coreCLR系列随笔 之ClrJit项目之alloc.cpp文件分析（1）

　　首先声明，自己对CLR了解得不多，只是个人爱好，可能有错误，请指出，文件源码如下（可能不是最新的）

//
// Copyright (c) Microsoft. All rights reserved.
// Licensed under the MIT license. See LICENSE file in the project root for full license information.
//
/*****************************************************************************/

#include "jitpch.h"
#ifdef _MSC_VER
#pragma hdrstop
#endif
/*****************************************************************************/

/*****************************************************************************/
void                allocatorCodeSizeBeg(){}
/*****************************************************************************/
#ifdef  DEBUG
/*****************************************************************************/

void    __cdecl     debugStop(const char *why, ...)
{
    va_list     args;

    va_start(args, why);

    printf("NOTIFICATION: ");
    if  (why)
        vprintf(why, args);
    else
        printf("debugStop(0)");

    printf("\n");

    va_end(args);

    BreakIfDebuggerPresent();
}

/*****************************************************************************/

/*
 * Does this constant need to be bigger?
 */
static  size_t    blockStop    = ;

/*****************************************************************************/
#endif // DEBUG
/*****************************************************************************/

size_t THE_ALLOCATOR_BASE_SIZE  = ;

bool   norls_allocator::nraInit(IEEMemoryManager* pMemoryManager, size_t pageSize, int preAlloc)
{
    bool    result = false;

    nraMemoryManager = pMemoryManager;

    nraPageList  =
    nraPageLast  = ;

    nraFreeNext  =
    nraFreeLast  = ;

    assert(THE_ALLOCATOR_BASE_SIZE != );

    nraPageSize  = pageSize ? pageSize : THE_ALLOCATOR_BASE_SIZE;

#ifdef DEBUG
    static ConfigDWORD fShouldInjectFault;
    nraShouldInjectFault = fShouldInjectFault.val(CLRConfig::INTERNAL_InjectFault) != ;
#endif    

    if  (preAlloc)
    {
        /* Grab the initial page(s) */

        setErrorTrap(NULL, norls_allocator *, pThis, this)  // ERROR TRAP: Start normal block
        {
            pThis->nraAllocNewPage();
        }
        impJitErrorTrap()  // ERROR TRAP: The following block handles errors
        {
            result = true;
        }
        endErrorTrap()  // ERROR TRAP: End
    }

    return  result;
}

/*---------------------------------------------------------------------------*/

void    *   norls_allocator::nraAllocNewPage(size_t sz)
{
    norls_pagdesc * newPage;
    size_t          sizPage;

    size_t          realSize = sz + sizeof(norls_pagdesc);
    if (realSize < sz)
        NOMEM();   // Integer overflow

    /* Do we have a page that's now full? */

    if  (nraPageLast)
    {
        /* Undo the "+=" done in nraAlloc() */

        nraFreeNext -= sz;

        /* Save the actual used size of the page */

        nraPageLast->nrpUsedSize = nraFreeNext - nraPageLast->nrpContents;
    }

    /* Make sure we grab enough to satisfy the allocation request */

    sizPage = nraPageSize;

    if  (sizPage < realSize)
    {
        /* The allocation doesn't fit in a default-sized page */

#ifdef  DEBUG
//      if  (nraPageLast) printf("NOTE: wasted %u bytes in last page\n", nraPageLast->nrpPageSize - nraPageLast->nrpUsedSize);
#endif

        sizPage = realSize;
    }

    /* Round to the nearest multiple of OS page size */

    if (!nraDirectAlloc())
    {
        sizPage +=  (DEFAULT_PAGE_SIZE - );
        sizPage &= ~(DEFAULT_PAGE_SIZE - );
    }

    /* Allocate the new page */

    newPage = (norls_pagdesc *)nraVirtualAlloc(, sizPage, MEM_COMMIT, PAGE_READWRITE);
    if  (!newPage)
        NOMEM();

#ifdef DEBUG
    newPage->nrpSelfPtr = newPage;
#endif

    /* Append the new page to the end of the list */

    newPage->nrpNextPage = ;
    newPage->nrpPageSize = sizPage;
    newPage->nrpPrevPage = nraPageLast;
    newPage->nrpUsedSize = ;  // nrpUsedSize is meaningless until a new page is allocated.
                               // Instead of letting it contain garbage (so to confuse us),
                               // set it to zero.

    if  (nraPageLast)
        nraPageLast->nrpNextPage = newPage;
    else
        nraPageList              = newPage;
    nraPageLast = newPage;

