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Subject: Re: highest bit, statically determined
From: Vadim Godunko <vgodunko@gmail.com>
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Date: 2012-09-30T08:01:54-07:00
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On Sunday, September 30, 2012 1:37:00 AM UTC+4, Georg Bauhaus wrote:
> 
> 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)!
> 
If you don't need static function, you can use following function (GCC can precompute its result for static value):

with Interfaces.C; use Interfaces.C;

function Bit (X : Unsigned) return Unsigned;
pragma Inline (Bit);

function Bit (X : Unsigned) return Unsigned is

   function CLZ (X : Unsigned) return Unsigned;
   pragma Import (Intrinsic, CLZ, "__builtin_clz");

begin
   if X = 0 then
      --  XXX Return what you want.

      return 0;

   else
      return Unsigned'Size - CLZ (X);
   end if;
end Bit;