[eigen] runtime API to configure the cache/blocking parameters

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I've committed a change allowing to configure the cache size and the related blocking size parameters at runtime. It use the trick that consists in storing runtime variables as static members of a global function. This way no need to have a shared binary lib just for that.

Here is the related "public" documented API:

/** \returns the currently set cpu cache size (in bytes) used to estimate the ideal blocking size parameters */
std::ptrdiff_t ei_cpuCacheSize();

/** Set the cpu cache size (in bytes) for blocking.
  * This function also automatically set the blocking size parameters for each scalar type using the following formula:
  * \code
  *  max_k = 4 * sqrt(cache_size/(64*sizeof(Scalar)));
  *  max_m = 2 * k;
  * \endcode
  * overwriting custom values set using the ei_setBlockingSizes function.
  * \sa ei_setBlockingSizes */
void ei_setCpuCacheSize(std::ptrdiff_t cache_size);

/** Set the blocking size parameters \a maxK and \a maxM for the scalar type \a Scalar.
  * Note that in practice there is no distinction between scalar types of same size.
  * \sa ei_setCpuCacheSize */
template<typename Scalar>
void ei_setBlockingSizes(std::ptrdiff_t maxK, std::ptrdiff_t maxM);

/** \returns in \a makK, \a maxM the blocking size parameters for the scalar type \a Scalar.
  * \sa ei_setBlockingSizes */
template<typename Scalar>
void ei_getBlockingSizes(std::ptrdiff_t& maxK, std::ptrdiff_t& maxM);

You can also see the complete diff there: http://bitbucket.org/eigen/eigen/changeset/af6d26ce2778 to see how it is implemented in practice.

This API is just a proposal, if someone has better ideas...


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