GCC extended asm, struct element offset encoding

Question

I am trying to write a small piece of my code in GCC style extended asm (x86-64 target) and am having trouble encoding struct offsets.

I have a struct s

Answer 1

Your only option is to use Intel syntax. GCC, of course, can generate insn like mov off(base, index, scale), but does this on the level of whole MEM RTL expressions, i.e. not having offset, base, etc like individual operands.

So, Intel syntax, compile the following with gcc -c -masm=intel x.c:

#include 

struct s
{
  int a, b;
};

int foo (struct s *s, int i)
{
  int r;
  asm (
       "mov %[r], dword ptr [%[s]+%[index]*8 + %[displ]] "
       : [r] "=r" (r)
       : [s] "r" (s) , [index] "r" (i),
         [displ] "e" (offsetof(struct s, b))
       :
       );

  return r;
}