    /* Set up the 'next' and 'last' pointers */

    nraFreeNext = newPage->nrpContents + sz;
    nraFreeLast = newPage->nrpPageSize + (BYTE *)newPage;

    assert(nraFreeNext <= nraFreeLast);

    return  newPage->nrpContents;
}

// This method walks the nraPageList forward and release the pages.
// Be careful no other thread is doing nraToss at the same time.
// Otherwise, the page specified by temp could be double-freed (VSW 600919).

void        norls_allocator::nraFree(void)
{
    /* Free all of the allocated pages */

    while   (nraPageList)
    {
        norls_pagdesc * temp;

        temp = nraPageList;
               nraPageList = temp->nrpNextPage;

        nraVirtualFree(temp, , MEM_RELEASE);
    }
}

// This method walks the nraPageList backward and release the pages.
// Be careful no other thread is doing nraFree as the same time.
// Otherwise, the page specified by temp could be double-freed (VSW 600919).
void        norls_allocator::nraToss(nraMarkDsc &mark)
{
    void    *   last = mark.nmPage;

    if  (!last)
    {
        if  (!nraPageList)
            return;

        nraFreeNext  = nraPageList->nrpContents;
        nraFreeLast  = nraPageList->nrpPageSize + (BYTE *)nraPageList;

        return;
    }

    /* Free up all the new pages we've added at the end of the list */

    while (nraPageLast != last)
    {
        norls_pagdesc * temp;

        /* Remove the last page from the end of the list */

        temp = nraPageLast;
               nraPageLast = temp->nrpPrevPage;

        /* The new last page has no 'next' page */

        nraPageLast->nrpNextPage = ;

        nraVirtualFree(temp, , MEM_RELEASE);
    }

    nraFreeNext = mark.nmNext;
    nraFreeLast = mark.nmLast;
}

/*****************************************************************************/
#ifdef DEBUG
/*****************************************************************************/
void    *           norls_allocator::nraAlloc(size_t sz)
{
    void    *   block;

    assert(sz !=  && (sz & (sizeof(int) - )) == );
#ifdef _WIN64
    //Ensure that we always allocate in pointer sized increments.
    /* TODO-Cleanup:
     * This is wasteful.  We should add alignment requirements to the allocations so we don't waste space in
     * the heap.
     */
    sz = (unsigned)roundUp(sz, sizeof(size_t));
#endif

#ifdef DEBUG
    if (nraShouldInjectFault)
    {
        // Force the underlying memory allocator (either the OS or the CLR hoster)
        // to allocate the memory. Any fault injection will kick in.
        void * p = DbgNew();
        if (p)
        {
            DbgDelete(p);
        }
        else
        {
            NOMEM();  // Throw!
        }
    }
#endif    

    block = nraFreeNext;
            nraFreeNext += sz;

    if  ((size_t)block == blockStop) debugStop("Block at %08X allocated", block);

    if  (nraFreeNext > nraFreeLast)
        block = nraAllocNewPage(sz);

#ifdef DEBUG
    memset(block, UninitializedWord<char>(), sz);
#endif

    return  block;
}

/*****************************************************************************/
#endif
/*****************************************************************************/

size_t              norls_allocator::nraTotalSizeAlloc()
{
    norls_pagdesc * page;
    size_t          size = ;

    for (page = nraPageList; page; page = page->nrpNextPage)
        size += page->nrpPageSize;

    return  size;
}

size_t              norls_allocator::nraTotalSizeUsed()
{
    norls_pagdesc * page;
    size_t          size = ;

    if  (nraPageLast)
        nraPageLast->nrpUsedSize = nraFreeNext - nraPageLast->nrpContents;

    for (page = nraPageList; page; page = page->nrpNextPage)
        size += page->nrpUsedSize;

    return  size;
}

/*****************************************************************************
 * We try to use this allocator instance as much as possible. It will always
 * keep a page handy so small methods won't have to call VirtualAlloc()
 * But we may not be able to use it if another thread/reentrant call
 * is already using it.
 */

static norls_allocator *nraTheAllocator;
static nraMarkDsc       nraTheAllocatorMark;
static LONG             nraTheAllocatorIsInUse = ;

// The static instance which we try to reuse for all non-simultaneous requests

static norls_allocator  theAllocator;

/*****************************************************************************/

void                nraInitTheAllocator()
{
    THE_ALLOCATOR_BASE_SIZE = norls_allocator::nraDirectAlloc() ?
        (size_t)norls_allocator::MIN_PAGE_SIZE : (size_t)norls_allocator::DEFAULT_PAGE_SIZE;
}

void                nraTheAllocatorDone()
{
    // We chose not to call nraTheAllocator->nraFree() and let the memory leak.
    // Below is the reason (VSW 600919).

