## MethodTable内存空间分配中加法运算算法解析

在分析MethodTable具体分配内存实现的时候，看到了计算MethodTable的大小，然后分配空间的算法。其中有个加法运算实现的非常赞，特地截取出来。

所有的MethodTable的分配，都是通过methodtable中的一个static方法AllocagteNewMT来实现的，该方法定义如下：

MethodTable * MethodTable::AllocagteNewMT(EEClass *pClass,

DWORD dwVtableSlots,

DWORD dwGCSize,

DWORD dwNumInterfaces,

DWORD numGenericArgs,

DWORD dwNumDicts,

DWORD cbDict,

BaseDomain *pDomain,

BOOL isInterface,

BOOL fHasGenericsStaticsInfo,

BOOL fNeedsRemotableMethodInfo,

BOOL fNeedsRemotingVtsInfo,

, AllocMemTracker *pamTracker

)

下面是该方法中计算大小的一段，采用模板来忽略类型带来的影响：

DWORD cbTotalSize = 0;

DWORD cbDicts = 0;

if (!ClrSafeInt<DWORD>::multiply(dwNumDicts, sizeof(TypeHandle*), cbDicts) ||

ThrowHR(COR_E_OVERFLOW);

// Returns true if safe, false on overflow

static inline bool addition(T lhs, T rhs, T &result)

{

//check for T first.

if(IsSigned())

{

//test for +/- combo

if(!IsMixedSign(lhs, rhs))

{

//either two negatives, or 2 positives, not mixed symbols

if(rhs < 0)

{

//two negatives

if(lhs < (T)(MinInt() - rhs)) //remember rhs < 0

{

return false;

}

//ok

}

else

{

//two positives

if((T)(MaxInt() - lhs) < rhs)

{

return false;

}

//OK

}

}

//else overflow not possible

result = lhs + rhs;

return true;

}

else //unsigned, and two symbols is mixed

{

if((T)(MaxInt() - lhs) < rhs)

{

return false;

}

result = lhs + rhs;

return true;

}

}

static bool IsSigned()

{

return( (T)-1 < 0 );

}

//Check if lhs and rhs is mixed Sign symbols

static bool IsMixedSign(T lhs, T rhs)

{

return ((lhs ^ rhs) < 0);

}

//both of the following should optimize away

static T MinInt()

{

if(IsSigned())

{

return (T)((T)1 << (BitCount()-1));

}

else

{

return ((T)0);

}

}

static T MaxInt()

{

if(IsSigned())

{

return (T)~((T)1 << (BitCount()-1));

}

//else

return (T)(~(T)0);

}

lbq1221110@Cnblogs.  11.5 ; first post at sscli.cnblogs.com

posted on 2008-11-05 18:12  lbq1221119  阅读(2053)  评论(5编辑  收藏  举报