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

[Klaus Rudolph]

Please use optimizer! Something like -O2 -O3 -Os ... as you need!

Simplify your delay loop:
void delay(volatile word cnt) { ...

Have fun!


Royce Pereira schrieb:
> 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.