[Vorbis-dev] Fixed x64 patch for libvorbis r15108

[Markus Ewald]

xiphmont at xiph.org wrote:
> Sorry, originally replied to only Ivo by accident:
>
> "Round toward closest" is the intended behavior cross-platform
> (including negative numbers), although Vorbis is not sensitive to
> specific roundoff behavior.  Noise normalization in the encoder will
> mostly override the effects of dodgy rounding.
>
>   
Thanks, already picked that up from the sources.

I seem to have none of the platforms available where 'truncate' is the 
default rounding scheme, but as long as floor() gives consistent results 
(:D) the new default rounding code you suggested should always work now.

The SSE2 code I introduced for x64 builds always performs 'toward 
closest' roundings (SSE2 actually uses different instructions for 
different rounding schemes, there's no error-prone switchable rounding 
mode like in the FPU).

--

Interestingly, even if I use trunc() to force a 'truncate' rounding, the 
original code would lead to incorrect results: -2.9 -> 2 and a 
conditional (x < 0) ? (int)(x-.5) : (int)(x+.5) produces correct 
results. At least with IEEE 754 floats, where only a bit indicates 
negative numbers, 'truncate' appears to be the same as 'towards zero' 
rounding, the behavior I get consistently on the machines I can test on 
(msvc-win32, msvc-win64, gcc-win32, gcc-linux).

> Monty
>   

-Markus-