AVR-Chat

The GCC compiler optimiser seems to be doing a weird thing to me... I'd like to understand it. it seems to require a volatile qualifier when I am not expecting it.

I have the following variables defined
unsigned char LedLevel0=0;
unsigned char LedLevel1=0;
volatile unsigned char AdcVal[4];

AdcVal holds readings from the ADC and is loaded in an interrupt routine

main loop:

	//the main loop
	while(1)
	{
		LedLevel0=AdcVal[0];
		LedLevel1=AdcVal[1];
	}

compiler output:

	//the main loop
	while(1)
	{
		LedLevel0=AdcVal[0];
 3ea:	80 91 86 01 	lds	r24, 0x0186
		LedLevel1=AdcVal[1];
 3ee:	80 91 87 01 	lds	r24, 0x0187
 3f2:	fb cf       	rjmp	.-10     	; 0x3ea <main+0x8>

i.e. it's loading the AdcVal into r24, but never stores it into the other variable.

if I change the main loop to add a call to a delay routine then it works correctly. Also if I declare LedLevel0/1 as volatile.
But in my mind it shouldn't need to be declared volatile, nothing else is changing it.

	//the main loop
	while(1)
	{
		LedLevel0=AdcVal[0];
		LedLevel1=AdcVal[1];
		TimeWait(1);
	}

compiler output:
	//the main loop
	while(1)
	{
		LedLevel0=AdcVal[0];
 3ea:	80 91 86 01 	lds	r24, 0x0186
 3ee:	80 93 06 01 	sts	0x0106, r24
		LedLevel1=AdcVal[1];
 3f2:	80 91 87 01 	lds	r24, 0x0187
 3f6:	80 93 07 01 	sts	0x0107, r24
		TimeWait(1);
 3fa:	81 e0       	ldi	r24, 0x01	; 1
 3fc:	90 e0       	ldi	r25, 0x00	; 0
 3fe:	f4 cf       	rjmp	.-24     	; 0x3e8 <main+0x6>


-- 
Tim Mitchell