Re: [avr-gcc-list] Problem with delay loop

[Royce Pereira]

Hi,

On Fri, 28 Sep 2007 14:47:26 +0530, David Brown <address@hidden> wrote:

>
> This is probably in the FAQ somewhere - if not, it should be!
>
> The compiler is smart enough to figure out that your delay function does
> no useful work - thus the optimiser does not generate any code.  This is
> correct compilation - it's your code that is wrong.  The difference is
> that the newer version of the compiler is smarter than the older version
> (or newer makefiles have higher optimisation enabled by default).
>
> The correct way to write such a loop is:
>
> void delay(unsigned int del_cnt) {
>       volatile unsigned int n = del_cnt;
>       while (n--);
> }

I tried that and -Yikes !! Here's what I get :( :
Compare it to the short & sweet loop of the earlier avr-gcc version (below)...
//-------------------------------------------
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
  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

000002e8 <wr_lcd_hi>:

       return;
    }
//=======================

Output of the older -WinAVR-20060421-version (and without the 'volatile' thingy 
too!)
>> //==========================================
>> 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;
>>     }
>> //=======================================
>>

Thanks,

--Royce.
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