[IPOL discuss] use of Eigen for PCA/SVD computation in demo blm_color_dimensional_filtering

[Nicolas Limare]

> We should do some experiments to compare performances before I'm
> convinced.  But in my view, chosing SSE instructions should be the
> task of the compiler, not of an optimized library.

In principle I agree, but I have an experience eith the ASIFT
code. The computation of distances between vectors (sums of absolute
values) was time consuming, and the compiler was not using vector
instructions. By modifying the data types and instructions order, we
could get the compiler to use the some vector instructions and it
speeded up the code bu a measurable factor. Without hardcoding these
optimizations, with portable, pure C and perfectly readable code.

> There is no reason to think that a trivial two-line C program that
> computes the dot product can not be as fast as a fancy library.  The
> compiler should be fancy enough to select the appropriate
> instructions by itself.

CPUs are now quite different fromt he typical Turing machine, and
properly choosing and ordering the instructions has an important
impact on the performances. Jérôme Darbon has much more expertise
and experience with these low-level details, he can probably explain
in detail (and correct my mistakes).

And, of course, I completely agree with Miguel that we don'ty want to
deal with this level of optimization. We just want to choose the best
possible solution if it is easily available, portable, perfectly
readable, etc... But it's good to know how to speed up a code,
sometimes we need it. If simply compiling with...
    cc foo.o -Lcblas -o program
...results in a program 20% faster than...
    cc foo.o gsl_cblas.o -o program
...why not benefit from this speedup?

-- 
Nicolas LIMARE - CMLA - ENS Cachan    http://www.cmla.ens-cachan.fr/~limare/
IPOL - image processing on line                          http://www.ipol.im/
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