来自不相关类的虚拟功能表条目 [英] Virtual Function Table entry from class that is not related

问题描述

我正在浏览一个简单的C ++ Windows程序(我没有源代码，只有二进制文件)的VFT(VMT)，这些程序是由Visual Studio编译并进行了某种优化的.

I am browsing through VFTs (VMTs) of a simple C++ Windows program (I don't have a source code, only binary), compiled by Visual Studio with some sort of optimization on.

我注意到这是使用继承和多态性.我找到了该程序具有的每个类的struct s_RTTIBaseClassArray的位置.在该位置，有一个指向struct _s_RTTIBaseClassDescriptor的指针数组.基类描述符数组应该为您提供有关当前类所源自的所有类的信息.

I noticed that is uses inheritance and polymorphism. I found the location of struct s_RTTIBaseClassArray for each class that the program has. In that location there is an array of pointers to struct _s_RTTIBaseClassDescriptor. The array of base class descriptors should give you information about all the classes that the current class is derived from.

虚函数(方法)表是一个表，其中包含指向当前类的所有虚函数的指针.但是在一些类的VFT中，我找到了一个指向虚拟方法的指针，该方法实际上属于另一个类(根据基类数组)，该类与当前类无关.下面的示例:

Virtual Function (Method) Table is a table that contains pointers to all the virtual functions of the current class. However in VFT of a few classes I found a pointer to a virtual method that actually belongs to a different class that is (acording to the Base Class Array) not related to the current class. Example below:

ClassA_BaseClassArray:
            dd offset ClassA_BaseClassDescriptor
            dd offset ClassB_BaseClassDescriptor ; ClassA inherits from ClassB

ClassB_BaseClassArray:
            dd offset ClassB_BaseClassDescriptor

ClassC_BaseClassArray:
            dd offset ClassC_BaseClassDescriptor

ClassA_VMT: 
            dd offset ClassA_VM1 ; virtual method of ClassA
            dd offset ClassA_VM2
            dd offset ClassB_VM2 ; virtual method of ClassB - override
            dd offset ClassC_VM3 ; virtual method of ClassC - NOTHING TO DO HERE
            dd offset ClassA_VM3

这个例子很简短，实际的类有更多的虚方法.

The example is short, the actual classes have much more virtual methods.

在检查ClassC_VM3之后，我发现它仅由两条指令组成:

After examination of ClassC_VM3 I noticed, that it consists of just two instructions:

mov    eax, [ecx+10h]
retn

到目前为止，我发现大约有3个类似于该示例的VMT，无关的方法总是这么短.

I found about 3 VMTs similar to this example so far, the unrelated method is always this short.

我的问题是:是什么原因造成的? ClassC_VM3的代码是否与某些ClassA方法的代码相同，所以编译器只是对其进行了优化?

My question is: what is causing this? Could the code of ClassC_VM3 be identical to the code of some ClassA method, so the compiler just optimized it out?

推荐答案

这可能是由 COMDAT折叠，该优化将具有完全相同的机器代码的功能合并为一个.由于它是一个简单的函数，因此机会很大.

This might be caused by COMDAT folding, an optimization that merges functions that have the same exact machine code into one. Since it's such a simple function the chances of that are good.

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