[maemo-developers] Performance of floating point instructions

[From: Laurent Desnogues]

On Wed, Mar 10, 2010 at 7:29 PM, Alberto Mardegan
<mardy at users.sourceforge.net> wrote:
> Alberto Mardegan wrote:
>>
>> Does one have any figure about how the performance of the FPU is, compared
>> to integer operations?
>
> I added some profiling to the code, and I measured the time spent by a
> function which is operating on an array of points (whose coordinates are
> integers) and trasforming each of them into a geographic coordinates
> (latitude and longitude, floating point) and calculating the distance from
> the previous point.
>
> http://vcs.maemo.org/git?p=maemo-mapper;a=shortlog;h=refs/heads/gps_control
> map_path_calculate_distances() is in path.c,
> calculate_distance() is in utils.c,
> unit2latlon() is a pointer to unit2latlon_google() in tile_source.c
>
>
> The output (application compiled with -O0):
>
>
> double:
>
> map_path_calculate_distances: 110 ms for 8250 points
> map_path_calculate_distances: 5 ms for 430 points
>
> map_path_calculate_distances: 109 ms for 8250 points
> map_path_calculate_distances: 5 ms for 430 points
>
>
> float:
>
> map_path_calculate_distances: 60 ms for 8250 points
> map_path_calculate_distances: 3 ms for 430 points
>
> map_path_calculate_distances: 60 ms for 8250 points
> map_path_calculate_distances: 3 ms for 430 points
>
>
> float with fast FPU mode:
>
> map_path_calculate_distances: 50 ms for 8250 points
> map_path_calculate_distances: 2 ms for 430 points
>
> map_path_calculate_distances: 50 ms for 8250 points
> map_path_calculate_distances: 2 ms for 430 points
>
>
> So, it seems that there's a huge improvements when switching from doubles to
> floats; although I wonder if it's because of the FPU or just because the
> amount of data passed around is smaller.
> On the other hand, the improvements obtained by enabling the fast FPU mode
> is rather small -- but that might be due to the fact that the FPU operations
> are not a major player in this piece of code.

The "fast" mode only gains 1 or 2 cycles per FP instruction.
The FPU on Cortex-A8 is not pipelined and the fast mode
can't change that :-)

> One curious thing is that while making these changes, I forgot to change the
> math functions to there float version, so that instead of using:
>
> float x, y;
> x = sinf(y);
>
> I was using:
>
> float x, y;
> x = sin(y);
>
> The timings obtained this way are surprisingly (at least to me) bad:
>
> map_path_calculate_distances: 552 ms for 8250 points
> map_path_calculate_distances: 92 ms for 430 points
>
> map_path_calculate_distances: 552 ms for 8250 points
> map_path_calculate_distances: 91 ms for 430 points
>
> Much worse than the double version. The only reason I can think of, is the
> conversion from float to double and vice versa, but is it really that
> expensive?

This looks odd given that the 2 additional instructions
take 5 and 7 cycles.

> Anyway, I'll stick to using 32bit floats. :-)

As long as it fits your needs that seems wise :)


Laurent