Re: [avr-gcc-list] How to (efficeiently !!!) test a bit within a multi-byte integer ?

[Ian Caddy]

Hi Vince,

All your new code is doing is checking that address is non-zero as it is 
a logical AND with a non-zero number, which would get optimised out.

I haven't tried this, but it might be better:

unsigned char temp;

temp = (address >> 16) & 0xFF;

if(temp & 0x04)
{
   /* Your code */
}

This is just off the top of my head...hope it works for you.

regards,

Ian Caddy



Vincent Trouilliez wrote:
> > I replaced the '&' operator with the more appropriate '&&' one.... works
> > much better... 5 instructions instead of 115 (3 + 6*18 + 4) for the
> > previous code.
> > So that's indeed about 20 times faster. Although I still don't get why
> > it can't just use one single SBRC/S instruction instead of these 4 cp or
> > cpc instructions... I guess I am still missing something in the bit test
> > department...
>
>
> >                 if (address && 0x00040000)
> >    24ec:        21 15           cp      r18, r1
> >    24ee:        31 05           cpc     r19, r1
> >    24f0:        41 05           cpc     r20, r1
> >    24f2:        51 05           cpc     r21, r1
> >    24f4:        11 f0           breq    .+4             ; 0x24fa
>
>
>
>
> Hmmm, '&&' might give much tighter code than '&', but my program no
> longer works ! So it's still not the proper way to test a bit then,
> dear... :o(((
>
>
> --
> Vince
>
>
>
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--
Ian Caddy
Goanna Technologies Pty Ltd
+61 8 9221 1860