Is there any way to get MSVC to pass structs arguments in registers on x64?

Question

For a function with signature:

struct Pair { void *v1, *v2 };
void f(Pair p);

compiled on x64, I would like Pair's fields to be passed via register, as if the function was:

void f(void *v1, void *v2);

Compiling a test with gcc 4.2.1 for x86_64 on OSX 10.6, I can see this is exactly what happens by examining the disassembly. However, compiling with MSVC 2008 for x64 on Windows, the disassembly shows that Pair is passed on the stack. I understand that platform ABIs can prevent this optimization; does anyone know any MSVC-specific annotations, calling conventions, flags, or other hacks that can get this to work?

Thank you!

Answer 1

what if you do this ?

   void f( void *ps ){
       struct Pair *p = (struct Pair *)ps;
   }

or

void f( unsigned long addr ){
    struct Pair *p = (struct Pair *)addr;
}

struct Pair pp;

f( reinterpret_cast<unsigned long>(&pp) );

Answer 2

My VS2008 x64 compiler doesn't do the same thing yours does. The struct fits in an XMM register, it passes a pointer to the copy of the pair object in a register:

    Pair p;
    f(p);
000000013F2B1189  movaps      xmm6,xmmword ptr [p] 
000000013F2B118E  lea         rcx,[p] 
000000013F2B1193  movdqa      xmmword ptr [p],xmm6 
000000013F2B1199  call        f (13F2B1000h) 

Nothing is passed on the stack here.