How to do inline assembly in C++ (Visual Studio 2010)

Question

I'm writing a performance-critical, number-crunching C++ project where 70% of the time is used by the 200 line core module.

I'd like to optimize the core using inline assembly, but I'm completely new to this. I do, however, know some x86 assembly languages including the one used by GCC and NASM.

All I know:

I have to put the assembler instructions in _asm{} where I want them to be.

Problem:

• I have no clue where to start. What is in which register at the moment my inline assembly comes into play?

Answer 1

You can access variables by their name and copy them to registers. Here's an example from MSDN:

int power2( int num, int power )
{
   __asm
   {
      mov eax, num    ; Get first argument
      mov ecx, power  ; Get second argument
      shl eax, cl     ; EAX = EAX * ( 2 to the power of CL )
   }
   // Return with result in EAX
}

Using C or C++ in ASM blocks might be also interesting for you.