C++ custom calling convention

While reverse engineering I came around a very odd program that uses a calling convention that passes one argument in eax ( very odd compiler ?? ). I want to call that function now and I don't know how to declare it, IDA defines it as

bool __usercall foo<ax>(int param1<eax>, int param2);

where param1 is passed in the eax register. I 开发者_Go百科tried something like

bool MyFoo(int param1, int param2) 
{
    __asm mov eax, param1;
    return  reinterpret_cast<bool(__stdcall *)(int)>(g_FooAddress)(param2);
}

However, unfortunately my compiler makes use of the eax register when pushing param2 on the stack, is there any way how I can make this clean without writing the whole call with inline assembler? (I am using Visual Studio if that matters)

There are "normal" calling conventions which pass arguments via registers. If you are using MSVC for example, __fastcall.

http://en.wikipedia.org/wiki/X86_calling_conventions#fastcall

You cannot define your own calling conventions, but I would suggest that you do create a wrapper function which does its own calling / cleanup via inline assembly. This is probably the most practical to achieve this effect, though you could also probably do it faster by using __fastcall, doing a bit of register swapping, then jmp to the correct function.

There's more to a calling convention than argument passing though, so option #1 is probably the best as you'll get full control over how the caller acts.