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 with a member size_t a[], a pointer to such a struct and an index both of which are generated within the asm block.

Now I need to address that element in asm

asm (
    "mov %[displ](%[s], %[index], 8), %%rbx"
    : [s] "+r" (s)
    , [index] "+r" (i)
    : "memory", "cc", "rax", "rbx"
);

How can I encode displ into the asm block? Passing offsetof(struct s, a) as an immediate prefixes it with $ and generates invalid assembly.

asm (
    "mov %[displ](%[s], %[index], 8), %%rbx"
    : [s] "+r" (s)
    , [index] "+r" (i)
    : [displ] "i" (offsetof(struct s, a))
    : "memory", "cc", "rax", "rbx"
);

Answer 1

It actually is possible, using the %c... operand modifier:

#include <stddef.h>
#include <stdint.h>

struct s
{
  int a, b;
};

int foo (struct s *s, int i)
{
  int r;
  asm (
       "movl %c[displ](%[s],%[index],8), %[r]\n\t"
       : [r] "=r" (r)
       : [s] "r" (s) , [index] "r" ((uintptr_t)i),
         [displ] "e" (offsetof(struct s, b))
       :
       );

  return r;
}

Thanks where thanks is due – found that here. There’s a gcc mailing list posting referring to this as well; the keywords there are “output substitution”.
The stackoverflow posting What does %c mean in GCC inline assembly code? also has an explanation about %c in particular.