[GRASS-user] r.neighbors velocity

[Markus Metz]

On Sat, Jun 29, 2013 at 1:26 PM, Hamish <hamish_b at yahoo.com> wrote:
> Markus Metz wrote:
>
>> Some more results with Sören's test program on a Intel(R) Core(TM) i5
>> CPU M450 @ 2.40GHz (2 real cores, 4 fake cores) with gcc 4.7.2 and
>> clang 3.3
>>
>> gcc -O3
>> v is 2.09131e+13
>>
>> real    2m0.393s
>> user    1m57.610s
>> sys    0m0.003s
>>
>> gcc -Ofast
>> v is 2.09131e+13
>>
>> real    0m7.218s
>> user    0m7.018s
>> sys    0m0.017s
>
>
> nice. one thing we need to remember though is that it's not entirely
> free, one thing -Ofast turns on is -ffast-math,
> """
>  This option is not turned on by any -O option besides -Ofast since it can
>  result in incorrect output for programs that depend on an exact
>  implementation of IEEE or ISO rules/specifications for math functions. It
>  may, however, yield faster code for programs that do not require the
>  guarantees of these specifications.
> """
>
> which may not be fit for our purposes.
>
>
> With the ifort compiler there is '-fp-model precise' which allows only
> optimizations which don't harm the results. Maybe gcc has something
> similar.

In gcc, you can turn of -ffoo with -fno-foo, maybe this way you can
use -Ofast -fno-fast-math to preserve IEEE specifications.
>
> Glad to see -floop-parallelize-all in gcc 4.7, it will help us identify
> places to focus OpenMP work on.
>
>
> Hamish
>