Re: [avr-gcc-list] optimization and interrupt

[Keith Gudger]

I have to say that the optimizer is brilliant!  It subtracts 113 from the
value, then subtracts 24 (113 + 24 = 137), and looks for the carry flag.
Should work, no?

Keith

On Wed, 9 Mar 2005, [iso-8859-1] Jörg Krein wrote:

> Hi,
> avr-gcc behaves strange when optimizing interrupt code.
> In my interrupt routine I compare a variable against two constants.
> I added the code at the end of this posting.
> After compiling I found only one comparison (113) in the assembler code:
> 
> movw r24,r20          
> sts (lastcapt)+1,r19  
> sts lastcapt,r18      
> subi r24,lo8(-(-113)) 
> sbci r25,hi8(-(-113)) 
> sbiw r24,24           
> brsh .L1              
> 
> After changing optimization level to 0 i got:
> 
> std Y+3,r24
> std Y+4,r25
> ldd r24,Y+1
> ldd r25,Y+2           
> sts (lastcapt)+1,r25  
> sts lastcapt,r24      
> ldd r24,Y+3           
> ldd r25,Y+4           
> cpi r24,113           
> cpc r25,__zero_reg__  
> brlo .L1              
> ldd r24,Y+3           
> ldd r25,Y+4           
> cpi r24,137           
> 
> So here a two comparisons (113, 137) as expected.
> It helps to declare diffcapt to volatile but that doesn't
> make sense to me as it is only used in the interrupt
> I hope someone has a logical answer, let me believe
> in gcc again.
> 
> Regards, Joerg
> 
> 
> volatile uint16_t     lastcapt;
> 
> SIGNAL( SIG_INPUT_CAPTURE1 )
> {
>       uint16_t actcapt;
>       uint16_t diffcapt;
> 
>       actcapt = ICR1;
> 
>       diffcapt = actcapt - lastcapt;
>       lastcapt = actcapt;
> 
>       if( (diffcapt > 112)  &&  (diffcapt < 137) )
>       {...}
> }
> 
> 
> 
> 
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