[Gdal-dev] Windows build from CVS

[Frank Warmerdam]

Clay, Bruce wrote:
> This is one of those really trivial things but since some of the
> operations in GDAL are performed millions or billions of times wouldn't
> it make since to not perform redundant operations where possible.
> 
> Example: in the code shown below the operation 127 * 127 is performed 4
> times X the number of times this method is called.  If the number was
> changed to 16129 several cycles could be saved.  While it is true that
> computers are getting faster and faster the databases are getting larger
> as well.
 >
 >        M[M13] = byte[4] * fabs((double) byte[4]) * M[M11] / (127*127);

Bruce,

All but the most brain dead of modern optimizing compilers will compute
constant expressions at compile time and the emitted code will have the
resulting value.  I often like to leave expressions like the above in
the code as it is easier to relate it to the original documented
equations or in general to understand the meaning.

I think a more compelling case could be made that I should have optimized
the fabs() call to just mask off the sign bit of the byte[4] which did not
occur to me when I was writing the code.  Converting a signed byte to
floating point and then invoking a floating point function just to clear
the sign deep in per-pixel calculations is pretty pricey.

In this same source file I also used pow() to compute powers of two and the
very general purpose pow() can be very expensive for something that should be
achievable with left integer bit shifts.  However, a brief attempt to
replace the pow() didn't work out, so I abandoned that effort.

I do try to be sensitive to performance issues but ultimately I only
apply careful optimization in code that I feel will be heavily used or
where a client has emphasized the importance of performance.

The good news is that AIRSAR images aren't getting any bigger.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent