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Date: Sat, 29 Sep 2012 23:36:59 +0200
From: Georg Bauhaus <rm.dash-bauhaus@futureapps.de>
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Subject: Re: highest bit, statically determined
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Date: 2012-09-29T23:36:59+02:00
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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;

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