    // The following race-condition exists during ExitProcess.
    // Thread A calls ExitProcess, which causes thread B to terminate.
    // Thread B terminated in the middle of nraToss()
    // (through the call-chain of nraFreeTheAllocator()  ==> nraRlsm() ==> nraToss())
    // And then thread A comes along to call nraTheAllocator->nraFree() which will cause the double-free
    // of page specified by "temp".

    // These are possible fixes:
    // 1. Thread A tries to get hold on nraTheAllocatorIsInUse lock before
    //    calling theAllocator.nraFree(). However, this could cause the deadlock because thread B
    //    has already gone and therefore it can't release nraTheAllocatorIsInUse.
    // 2. Fix the logic in nraToss() and nraFree() to update nraPageList and nraPageLast in a thread safe way.
    //    But it needs careful work to make it high performant (e.g. not holding a lock?)
    // 3. The scenario of dynamically unloading clrjit.dll cleanly is unimportant at this time.
    //    We will leak the memory associated with other instances of morls_allocator anyway.

    // Therefore we decided not to call the cleanup code when unloading the jit. 

}

/*****************************************************************************/

norls_allocator *   nraGetTheAllocator(IEEMemoryManager* pMemoryManager)
{
    if (InterlockedExchange(&nraTheAllocatorIsInUse, ))
    {
        // Its being used by another Compiler instance
        return NULL;
    }

    if (nraTheAllocator == NULL)
    {
        // Not initialized yet

        bool res = theAllocator.nraInit(pMemoryManager, , );

        if (res)
        {
            // failed to initialize
            InterlockedExchange(&nraTheAllocatorIsInUse, );
            return NULL;
        }

        nraTheAllocator = &theAllocator;

        assert(nraTheAllocator->nraTotalSizeAlloc() == THE_ALLOCATOR_BASE_SIZE);
        nraTheAllocator->nraMark(nraTheAllocatorMark);
    }
    else
    {
        if (nraTheAllocator->nraGetMemoryManager() != pMemoryManager)
        {
            // already initialize with a different memory manager
            InterlockedExchange(&nraTheAllocatorIsInUse, );
            return NULL;
        }
    }

    assert(nraTheAllocator->nraTotalSizeAlloc() == THE_ALLOCATOR_BASE_SIZE);
    return nraTheAllocator;
}

void                nraFreeTheAllocator()
{
    assert (nraTheAllocator != NULL);
    assert(nraTheAllocatorIsInUse == );

    nraTheAllocator->nraRlsm(nraTheAllocatorMark);
    assert(nraTheAllocator->nraTotalSizeAlloc() == THE_ALLOCATOR_BASE_SIZE);

    InterlockedExchange(&nraTheAllocatorIsInUse, );
}

/*****************************************************************************/

　　开始吧，alloc.cpp是在ClrJit项目当中的，其中alloc.cpp会引用jitpch.h这个头文件。首先会定义：_MSC_VER 这个活动预处理块。如下图，关于，pragma hdrstop 的介绍可以点击链接去看看，这里就不再赘述。

　　这里有一个空方法叫做：

void                allocatorCodeSizeBeg(){}

　　我们碰到这种情况怎么去学习呢，很多人就被卡住了，其实这个问题解决方法很简单，可以利用VS的全局搜索功能。

　　很遗憾，没有地方用到，这只能证明2点，第一，这个函数可能是给外部调用的，第二，这可能是内部机制的一部分，但是我并不了解，从名字上看，是分配器的相关功能，请自行补脑，我不做解释，怕误导你们。

　　下面继续看代码：关于__cdecl 请看这里的介绍，如果有不懂的人，这个函数顾名思义，是debug调试停止的时候触发的函数，其中带2个参数，第一个是指针类型的char,你可以理解为string 常量（对应C#中的readonly string）,第二个参数是任意类型的任意参数，即你可以传入多个参数（类似C#中的params 参数）。

void    __cdecl     debugStop(const char *why, ...)
{
    va_list     args;

    va_start(args, why);

    printf("NOTIFICATION: ");
    if  (why)
        vprintf(why, args);
    else
        printf("debugStop(0)");

    printf("\n");

    va_end(args);

