Gem5 实现与架构 ( Impl and Arch )

这一系列的目的是为了梳理清楚gem5是如何将不同的体系结构的代码进行构建的，记录学习的过程。如有错误，请不吝赐教。

1 Impl

./src/cpu/o3/impl.hh
这个文件中定义了CPUPolicy，BaseO3DynInst和FullO3CPU (其模板参数又为O3CPUImpl，看起来会比较奇怪)。

struct O3CPUImpl
{
   
   
	/*.....*/
    /** The CPU policy to be used, which defines all of the CPU stages. */
    typedef SimpleCPUPolicy<O3CPUImpl> CPUPol;
    /** The DynInst type to be used. */
    typedef BaseO3DynInst<O3CPUImpl> DynInst;

    /** The refcounted DynInst pointer to be used.  In most cases this is
     *  what should be used, and not DynInst *.
     */
    typedef RefCountingPtr<DynInst> DynInstPtr;
    typedef RefCountingPtr<const DynInst> DynInstConstPtr;

    /** The O3CPU type to be used. */
    typedef FullO3CPU<O3CPUImpl> O3CPU;
	/*.....*/
};

我们来看看CPUPolicy是什么
./src/cpu/o3/cpu_policy.hh

template<class Impl>
struct SimpleCPUPolicy
{
   
   
    /** Typedef for the freelist of registers. */
    typedef UnifiedFreeList FreeList;
    /** Typedef for the rename map. */
    typedef UnifiedRenameMap RenameMap;
    /** Typedef for the ROB. */
    typedef ::ROB<Impl> ROB;
    /** Typedef for the instruction queue/scheduler. */
    typedef InstructionQueue<Impl> IQ;
    /** Typedef for the memory dependence unit. */
    typedef ::MemDepUnit<StoreSet, Impl> MemDepUnit;
    /** Typedef for the LSQ. */
    typedef ::LSQ<Impl> LSQ;
    /** Typedef for the thread-specific LSQ units. */
    typedef ::LSQUnit<Impl> LSQUnit;

    /** Typedef for fetch. */
    typedef DefaultFetch<Impl> Fetch;
    /** Typedef for decode. */
    typedef DefaultDecode<Impl> Decode;
    /** Typedef for rename. */
    typedef DefaultRename<Impl> Rename;
    /** Typedef for Issue/Execute/Writeback. */
    typedef DefaultIEW<Impl> IEW;
    /** Typedef for commit. */
    typedef DefaultCommit<Impl> Commit;
    /**......................*/
}

这个CPU Policy并没有实例化或者声明CPU的部件，只是起了别名。
而真正的创建了CPU各个单元的代码则是：
./src/cpu/o3/cpu.hh
如果同样是O3CPU，即使是同样的ISA，想要改变比如decode，fetch的方式，应当是可以直接增加新的fetch，decode类，建一个不同的CPUPolicy，然后更改O3CPUImpl中使用的CPUPolicy，即可以实现不同的decode的类型的切换。

/**
 * FullO3CPU class, has each of the stages (fetch through commit)
 * within it, as well as all of the time buffers between stages.  The
 * tick() function for the CPU is defined here.
 */
template <class Impl>
class FullO3CPU : public BaseO3CPU
{
   
   
  public:
    // Typedefs from the Impl here.
    typedef typename Impl::CPUPol CPUPolicy;
    typedef