When is optimisation premature?

Question

As Knuth said,

We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil.

Answer 1

What you seem to be talking about is optimization like using a hash-based lookup container vs an indexed one like an array when a lot of key lookups will be done. This is not premature optimization, but something you should decide in the design phase.

The kind of optimization the Knuth rule is about is minimizing the length the most common codepaths, optimizing the code that is run most by for example rewriting in assembly or simplifying the code, making it less general. But doing this has no use until you are certain which parts of code need this kind of optimization and optimizing will (could?) make the code harder to understand or maintain, hence "premature optimization is the root of all evil".

Knuth also says it is always better to, instead of optimizing, change the algorithms your program uses, the approach it takes to a problem. For example whereas a little tweaking might give you a 10% increase of speed with optimization, changing fundamentally the way your program works might make it 10x faster.

In reaction to a lot of the other comments posted on this question: algorithm selection != optimization