[cairo] Re: Some other bits...

[Behdad Esfahbod]

On Thu, 18 Aug 2005, [ISO-8859-1] Kristian Høgsberg wrote:

> There's a standard trick for this, though.  If you want to guarantee
> arithmetic (signed) right shift, you can say:
>
> 	a = (b >> shift) | ~(-1 >> shift);
>
> On architectures that implement >> as arithmetic right shift, the last
> part of the expression, ~(-1 >> shift), should optimize to 0 and not
> cause any overhead.  On architectures that implement >> as logical
> shift, the ~(-1 >> shift) evaluates to exactly the missing bits.

Then you fall into another hole:  it assumes 2-complete arithmetic ;).


> It does feel a bit like a theoretical exercise, though.
>
> Kristian

--behdad
http://behdad.org/