AVR Freaks

It is working now the error is due to TIMSK, it shall be set with OCIE0A.  I have one more question which confusing to me due to my stupidity.  

This Timer 0 I wanted to have millis working in 1 millis sec.   I have the Timer1 which i need it to work in PLL mode.  The fuse I need to set to PLL Clock and CKDIV by 8 have to be untick. I did not use external crystal. 

At this PLL mode fuse, what is my F_CPU for the Timer0 ?   'Will I still get 1 millis sec in millis ?  

Anytip where can i learn more about simulator in AS7.  I face quite a lots of strange stuff like variable Value in simulator is unknown location and some variable show optimised away.  

My millis() code is working but during the build, there is still two warning but build successful..  These are the warning I hilighted in the below picture.t 

Is this two warning dangerous and could cause some problem ?

Attached is my code, Kindly help. 

* millis.c
 * ATTiny861 test the millis functions in Atmel Studio 7
 * using Timer0 at PLL, the F_CPU will be in 16MHz
 * Fuse setting L:D1, H:D7, E: FF
 */

#define F_CPU 16000000UL
#include <avr/io.h>
#include <util/delay.h>
#include <util/atomic.h>
#include <avr/interrupt.h>


// Store value of millisecond since uC started
volatile unsigned long millis_value;
unsigned long interval_value;
unsigned long elapsed_time;

// This function is used to get millis_value
unsigned long millis()
{
	unsigned long m;
	uint8_t oldSREG = SREG;
	// Disable interrupts while we read millis_value or we might get an
	// inconsistent value (e.g. in the middle of a write to millis_value)
	ATOMIC_BLOCK(ATOMIC_FORCEON){
	cli();
	m = millis_value;
	SREG = oldSREG;
	
	return millis_value;
	}
}


// Timer 0 output compare interrupt service routine
ISR(TIMER0_COMPA_vect)
{
	// Increment millisecond every 1ms
	millis_value++;	
		
}


void init_millis()
{
    
	/* Set timer 0 on CTC mode that will compare match every 1ms */
	// Timer/Counter 0 initialization
	// Mode: CTC top=OCR0A
	// TCCR0A = (1<<TCW0) if use 16 bit mode
		
	// TCCR0A = TCW0 | ICEN0 | INCC0 | ICES0 | ACIC0 | - | - | CTC0
	//            0  |  0    |    0  |     0 |  0    | 0 | 0 | 1
 	TCCR0A |= 1<< CTC0;
	
	// TCCR0B = |- | -| - | TSM | PSR0 | CS02 | CS01 | CS00 
	//          | 0|0 | 0 |  0  | 0    | 0    |  1   | 1   //64    
	//if 16MHz divide by 64 will get 249
	//if 8MHz divide by 64 since we only use 8 bits, cs0=1 and cs1 =1
	//if 1MHz devide by 8 since we only use 8 bits, cs01 =1
	TCCR0B |= (1<<CS00) | (1<<CS01) ;
	
	TCNT0L = 0x00;
	
	// 0CR0A value = (0.001 s x Fcpu/N) - 1
	// if 16MHz set at 254 (FE) fuse setting on PLL
	// if 8MHz set at 124 (7C) 
	OCR0A = 0xFE;//DEC IS 254
    
	/* Enable compare match interrupt for timer 0 */
	// Enable Compare A Interrupt(s) initialization
	TIMSK |= 1 << OCIE0A;//OCIE0A IS SET
	
	/* Remember to Enable global interrupt by using sei */
}
	
int main(){
	DDRB = 0xFF;
	init_millis();
    sei();  
	unsigned long previous_millis = 0 ;
	while(1)
	{
		PORTB |= (1<<PB4);
		unsigned long current_millis = millis();
		elapsed_time = current_millis - previous_millis;
    if (elapsed_time > 1000)
	{
    PORTB &= ~(1<<PB4);
     _delay_ms(100);    
    previous_millis = millis();
	}
	}
  return 0;
}

Thank you Kartman. The error all gone.  You are genious !