    BreakIfDebuggerPresent();
}

　　要研究上面的代码，你必须了解va_list这个东西，下面是va_list的定义：下面的翻译成中文就是，如果没有定义_VA_LIST_DEFINED，那么定义_VA_LIST_DEFINED，如果定义了_M_CEE_PURE 把va_list定义为ArgIterator,否则是char类型的指针变量。这里我们是第二种情况，第一种情况你就当空气，因为我也不太懂。另外关于_M_CEE_PURE的解释我在网上并没有找到，只知道在Math.h里面有过类似的定义，有知道的小伙伴可以提示下我。

#ifndef _VA_LIST_DEFINED
    #define _VA_LIST_DEFINED
    #ifdef _M_CEE_PURE
        typedef System::ArgIterator va_list;
    #else
        typedef char* va_list;
    #endif
#endif

　　好吧，现在我们知道了，va_list其实就是一个char指针，类似C#当中的String，简单点理解。下面我们来分析下下面的代码。

 va_start(args, why);

　　其中va_start的源码如下，它来自于stdarg.h文件，这是Visual studio的一个内置文件，：

#define va_start __crt_va_start

　　我们 来看看__crt_va_start的真容：它分为__vcrt_va_start_verify_argument_type和__crt_va_start_a

#define __crt_va_start(ap, x) ((void)(__vcrt_va_start_verify_argument_type<decltype(x)>(), __crt_va_start_a(ap, x)))

　　下面是上面所提到的2种类型的一些源码，可以看到，__vcrt_va_start_verify_argument_type只是做了一个类似“断言”的功能，否就抛出异常；而第二个函数__va_start里的真实代码如下图所示，有四种不同的表现形式，分布在4个.h头文件中，其中4种不同的情况下去调用。分别有_M_X64，_M_ARM64和_M_CEE_PURE || (defined _M_CEE && !defined _M_ARM && !defined _M_ARM64) 和_M_ARM，这其中牵扯到和汇编的相关知识，有兴趣的可以先学学汇编去.

        template <typename _Ty>
        void __vcrt_va_start_verify_argument_type() throw()
        {
            static_assert(!__vcrt_va_list_is_reference<_Ty>::__the_value, "va_start argument must not have reference type and must not be parenthesized");
        }

#define __crt_va_start_a(ap, x) ((void)(__va_start(&ap, x)))

　　中间的代码我就跳过了，都是一些print语句，打印日志的，其中va_end的真实代码为，其实很好理解，就是清空，因为C++中没有垃圾回收机制。

#define va_end   __crt_va_end

#define __crt_va_end(ap)        ((void)(ap = (va_list)0))

　　这个方法中的最后一个方法是：BreakIfDebuggerPresent，它的代码如下，它的意思是至少执行一次，如果有一只处于调试状态，它会进行判断，如果为0,FALSE那么继续循环，

#define BreakIfDebuggerPresent()                                            \
    do { if (IsDebuggerPresent()) DebugBreak(); }                           \
    while ()
#endif

　　其中IsDebuggerPresent和DebugBreak为windows的API函数，应该类似于WINDOWS进程挂起，其实我也没研究过。

#if (_WIN32_WINNT >= 0x0400) || (_WIN32_WINDOWS > 0x0400)
WINBASEAPI
BOOL
WINAPI
IsDebuggerPresent(
    VOID
    );

#endif

　　#if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP)

　　WINBASEAPI

　　VOID
　　WINAPI
　　DebugBreak(
　　VOID
　　);

#endif /* WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP) */

　　下面有一个有趣的东西，作者问了个问题，这个常量是不是应该更大？欢迎各位来讨论

/*
 * Does this constant need to be bigger?
 */
static  size_t    blockStop    = ;

　　其中size_t的源码如下，其实就是类似于blockStop的一个定义域吧，如果是64位windows那么就是无符号的long,否则为无符号的int类型。这个很好理解吧。

#ifdef _WIN64
    typedef unsigned __int64 size_t;
    typedef __int64          ptrdiff_t;
    typedef __int64          intptr_t;
#else
    typedef unsigned int     size_t;
    typedef int              ptrdiff_t;
    typedef int              intptr_t;
#endif

　　下面初始化了变量：

size_t THE_ALLOCATOR_BASE_SIZE  = ;

