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 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.

Answer 2

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 <stddef.h>

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;
}