Occasionally, I like to peruse uops.info. It is a great resource for micro-optimization: benchmark every x86 instruction on every architecture, and compile the results. Every time I look at this table, there is one thing that sticks out to me: the DIV instruction. On a Coffee Lake CPU, an 8-bit DIV takes a long time: 25 cycles. Cannon Lake and Ice Lake do a lot better, and so does AMD. We know that divider architecture is different between architectures, and aggregating all of the performance numbers for an 8-bit DIV, we see:

This is the second part in a 2-part series on the “Fibonacci” interview problem. We are building off of a previous post, so take a look at Part I if you haven’t seen it.
Previously, we examined the problem and constructed a logarithmic-time solution based on computing the power of a matrix. Now we will derive a constant time solution using some more linear algebra. If you had trouble with the linear algebra in part I, it may help to read up on matrices, matrix multiplicaiton, and special matrix operations (specifically determinants and inverses) before moving on.

Few interview problems are as notorious as the “Fibonacci” interview question. At first glance, it seems good: Most people know something about the problem, and there are several clever ways to achieve a linear time solution. Usually, in interviews, the linear time solution is the expected solution. However, the Fibonacci problem is unique among interview problems in that the expected solution is not the optimal solution. There is an $O(1)$ solution, and to get there, we need a little bit of linear algebra.