　　然后是初始化nra:

bool   norls_allocator::nraInit(IEEMemoryManager* pMemoryManager, size_t pageSize, int preAlloc)
{
    bool    result = false;

    nraMemoryManager = pMemoryManager;

    nraPageList  =
    nraPageLast  = ;

    nraFreeNext  =
    nraFreeLast  = ;

    assert(THE_ALLOCATOR_BASE_SIZE != );

    nraPageSize  = pageSize ? pageSize : THE_ALLOCATOR_BASE_SIZE;

#ifdef DEBUG
    static ConfigDWORD fShouldInjectFault;
    nraShouldInjectFault = fShouldInjectFault.val(CLRConfig::INTERNAL_InjectFault) != ;
#endif    

    if  (preAlloc)
    {
        /* Grab the initial page(s) */

        setErrorTrap(NULL, norls_allocator *, pThis, this)  // ERROR TRAP: Start normal block
        {
            pThis->nraAllocNewPage();
        }
        impJitErrorTrap()  // ERROR TRAP: The following block handles errors
        {
            result = true;
        }
        endErrorTrap()  // ERROR TRAP: End
    }

    return  result;
}

　　我们先来看看这个方法的声明，其中后2个参数有默认值：

bool            nraInit (IEEMemoryManager* pMemoryManager, size_t pageSize = , int preAlloc = );

　　我们先来看看IEEMemoryManager这个东西是何方神圣，先放整体代码，大家先不要看整体代码，怕搞不懂，我一句句来剖析：

#if defined(__cplusplus) && !defined(CINTERFACE)

    MIDL_INTERFACE("17713B61-B59F-4e13-BAAF-91623DC8ADC0")
    IEEMemoryManager : public IUnknown
    {
    public:
        virtual LPVOID STDMETHODCALLTYPE ClrVirtualAlloc(
            /* [in] */ LPVOID lpAddress,
            /* [in] */ SIZE_T dwSize,
            /* [in] */ DWORD flAllocationType,
            /* [in] */ DWORD flProtect) = ;

        virtual BOOL STDMETHODCALLTYPE ClrVirtualFree(
            /* [in] */ LPVOID lpAddress,
            /* [in] */ SIZE_T dwSize,
            /* [in] */ DWORD dwFreeType) = ;

        virtual SIZE_T STDMETHODCALLTYPE ClrVirtualQuery(
            /* [in] */ const void *lpAddress,
            /* [in] */ PMEMORY_BASIC_INFORMATION lpBuffer,
            /* [in] */ SIZE_T dwLength) = ;

        virtual BOOL STDMETHODCALLTYPE ClrVirtualProtect(
            /* [in] */ LPVOID lpAddress,
            /* [in] */ SIZE_T dwSize,
            /* [in] */ DWORD flNewProtect,
            /* [in] */ DWORD *lpflOldProtect) = ;

        virtual HANDLE STDMETHODCALLTYPE ClrGetProcessHeap( void) = ;

        virtual HANDLE STDMETHODCALLTYPE ClrHeapCreate(
            /* [in] */ DWORD flOptions,
            /* [in] */ SIZE_T dwInitialSize,
            /* [in] */ SIZE_T dwMaximumSize) = ;

        virtual BOOL STDMETHODCALLTYPE ClrHeapDestroy(
            /* [in] */ HANDLE hHeap) = ;

        virtual LPVOID STDMETHODCALLTYPE ClrHeapAlloc(
            /* [in] */ HANDLE hHeap,
            /* [in] */ DWORD dwFlags,
            /* [in] */ SIZE_T dwBytes) = ;

        virtual BOOL STDMETHODCALLTYPE ClrHeapFree(
            /* [in] */ HANDLE hHeap,
            /* [in] */ DWORD dwFlags,
            /* [in] */ LPVOID lpMem) = ;

        virtual BOOL STDMETHODCALLTYPE ClrHeapValidate(
            /* [in] */ HANDLE hHeap,
            /* [in] */ DWORD dwFlags,
            /* [in] */ const void *lpMem) = ;

        virtual HANDLE STDMETHODCALLTYPE ClrGetProcessExecutableHeap( void) = ;

    };

　　首先先决条件是，定义了C++而且没有定义CINTERFACE，CINTERFACE是什么我暂时还不太懂，不过没关系，这并不妨碍我学习源码。

#if defined(__cplusplus) && !defined(CINTERFACE)

