But the stackoverflow answer is right, and accounts for the fact that in the majority of cases the person who is asking a confused question is indeed confused.
If what you are trying to do is to get best performance, make use of work that other people already did, rather than wasting your time on a solved problem.
If you want to learn about how efficient matrix multiplication can be implemented, that is a different problem.
The problem with Stackoverflow answers of the form "You asked about doing X, but it looks like you are trying to do X to achieve Y, and X is not a good way to achieve Y. You want to do Z. Here's how to do Z" is that later when people who actually do need to do X are looking for help, their searches keep turning up these damned answers that don't tell how to do X.
SO needs a flag on questions that is set if there is an answer to the actual question as asked and clear otherwise, and that can be used as a search filter.
By the way "make use of work that other people already did" might be nice for getting something built fast but it may not be the best thing.
Any issue that involves writing fast code involves tradeoffs. If you sit down to write something new you may have different views on the tradeoffs than whoever wrote "the fastest" one.
Life ain't so black and white. In fact even these 'best of field' products tend to be ugly inside and unoptimized in places. (source: I develop linear algebra libraries)
Bonus: for low level ASM math, every "solved" problem (which by the way it wasn't) becomes unsolved the second Intel or AMD or whoever releases a new chip or coprocessor.
This is something I'm interested in contributing to. Can you name a few libraries (especially ones implementing new and interesting work) that would welcome open source contributors? Alternatively you can just contact me (via my profile info) if you're working on something in particular but would rather not be identified publicly.
If what you are trying to do is to get best performance, make use of work that other people already did, rather than wasting your time on a solved problem.
If you want to learn about how efficient matrix multiplication can be implemented, that is a different problem.