Question

I know that when calling a non-static member function of an object in c++, the this pointer is passed in ecx register. What about static functions, as there is no this pointer, does the compiler use ecx register to pass one of the regular parameters in this case?

Edit – I’m talking about the cdecl calling convention here.

Answer 1

What ecx (or any other register) is used for when calling functions depends on the calling convention.

For instance, consider the C++ code

struct S {
    static void f( int, int ) { }
    void g( int ) { }
};

void g() {
    S::f( 3, 4 );
    S s;
    s.g( 3 );
}

Compiling this with Microsoft Visual Studio 2010 (64bit) via

cl /c main.cpp

And then dumping the assembly via

dumpbin /disasm main.obj

Gives this for the g function:

?g@@YAXXZ (void __cdecl g(void)):
  0000000000000000: 48 83 EC 38        sub         rsp,38h
  0000000000000004: BA 04 00 00 00     mov         edx,4
  0000000000000009: B9 03 00 00 00     mov         ecx,3
  000000000000000E: E8 00 00 00 00     call        ?f@S@@SAXHH@Z
  0000000000000013: BA 03 00 00 00     mov         edx,3
  0000000000000018: 48 8D 4C 24 20     lea         rcx,[rsp+20h]
  000000000000001D: E8 00 00 00 00     call        ?g@S@@QEAAXH@Z
  0000000000000022: 48 83 C4 38        add         rsp,38h
  0000000000000026: C3                 ret

Notice how the first call to the static S::f method has the first argment (3) passed in ecx and the second argment (4) in edx. So the answer to your question is:

Yes. For this particular compiler, ecx is used to pass one of the parameters in a static function call.