　　从下面的我们 可以得到一个结论，它是一个类，并且它被IUnknown所继承。

IEEMemoryManager : public IUnknown

　　它有几个方法分别为：

• ClrVirtualAlloc
• ClrVirtualFree
• ClrVirtualQuery
• ClrVirtualProtect
• ClrGetProcessHeap
• ClrHeapCreate
• ClrHeapDestroy
• ClrHeapAlloc
• ClrHeapFree
• ClrHeapValidate
• ClrGetProcessExecutableHeap

英语好的同学，理解这些东西应该不是很难，其实你也没必要深入研究，反正认为：这货就是在CLR生命周期中一些必须做的“事情”而已。下面我们再回到nraInit方法里，下面的应该不用我多说了，初始化而已。

 nraMemoryManager = pMemoryManager;

    nraPageList  =
    nraPageLast  = ;

    nraFreeNext  =
    nraFreeLast  = ;

    assert(THE_ALLOCATOR_BASE_SIZE != );

    nraPageSize  = pageSize ? pageSize : THE_ALLOCATOR_BASE_SIZE;

　　如果处于调试模式，还会走如下代码，会从CLR的配置文件中（我也不知道在哪），寻找配置。

#ifdef DEBUG
    static ConfigDWORD fShouldInjectFault;
    nraShouldInjectFault = fShouldInjectFault.val(CLRConfig::INTERNAL_InjectFault) != ;
#endif 

　　下面是INTERNAL_InjectFault的源码。

CONFIG_DWORD_INFO_EX(INTERNAL_InjectFault, W("InjectFault"), , "", CLRConfig::REGUTIL_default)

　　其中不光是我困惑，看来大大们也困惑，大大还在犹豫是否要“注入”失败”的标志位，具体原因当然我也不知道。

#ifdef DEBUG
    bool            nraShouldInjectFault; // Should we inject fault?
#endif

　　如果参数里面preAlloc为true.

    if  (preAlloc)
    {
        /* Grab the initial page(s) */

        setErrorTrap(NULL, norls_allocator *, pThis, this)  // ERROR TRAP: Start normal block
        {
            pThis->nraAllocNewPage();
        }
        impJitErrorTrap()  // ERROR TRAP: The following block handles errors
        {
            result = true;
        }
        endErrorTrap()  // ERROR TRAP: End
    }

　下面我来分析下第一个函数setErrorTrap，其实这只是初始化JIT的一个异常处理机制的部分，从下面的代码中可以窥探下JIT的一些东西。代码没有截全。有兴趣的可以搜索下error.h文件，本文不再分析。

#define                 setErrorTrap(compHnd, ParamType, paramDef, paramRef) \
    struct __JITParam : ErrorTrapParam                                      \
    {                                                                       \
        ParamType param;                                                    \
    } __JITparam;                                                           \
    __JITparam.errc = CORJIT_INTERNALERROR;                                 \
    __JITparam.jitInfo = compHnd;                                           \
    __JITparam.param = paramRef;                                            \
    PAL_TRY(__JITParam *, __JITpParam, &__JITparam)                         \
    {                                                                       \
        ParamType paramDef = __JITpParam->param;

　　下面让我们看下nraAllocNewPage这个方法：

void    *   norls_allocator::nraAllocNewPage(size_t sz)
{
    norls_pagdesc * newPage;
    size_t          sizPage;

    size_t          realSize = sz + sizeof(norls_pagdesc);
    if (realSize < sz)
        NOMEM();   // Integer overflow

    /* Do we have a page that's now full? */

    if  (nraPageLast)
    {
        /* Undo the "+=" done in nraAlloc() */

        nraFreeNext -= sz;

        /* Save the actual used size of the page */

        nraPageLast->nrpUsedSize = nraFreeNext - nraPageLast->nrpContents;
    }

    /* Make sure we grab enough to satisfy the allocation request */

    sizPage = nraPageSize;

    if  (sizPage < realSize)
    {
        /* The allocation doesn't fit in a default-sized page */

#ifdef  DEBUG
//      if  (nraPageLast) printf("NOTE: wasted %u bytes in last page\n", nraPageLast->nrpPageSize - nraPageLast->nrpUsedSize);
#endif

        sizPage = realSize;
    }

    /* Round to the nearest multiple of OS page size */

    if (!nraDirectAlloc())
    {
        sizPage +=  (DEFAULT_PAGE_SIZE - );
        sizPage &= ~(DEFAULT_PAGE_SIZE - );
    }

    /* Allocate the new page */

    newPage = (norls_pagdesc *)nraVirtualAlloc(, sizPage, MEM_COMMIT, PAGE_READWRITE);
    if  (!newPage)
        NOMEM();

