[cairo] Patches for speeding up _cairo_fixed_from_double

[Pedro de Medeiros]

On 11/6/06, Behdad Esfahbod <behdad at behdad.org> wrote:
> On Mon, 2006-11-06 at 15:49 -0800, Daniel Amelang wrote:
> >
> > > So, can you elaborate?  How and why does it fail?
> >
> > In single-precision mode, the new function returns zero for all input.
> > This is because the large add that is performed overflows the FPU when
> > it's only operating on 32-bits.
>
> Got it.  Thanks.


A similar approach for single is possible, but losing precision,
because the size of single and cairo_fixed_t are the same and there
are exponent and signal bit, both being left out:

==================================
#define CAIRO_MAGIC_NUMBER_FIXED_16_6 (196608.0)

cairo_fixed_t
_cairo_fixed_from_single(float f)
{
   union {
      float f;
      uint32_t n;
   } u;

   u.f = f + CAIRO_MAGIC_NUMBER_FIXED_16_6;

   return (u.n & 0x3FFFFF) << 10;
}
==================================

(This little code is not endianness aware though.)


Cheers.
--
Pedro de Medeiros - Computer Science - University of Brasília
Home Page: http://www.nonseq.net - Linux User No.: 234250