LED blinking while loop not working AVR timer 0

Question

It is strange and rare. I am trying to blink a LED at PD0 at 2hz, but it doesn't work. Seems to me that while loop doesn't work at all. I have triple checked the code. what is the problem here?

uint8_t c=0; 

int main(void){

DDRD= 1<<0 ;
TCCR0 = (1 << CS02) |  (1 << CS00); // (clk_i/o )/1024 ( Prescaler) 

TIMSK |= 1<< TOIE0;
sei();

  while (1){ 
  if ( c >=30 ) { PORTD ^= 1<<0; c=0; }                //61.035/30= 2.034 hz
  }

  }

ISR (TIMER0_OVF_vect){
  c++;   //16000000/1024/256=61.035 hz
  }  

Of course, I know the solution. It's by adding a random line of code to the while loop, and it works again. Strange!! What would be the problem? compiler bug??

uint8_t c=0; 

int main(void){

DDRD= 1<<0 ;
TCCR0 = (1 << CS02) |  (1 << CS00); // (clk_i/o )/1024 ( Prescaler) 

TIMSK |= 1<< TOIE0;
sei();

  while (1){ 
  if ( c >=30 ) { PORTD ^= 1<<0; c=0; }                //61.035/30= 2.034 hz
  _NOP();
  }

  }

ISR (TIMER0_OVF_vect){
  c++;   //16000000/1024/256=61.035 hz
  }  

Answer 1

By NOT declaring it volatile you give the compiler permission to optimise.

In the first loop, the compiler knows the value of C at all times, because nothing else in the loop can modify C (and it isn't volatile) so, because C is always 0, the loop doesn't need to do anything. The code is working perfectly because that's what you asked for.

In the second loop, you call a procedure (oops, void function) NOP() which could do absolutely anything. So, unless the compiler does whole program analysis over all the libraries (and it doesn't), how does it know if NOP() modifies C?

It doesn't, so it can't make any assumptions about C's value and has to re-fetch it from memory.

Ths second program isn't working, it does what you expect by accident.

If the "random additional line" didn't involve a call - e.g. it was an inline assembly NOP instruction, you would probably see different behaviour.

Declare C volatile.