Question

Question: How can I access a member variable in assembly from within a non-POD class?

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Elaboration:

I have written some inline assembly code for a class member function but what eludes me is how to access class member variables. I’ve tried the offsetof macro but this is a non-POD class.

The current solution I’m using is to assign a pointer from global scope to the member variable but it’s a messy solution and I was hoping there was something better that I dont know about.

note: I’m using the G++ compiler. A solution with Intel syntax Asm would be nice but I’ll take anything.

example of what I want to do (intel syntax):

class SomeClass
{
  int* var_j;
  void set4(void)
  {
    asm("mov var_j, 4"); // sets pointer SomeClass::var_j to address "4"
  }
};

current hackish solution:

int* global_j;
class SomeClass
{
  int* var_j;
  void set4(void)
  {
    asm("mov global_j, 4"); // sets pointer global_j to address "4"
    var_j = global_j;       // copy it back to member variable :(
  }
};

Those are crude examples but I think they get the point across.

Answer 1

This is all you need:

__asm__ __volatile__ ("movl $4,%[v]" : [v] "+m" (var_j)) ;

Edited to add: The assembler does accept Intel syntax, but the compiler doesn’t know it, so this trick won’t work using Intel syntax (not with g++ 4.4.0, anyway).