How do member functions access and modify object data in x86 assembly?

Understanding Object Storage and Member Function Access in x86 Assembly

Object Storage

In x86 assembly, objects, including both structs and classes, are stored as contiguous blocks of memory. Within these blocks, member variables are arranged sequentially in the order they were declared. The address of each member variable increases as the block is traversed.

Member Function Access

Member functions can access objects via the this pointer. In non-member functions, the object's address is passed implicitly as the first argument. However, this is not the case for member functions due to the implicit this pointer.

The this pointer points to the beginning of the memory block where the object is stored. Using this pointer, member functions can directly access and modify the object's data members.

For example, if we have a class named foo with member variables m_a and m_b, and a member function inc_a that increments m_a, the assembly code for inc_a might look like this:

foo::inc_a():
    mov eax, DWORD PTR [rdi+4]      # eax = this->m_a
    lea edx, [rax+1]                # edx = eax + 1
    mov DWORD PTR [rdi+4], edx      # this->m_a = edx
    ret

Virtual Member Functions

In the case of classes with virtual member functions, an additional level of indirection is introduced. Each instance of the class stores a pointer to a virtual function table (vtable). The vtable contains pointers to the actual implementations of the virtual member functions.

When a virtual member function is called, the program first jumps to the vtable entry for that function. The jump target is the actual function implementation, which is then executed.

Object Storage Optimization

While objects are typically stored in memory, they may also be stored in registers. Compilers can optimize code to avoid placing objects in memory if they can be kept in registers throughout their use. This optimization is possible when the object is small enough to fit in a register and its members are actively used.

For example, a function that returns a small struct by value may not allocate memory for it. Instead, the compiler may pack the struct's members into registers and return them directly.

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