->size();
   }
 
   // Now all live objects will have been evacuated into the to-space,
   // and we don't need the data in the from-space anymore.
 
   swap_spaces();
 }

Cheney算法的简单优雅之处来自它通过隐式队列来实现广度优先遍历，但它的缺点之一却也在此：广度优先的拷贝顺序使得GC后对象的空间局部性（memory locality）变差了。但是如果要改为真的深度优先顺序就会需要一个栈，无论是隐式（通常意味着递归调用）或者是显式。

HotSpot Seial GC => rednaxelafx的Cheney算法伪代码 
* saved mark => scanned 
* FastScanClosure => process_reference() 
* GenCollectedHeap::gen_process_strong_roots()、SharedHeap::process_strong_roots() => 遍历根集合的循环逻辑 
* FastEvacuateFollowersClosure => 遍历扫描队列的循环 
* oopDesc::oop_iterate() => 遍历对象的引用类型字段的循环，Object::object_fields() 
* DefNewGeneration::copy_to_survivor_space() => 拷贝对象的逻辑，Heap::evacuate()

ParalleScavenge总体过程虽然也是这样，但是因为多线程模型细节复杂很多。除strong reference之外，其他的soft，weak，phantom等reference也要处理。

rednaxelafx的完整回答

http://hllvm.group.iteye.com/group/topic/39376

其他Cheney算法的概述

https://en.wikipedia.org/wiki/Cheney%27s_algorithm

http://www.cs.umd.edu/class/fall2002/cmsc631/cheney/cheney.html