Hi,

We know that a matrix in Eigen is either column indexed or row indexed, default is column indexed.

when we have a matrix X, it could be on either left side or right side of a multiply operation:

$a=Xv$ <-- X is on the left side, from cache locality perspective, it is better that X is row indexed this case
or
$N = M*X$ <--- X is on the right side, from cache locality perspective, it is better that X is column index-ed in this case.

If my program run both above operations continuously all the time, how does Eigen manage to do it efficiently? does Eigen keep a copy of $X^{T}$ all the time?

Any thoughts on this? Thanks