#ifdef DEBUG
    newPage->nrpSelfPtr = newPage;
#endif

    /* Append the new page to the end of the list */

    newPage->nrpNextPage = ;
    newPage->nrpPageSize = sizPage;
    newPage->nrpPrevPage = nraPageLast;
    newPage->nrpUsedSize = ;  // nrpUsedSize is meaningless until a new page is allocated.
                               // Instead of letting it contain garbage (so to confuse us),
                               // set it to zero.

    if  (nraPageLast)
        nraPageLast->nrpNextPage = newPage;
    else
        nraPageList              = newPage;
    nraPageLast = newPage;

    /* Set up the 'next' and 'last' pointers */

    nraFreeNext = newPage->nrpContents + sz;
    nraFreeLast = newPage->nrpPageSize + (BYTE *)newPage;

    assert(nraFreeNext <= nraFreeLast);

    return  newPage->nrpContents;
}

　　我只介绍几个重要的地方，第一个为norls_pagdesc这个结构体，它的访问权限是private。个人感觉这个东西是做链表功能用的，具体作用不详。

    struct norls_pagdesc
    {
        norls_pagdesc * nrpNextPage;
        norls_pagdesc * nrpPrevPage;
#ifdef DEBUG
        void    *       nrpSelfPtr;
#endif
        size_t          nrpPageSize;    // # of bytes allocated
        size_t          nrpUsedSize;    // # of bytes actually used. (This is only valid when we've allocated a new page.)
                                        // See norls_allocator::nraAllocNewPage.
        BYTE            nrpContents[];
    };

　　其中因为传入参数为0，所以真实大小为sizeof(norls_pagdesc)即上面结构体的成员变量的大小之和。

 size_t          realSize = sz + sizeof(norls_pagdesc);

　　如果出现参数异常，那么执行NOMEM，通知异常发生。

if (realSize < sz)
        NOMEM();   // Integer overflow

void DECLSPEC_NORETURN NOMEM()
{
#if MEASURE_FATAL
    fatal_NOMEM += ;
#endif // MEASURE_FATAL

    fatal(CORJIT_OUTOFMEM);
}

　　如果是到了链表表尾，其实作者很怀疑是否真正是这样的？

  if  (nraPageLast) /* Do we have a page that's now full? */
    {
        /* Undo the "+=" done in nraAlloc() */

        nraFreeNext -= sz;

        /* Save the actual used size of the page */

        nraPageLast->nrpUsedSize = nraFreeNext - nraPageLast->nrpContents;
    }

　　我们先来看看nraFreeNext 的介绍，作者说到：如果不为0，那么永远指向LAST。

    BYTE    *       nraFreeNext;        // these two (when non-zero) will
    BYTE    *       nraFreeLast;        // always point into 'nraPageLast'

　　好，现在回到alloc.cpp文件中，它的主要作用是为了更新新的使用过的空间大小。

        nraPageLast->nrpUsedSize = nraFreeNext - nraPageLast->nrpContents;

　　下面的代码是把norls_allocator里的nraPageSize赋值给sizePage以保障空间足够。

    /* Make sure we grab enough to satisfy the allocation request */

    sizPage = nraPageSize;

　　如果sizPage小于真实大小，那么把真实大小赋值给sizPage，这是什么原因造成的，其实我觉得这么写不太合理，作者也许做了一个硬编码吧。

    if  (sizPage < realSize)
    {
        /* The allocation doesn't fit in a default-sized page */

#ifdef  DEBUG
//      if  (nraPageLast) printf("NOTE: wasted %u bytes in last page\n", nraPageLast->nrpPageSize - nraPageLast->nrpUsedSize);
#endif

        sizPage = realSize;
    }

　　下面我们来看看，如果nraDirectAlloc是FALSE，那么执行如下代码，我们先来看看，sizePage是先+了DEFAULT_PAGE_SIZE 个单位，然后按位与了DEFAULT_PAGE_SIZE-1的反码个单位。如果你还不清楚什么是按位计算，那么请你复习下大学基础知识，本文不再做深入讨论。

    if (!nraDirectAlloc())
    {
        sizPage +=  (DEFAULT_PAGE_SIZE - );
        sizPage &= ~(DEFAULT_PAGE_SIZE - );
    }

