__fastcall

In __fastcall calling convention, the arguments are passed to the register if possible.

• The first two arguments are passed in register ECX and EDX. The remaining arguments are passed on the stack from Right to Left.
• Callee cleans the stack.

Going back to the assembly code of the main function, the statement

main()::return = fastcallAdd(7, 8, 9, 10);

In fastcallAdd, the arguments are pushed onto the stack in Right to Left order, then the 2 arguments that remain are pushed onto registers EDX and ECX respectively. (Or we can say the first 2 arguments are passed onto ECX and EDX in Left to Right order)

The function with the calling convention __fastcall is fastcallAdd(), which is called on line 48 in the main function. We have seen how the arguments are passed onto the stack. We know that since the caller i.e. main is not doing anything about the stack pointer, it is the callee’s job to clean up the stack.

fastcallAdd()

Explanation

Here, we can see that, the stack pointer is incremented with 8 bytes only. We are only popping two 4-byte integers from the stack. This is because the other 2 arguments are passed into ECX and EDX registers. So, we only need to pop the 2 arguments that are on the stack.

In C/C++ programming, a calling convention is a set of rules that specify how a function will be called. You might have seen keywords like __cdecl or __stdcall when you get linking errors. For example:

error LNK2019: unresolved external symbol "void __cdecl A(void)" (?A@@YAXXZ) referenced in function _main

Here, we see __cdecl being used. It is one of the calling conventions in C/C++. You can also see code like this in 3rd party libraries.

For example:

extern __m128i __cdecl _mm256_mask_cvtepi32_epi16(__m128i, __mmask8, __m256i);

What are caller and callee?

When we call a function, a stack frame is allocated for that function, arguments are passed to the function, and after the function does its work, the allocated stack frame is deallocated and control is passed to the calling function.

The function that calls the subroutine is called the caller. The function that gets called(i.e. subroutine) by the caller is called the callee.

In the above code, main() is the caller and func() is the callee.