On Sat, Aug 16, 2014 at 8:41 PM, peter green <plugwash@p10link.net> wrote:
> Bob Ammerman wrote:
>> I think Peter's response to this will get you going. However, it is wort=
h
>> noting that a shift is likely to be _much_ faster than a multiply (unles=
s of
>> course the compiler is smart enough to see that this multiply can be
>> implemented by a shift):

Actually, I tried this as a shift first...when that wasn't working, I
switched to the multiply approach! Now I have a better idea of why it
wasn't working...

> I would hope the compiler for an 8-bit microcontroller would be smart
> enough to turn either shifts or multiplys into byte copies.
>
> Interestingly C18 will sometimes turn a shift into a multiply. The PIC18
> is actually better at multiplication than it is at shifting since it has
> a multiplier but not a barrel shifter.

As a test, I looked at the list output of both options...and they are
identical. Here's an example:

   535                           ;ws2812_lib.c: 81: redtemp =3D red * 0x100=
00;
   536  000284  C03E  F041             movff    set_LED@red,??_set_LED
   537  000288  6A42                   clrf    (??_set_LED+1)& (0+255),c
   538  00028A  6A43                   clrf    (??_set_LED+2)& (0+255),c
   539  00028C  6A44                   clrf    (??_set_LED+3)& (0+255),c
   540  00028E  C042  F044             movff    ??_set_LED+1,??_set_LED+3
   541  000292  C041  F043             movff    ??_set_LED,??_set_LED+2
   542  000296  6A42                   clrf    ??_set_LED+1,c
   543  000298  6A41                   clrf    ??_set_LED,c
   544  00029A  C041  F04D             movff    ??_set_LED,set_LED@redtemp
   545  00029E  C042  F04E             movff    ??_set_LED+1,set_LED@redtem=
p+1
   546  0002A2  C043  F04F             movff    ??_set_LED+2,set_LED@redtem=
p+2
   547  0002A6  C044  F050             movff    ??_set_LED+3,set_LED@redtem=
p+3

Doing a << 16 produces the same code as above.

> greentemp =3D (uint32_t)green << 16;
> redtemp =3D (uint32_t)red << 8;
> bluetemp =3D blue;

Yup! that works!

Thank you very much for the help guys!

Josh
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