volatile keyword does not work for gcc inline assembly

Question

I need to run this simple inline assembly code:

#include <stdio.h>

int count;

int main() {

  count = 0;
  for (int i = 0; i < 10; i++) {
    asm volatile ("incl count");  // count++
  }

  printf("count=%d\n", count); 

  return 0;
}

It works fine (printing count=10) until I turn on optimization (gcc -O1), in which case it prints count=0. I read that the "volatile" qualifier will prevent the optimizer to put the code out of loop. But it seems to have no effect here.

Answer 1

Problem found. I had to define the global variable "count" as volatile. The problem was not putting asm block out of the loop. The compiler simply replaced count with 0 in printf.

volatile int count;

int main() {

  count = 0;
  for (int i = 0; i < 10; i++) {
    asm volatile ("incl count");  // count++
  }

  printf("count=%d\n", count); 

  return 0;
}