[avr-gcc-list] [Bug] Initialization for _delay_loop_2 OptimizedOut

[Bjarne Laursen]

I have seen this problem too. The intialization of the counter register is 
not present when using the original code:

/* 16-bit count, 4 cycles/loop */
static inline void
_delay_loop_2(unsigned int __count)
{
        asm volatile (
                "1: sbiw %0,1" "\n\t"
                "brne 1b"
                : "=w" (__count)
                : "0" (__count)
        );
}

However, if you change the function to:

/* 16-bit count, 4 cycles/loop */
static inline void
Mydelay_loop_2(unsigned int __count)
{
        asm volatile (
                "1: sbiw %0,1" "\n\t"
                "brne 1b"
                :
                : "w" (__count)
                : "0"
        );
}

it seems to work.
I don't know why the original doesn't work. But maybe, since the out is not 
used, the compiler thinks it does not need to supply the input. Funny that 
the loop haven't been optimized away too. (But that is be cause of the 
'volatile' I think)
The later code does not generate an output but destroys the input registers.


Unfortunely it doesnt work. I now tested this code:

           for (d=0; d<100; d++) Mydelay_loop_2(617); //delay 100ms
 bcc:   80 e0           ldi     r24, 0x00       ; 0
 bce:   e9 e6           ldi     r30, 0x69       ; 105
 bd0:   f2 e0           ldi     r31, 0x02       ; 2
 bd2:   31 97           sbiw    r30, 0x01       ; 1
 bd4:   f1 f7           brne    .-4             ; 0xbd2
 bd6:   8f 5f           subi    r24, 0xFF       ; 255
 bd8:   84 36           cpi     r24, 0x64       ; 100
 bda:   d8 f3           brcs    .-10            ; 0xbd2

The compiler thinks that the inline function doesnt change the input 
registers, so they get initialized only once outside the for-loop.