　　下面我们再来看看nraDirectAlloc这个方法吧。其实就是返回一个BOOL类型的东西，至于这个方法的整体作用，可以看下面的英文注释。和上面的Config一样，都是做了配置。

inline bool norls_allocator::nraDirectAlloc()
{
    // When JitDirectAlloc is set, all JIT allocations requests are forwarded
    // directly to the OS. This allows taking advantage of pageheap and other gflag
    // knobs for ensuring that we do not have buffer overruns in the JIT.

    static ConfigDWORD fJitDirectAlloc;
    return (fJitDirectAlloc.val(CLRConfig::INTERNAL_JitDirectAlloc) != );
}

　　下面是分配新的page.

    newPage = (norls_pagdesc *)nraVirtualAlloc(, sizPage, MEM_COMMIT, PAGE_READWRITE);

　　方法如下：如果满足nraDirectAlloc==true,那么执行HeapAlloc方法，其中参数GetProcessHeap和HeapAlloc(堆分配)为调用WINDOW API。有兴趣的可以自己看看源码。

HeapAlloc(
    _In_ HANDLE hHeap,
    _In_ DWORD dwFlags,
    _In_ SIZE_T dwBytes
    );

　如果不满足nraDirectAlloc，那庅调用DbgNew方法，源码为：

inline void * DbgNew(size_t size)
{
    return ClrAllocInProcessHeap(, S_SIZE_T(size));
}

我把ClrAllocInProcessHeap的代码贴一下，有兴趣的可以自己去看看。我就不多做解释了。

inline LPVOID ClrAllocInProcessHeap(DWORD dwFlags, S_SIZE_T dwBytes)
{
    STATIC_CONTRACT_SUPPORTS_DAC_HOST_ONLY;
    if (dwBytes.IsOverflow())
    {
        return NULL;
    }

#ifndef SELF_NO_HOST
    return __ClrAllocInProcessHeap(dwFlags, dwBytes.Value());
#else
#undef HeapAlloc
#undef GetProcessHeap
    static HANDLE ProcessHeap = NULL;
    if (ProcessHeap == NULL)
        ProcessHeap = GetProcessHeap();
    return ::HeapAlloc(ProcessHeap,dwFlags,dwBytes.Value());
#define HeapAlloc(hHeap, dwFlags, dwBytes) Dont_Use_HeapAlloc(hHeap, dwFlags, dwBytes)
#define GetProcessHeap() Dont_Use_GetProcessHeap()
#endif
}

    LPVOID          nraVirtualAlloc(LPVOID lpAddress, SIZE_T dwSize, DWORD flAllocationType, DWORD flProtect)
    {
#if defined(DEBUG)
        assert(lpAddress ==  && flAllocationType == MEM_COMMIT && flProtect == PAGE_READWRITE);
        if (nraDirectAlloc())
        {
#undef GetProcessHeap
#undef HeapAlloc
            return ::HeapAlloc(GetProcessHeap(), , dwSize);
        }
        else
            return DbgNew(dwSize);
#else
        return nraMemoryManager->ClrVirtualAlloc(lpAddress, dwSize, flAllocationType, flProtect);
#endif
    }

　　下面的代码就是做一些基本的赋值和检查。

    if  (!newPage)
        NOMEM();

#ifdef DEBUG
    newPage->nrpSelfPtr = newPage;
#endif

　　下面就是把新的page追加到list的后端。

    /* Append the new page to the end of the list */

    newPage->nrpNextPage = ;
    newPage->nrpPageSize = sizPage;
    newPage->nrpPrevPage = nraPageLast;
    newPage->nrpUsedSize = ;  // nrpUsedSize is meaningless until a new page is allocated.
                               // Instead of letting it contain garbage (so to confuse us),
                               // set it to zero.

    if  (nraPageLast)
        nraPageLast->nrpNextPage = newPage;
    else
        nraPageList              = newPage;
    nraPageLast = newPage;

　　最后重新设置一下next和last指针，总之这个是个公共方法，只是nraInit里面只用到了为0的情况： pThis->nraAllocNewPage(0);

    /* Set up the 'next' and 'last' pointers */

    nraFreeNext = newPage->nrpContents + sz;
    nraFreeLast = newPage->nrpPageSize + (BYTE *)newPage;

    assert(nraFreeNext <= nraFreeLast);

    return  newPage->nrpContents;

　　回到init函数然后执行如下，结束nraInit方法

        impJitErrorTrap()  // ERROR TRAP: The following block handles errors
        {
            result = true;
        }
        endErrorTrap()  // ERROR TRAP: End

　　写完了，知道写得不太好，请指出错误，轻喷。晚安。