[maemo-developers] Performance of floating point instructions

[From: Sivan Greenberg]

Hi Alberto!

On Wed, Mar 10, 2010 at 9:55 AM, Alberto Mardegan <
mardy at users.sourceforge.net> wrote:

> Hi all,
>  in maemo-mapper I have a lot of code involved in doing transformations
> from latitude/longitude to Mercator coordinates (used in google maps, for
> example), calculation of distances, etc.
>
> I'm trying to use integer arithmetics as much as possible, but sometimes
> it's a bit impractical, and I wonder if it's really worth the trouble.
>
> Does one have any figure about how the performance of the FPU is, compared
> to integer operations?
>
> A practical question: should I use this way of computing the square root:
>
>
> http://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Binary_numeral_system_.28base_2.29
>
> (but operating on 32 or even 64 bits), or would I be better using sqrtf()
> or sqrt()?
>
>
> Does anyone know any tricks to optimize certain operations on arrays of
> data?
>


Basically, what we did with ThinX OS, is have a full blown soft-float
toolchain which then used the already proven and highly optimized GCC's
stack floating point operations. However , Maemo is not soft float, so I'd
recommend to experiment with rebuilding Mapper using such a soft float
enabled toolchain, statically linked to avoid glitches to system's libc or
have a seperat LD_LIBRARY_PATH to avoid memory hogging, and see where it
gets you.

IMHO this is the best way to do FP optimization. We have experimented with
it alot, including sqrtf and friend to no significant improvement.

Sivan
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