I'm pasting the famous passage below, and I don't know how to interpret it as anything other than a call to correctly identify the performance critical part of

I'm pasting the famous passage below, and I don't know how to interpret it as anything other than a call to correctly identify the performance critical part of one's code before spending significant time on optimization. If you can intuit that, more power to you! If you can't, you must measure, and Knuth gives the suggestion that your compiler should do this automatically for you: "There is no doubt that the grail of efficiency leads to abuse. Programmers waste enormous amounts of time thinking about, or worrying about, the speed of noncritical parts of their programs, and these attempts at efficiency actually have a strong negative impact when debugging and maintenance are considered. 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%. A good programmer will not be lulled into complacency by such reasoning, he will be wise to look carefully at the critical code; but only after that code has been identified. It is often a mistake to make a priori judgments about what parts of a program are really critical, since the universal experience of programmers who have been using measurement tools has been that their intuitive guesses fail. After working with such tools or seven years, I've become convinced that all compilers written from now on should be designed to provide all programmers with feedback indicating what parts of their programs are costing the most; indeed, this feedback should be supplied automatically unless it has been specifically turned off." I certainly agree with the interpretation that the _cost_ is maintainability of prematurely optimised code and this is the evil he's describing. But this is also all in the context of efficiency as a virtue in itself - i.e. something you should be seeking, but seeking in the correct places.