How are Objects Stored and Accessed in x86 Assembly?

Understanding Object Storage in Assembly (x86)

In x86 assembly, objects are stored as contiguous blocks of memory. They are not inherently associated with classes like in higher-level languages; instead, they're simply sections of bytes that can be manipulated by the program.

Struct and Class Storage

Structs and classes are stored identically, except that classes with virtual members have an implicit "vtable" added as the first element. This vtable contains pointers to each virtual function's implementation, allowing for dynamic binding.

Member Function Access

Member functions are accessed through an implicit "this" pointer, which is passed as the first argument to the function. This pointer points to the object whose members are being accessed.

Virtual Function Dispatch

When a virtual function is called, the compiler checks the object's vtable to determine the correct function implementation. This dynamic behavior allows for polymorphic behavior in which different derived classes can override base class virtual functions.

Register Optimization

The compiler may choose to optimize away memory allocation for structs, especially for small structures or local variables. It can keep the struct's members in CPU registers, eliminating the need for memory access.

Example

Consider the following code:

struct foo {
  int m_a;
  int m_b;

  void inc_a() { m_a++; }

  void inc_b() { m_b++; }
};

The assembly code for the inc_b function would be:

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

In this code, the implicit "this" pointer is passed in the rdi register, and the values are stored 4 bytes apart within the object, as determined by the compiler.

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