val+ref.as

/*
  your obj pointer came in the first position of the stack
  so you will load it on a register
*/

movl  (%esp), %eax ;

/*
  then you want to do obj.a += 6, if we suppose
  that we're talking about records not dictionaries
  and we know that obj.a is on position 4
  on x86 you can do
*/
addl  $6, 4(%eax) ;

/*
  internally this is what is happening, but atomically
  so the cost of indirection is the same as if it was passed by value
  because the only reference to the object is the first mov
*/
movl  4(%eax), %ebx ; /* load */
addl  $6, %ebx ; /* op */
movl  %ebx, 4(%eax) /* store */