barriergen.c

// Example of unwanted reordering gcc 9.3
// https://godbolt.org/z/5AL_wh
// 
// The flag |ready| should be set after the load to the message.
//
// If I use size_t as type for |i| then all three functions give
// the correct ordering, so the compiler needs "something to chew on",
// in this case the sign extension to a 64 bit type, for it to emit 
// the stores in the wrong order.

#define READ_ONCE(x) (*(volatile typeof(x) *) &(x))
#define WRITE_ONCE(x, val) ((*(volatile typeof(x) *) &(x)) = (val))

volatile int ready;
int message[100];

// Ready is stored before message.
void faulty(int i) {
  message[i] = 42;
  ready = 1;
}

// The compiler barrier ensure that stores to message and ready can't be reordered. 
void ordered(int i) {
  message[i] = 42;
  asm("" : : : "memory");
  ready = 1;
}

// The volatile loads can't be reordered so message and ready are stored in the right order.
void volatile_ordered(int i) {
  WRITE_ONCE(message[i], 42);
  WRITE_ONCE(ready,1);
}