Then you can shave off some execution time by using a slightly different construct, though bigger wins usually lie elsewhere.
There are some important distinctions though, which gets lost if one takes the quote at face value without much context. For example, I think one should strive to avoid O(n^2) loops and similar unless one is absolutely positively certain n will forever be bounded to some sane value.
My point being that O(n^2) code can make the application completely unusable, which makes it something more than a pure optimization problem.
Bear in mind that the given quote is only the most memorable part of Knuth's longer statement:
> We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil.
> Yet we should not pass up our opportunities in that critical 3%.
FWIW, I've read that Knuth attributes it to Hoare, and Hoare attributes it to Knuth.
Looking now, I see that Knuth also wrote a similar comment in "Computer Programming as an Art", at http://faculty.salisbury.edu/~xswang/Research/Papers/turning... :
> The real problem is that programmers have spent far too much time worrying about efficiency in the wrong places and at the wrong times; premature optimization is the root of all evil (or at least most of it) in programming.
That was published the same year and month as the first paper, so I don't know which gets priority for any attributions.