Example of using gcc inline assembly syntax. NOTE: last time I checked clang, didn't support Intel assembly syntax, so AT&T it is.

inline_asm.c

static void test_asm(u8 *data, u64 bytes) {
#if defined(__aarch64__)
  __asm__ volatile (
      "cbz %[bytes], 1f\n\t"
      "mov x9, #0\n\t"
      "0: strb w9, [%[data], x9]\n\t"
      "add x9, x9, #1\n\t"
      "cmp %[bytes], x9\n\t"
      "b.ne 0b\n\t"
      "1:\n\t"
      : /* no output */
      : [data] "r" (data), [bytes] "r" (bytes)
      : "memory", "x9");
#elif defined(__x86_64__)
  __asm__ volatile (
      "testq %[bytes], %[bytes]\n\t"
      "je 1f\n\t"
      "xorl %%eax, %%eax\n\t"
      "0: movb %%al, (%[data],%%rax)\n\t"
      "incq %%rax\n\t"
      "cmpq %%rax, %[bytes]\n\t"
      "jne 0b\n\t"
      "1:\n\t"
      : /* no output */
      : [data] "r" (data), [bytes] "r" (bytes)
      : "memory", "rax");
#else
  #error Not implemented
#endif // #if defined(__aarch64__)
}