How do Objects Work at the Assembly Level in x86?

How Objects Work in x86 at the Assembly Level

In x86 assembly, objects are stored contiguously in memory, similar to structs. They occupy a block of memory, and individual members can be accessed by calculating their offsets from the base address of the object.

Regarding classes (with member functions), the situation is slightly different when virtual member functions are involved. These functions require an additional vtable pointer to be stored as the first member of the object. The vtable contains pointers to the actual implementations of the member functions.

Class Layout with Virtual Function:

class foo {
  int m_a;
  int m_b;
  virtual void inc_a(void);  // virtual member function
  void inc_b(void);
};

In memory, this class will be laid out as follows:

• Vtable Pointer (8 bytes): Points to the vtable for the actual type of the object.
• m_a (4 bytes): Member variable a.
• m_b (4 bytes): Member variable b.

Virtual Function Dispatch:

When calling a virtual function, the following steps occur:

1. The vtable pointer is loaded from the object's memory location into a register.
2. The register value is used to index into the vtable and retrieve the address of the correct function implementation.
3. A memory-indirect jump is performed to the target address.

Optimization: Devirtualization

Compilers can sometimes Devirtualize calls to virtual functions if they can determine the actual type of the object at compile time. In such cases, the overhead of the virtual function dispatch can be eliminated by directly calling the specific function implementation.

Objects and Inlining

In some cases, objects may not occupy physical memory. For example, when a struct is returned by value from an inline function, the compiler may optimize by passing and returning the individual members directly in registers.

In conclusion, objects in x86 assembly are conceptually similar to structs, with the addition of a vtable pointer for classes with virtual member functions. The layout of objects in memory and the process of virtual function dispatch have significant implications for performance and code optimization.

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