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Re: Problem with delay loop

by David Brown-6 :: Rate this Message:

There are good reasons for much of the stuff produced by the use of
"volatile" - since the local variable is volatile, it has to be put on
the stack. Generating a stack frame on the AVR needs a bit of messing
around in the prologue and epilogue, including turning off interrupts
while modifying the stack pointer (blame Atmel for that whoopsie).

What would be really nice is if gcc accepted the concept of "register
volatile" variables. I've used them on other compilers in the past to
get exactly the effect we are looking for here. But that was in the
days when "register" was a useful hint to an otherwise fairly poor
optimiser - gcc, I believe, ignores "register" for local variables.

Slightly better code can be generated with:

void delay2(unsigned int n) {
static volatile unsigned int x;
x = n;
while (x--) ;
}

But even better is:

void delay3(volatile unsigned int n) {
while (n--) {
asm volatile(" " : : );
};
}

mvh.,

David

Royce Pereira wrote:

> Hi all,
>
> OK fine I agree.
>
> we have to use 'volatile' and all.
>
> But why must it generate such horrid code...
> (I reproduce the comparison again below to get the *real* issue into focus)
>
> The compiler output with the 'correct' code ('volatile' used):
> //---------------------------------------------------
> void delay(unsigned int del_cnt)
> {
> 2aa: cf 93 push r28
> 2ac: df 93 push r29
> 2ae: cd b7 in r28, 0x3d ; 61
> 2b0: de b7 in r29, 0x3e ; 62
> 2b2: 22 97 sbiw r28, 0x02 ; 2
> 2b4: 0f b6 in r0, 0x3f ; 63
> 2b6: f8 94 cli ;<-----disabling interrupts? Why?
> 2b8: de bf out 0x3e, r29 ; 62
> 2ba: 0f be out 0x3f, r0 ; 63
> 2bc: cd bf out 0x3d, r28 ; 61
> volatile unsigned int n = del_cnt;
> 2be: 9a 83 std Y+2, r25 ; 0x02
> 2c0: 89 83 std Y+1, r24 ; 0x01
> while(n--);
> 2c2: 89 81 ldd r24, Y+1 ; 0x01
> 2c4: 9a 81 ldd r25, Y+2 ; 0x02
> 2c6: 01 97 sbiw r24, 0x01 ; 1
> 2c8: 9a 83 std Y+2, r25 ; 0x02
> 2ca: 89 83 std Y+1, r24 ; 0x01
> 2cc: 89 81 ldd r24, Y+1 ; 0x01
> 2ce: 9a 81 ldd r25, Y+2 ; 0x02
> 2d0: 8f 5f subi r24, 0xFF ; 255
> 2d2: 9f 4f sbci r25, 0xFF ; 255
> 2d4: b1 f7 brne .-20 ; 0x2c2 <delay+0x18>
> 2d6: 22 96 adiw r28, 0x02 ; 2
> 2d8: 0f b6 in r0, 0x3f ; 63
> 2da: f8 94 cli
> 2dc: de bf out 0x3e, r29 ; 62
> 2de: 0f be out 0x3f, r0 ; 63
> 2e0: cd bf out 0x3d, r28 ; 61
> 2e2: df 91 pop r29
> 2e4: cf 91 pop r28
> 2e6: 08 95 ret
>
> return;
> }
> //=======================
>
> Output of the older -WinAVR-20060421-version (with the alleged 'wrong' C code)
>>> //==========================================
>>> void delay(word cnt)
>>> {
>>> while(cnt--);
>>> 286: 01 97 sbiw r24, 0x01 ; 1
>>> 288: 2f ef ldi r18, 0xFF ; 255
>>> 28a: 8f 3f cpi r24, 0xFF ; 255
>>> 28c: 92 07 cpc r25, r18
>>> 28e: d9 f7 brne .-10 ; 0x286 <delay>
>>> 290: 08 95 ret
>>> return;
>>> }
>>> //=======================================
>>>
>
> So which is the right output we want? Obviously the second.
>
> Agreed not using 'volatile' optimises the code to a mere 'ret'. I'm ok with that.
>
> But should'nt the 'correct' code produce the same output (the shorter version above)??
>
> Thanks,
>
> --Royce.
>
>
>

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