Re: highest bit, statically determined

[Bill Findlay <yaldnif.w@blueyonder.co.uk>]

On 29/09/2012 22:36, in article
506769fb$0$6580$9b4e6d93@newsspool3.arcor-online.net, "Georg Bauhaus"
<rm.dash-bauhaus@futureapps.de> wrote:

> On 29.09.12 21:16, Bill Findlay wrote:
> 
> functionfirst_1_bit
> 
> :-)
> 
>> I can now confirm that with GNAT GPL 2012 at -O3 it does inline and fold,
>> but I now see that you want the result to be static as well as the operand,
>> and this does not achieve that.
> 
> Daringly, I have tried to steal the idea and try a comparison (out of
> curiosity, not for the static thing). GCC performs simple tail call
> elimination (explaining the Shift parameter)!
> 
>     function First_1_Bit_A (y : octet; Shift : Integer := 0)  return Natural
> is
>     begin
>        if y >= 2**4  then
>           if y >= 2**6 then
>              return Shift + 7 + Boolean'Pos (y >= 2**7);
>           else
>              return Shift + 5 + Boolean'Pos (y >= 2**5);
>           end if;
>        else
>           if Y = 0 then
>              return 0;
>           else
>              return First_1_Bit_A (y * 2**4, Shift => -4);
>           end if;
>        end if;
>     end First_1_Bit_A;

These bogus parameters, used to pretend that data is not intrinsically
variable, are as objectionable as using goto statements to synthesize while
loops, IMNSHO.

> Don't know if that's as readily adaptable to other word sizes, but it might
> make the functionist happier. ;-)

Fortunately, I have no interest in keeping functionists (or the adherents of
any other religion) happy. 8-)

-- 
Bill Findlay
with blueyonder.co.uk;
use  surname & forename;