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Re: declaring lexical variables in shortest scope: performance?

by eyepopslikeamosquito (Bishop)
on Apr 01, 2020 at 07:56 UTC ( #11114885=note: print w/replies, xml ) Need Help??

in reply to declaring lexical variables in shortest scope: performance?

Don't diddle code to make it faster -- find a better algorithm

-- The Elements of Programming Style

Donít Optimize Code -- Benchmark It

-- from Ten Essential Development Practices by Damian Conway

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

-- Donald Knuth

Donít pessimize prematurely. All other things being equal, notably code complexity and readability, certain efficient design patterns and coding idioms should just flow naturally from your fingertips and are no harder to write than the pessimized alternatives. This is not premature optimization; it is avoiding gratuitous pessimization.

-- Andrei Alexandrescu and Herb Sutter

Rule 1: Bottlenecks occur in surprising places, so don't try to second guess and put in a speed hack until you have proven that's where the bottleneck is.
Rule 2: Measure. Don't tune for speed until you've measured, and even then don't unless one part of the code overwhelms the rest.
Rule 3. Fancy algorithms are slow when n is small, and n is usually small. Fancy algorithms have big constants. Until you know that n is frequently going to be big, don't get fancy. (Even if n does get big, use Rule 2 first.)
Rule 4. Fancy algorithms are buggier than simple ones, and they're much harder to implement. Use simple algorithms as well as simple data structures.
Rule 5. Data dominates. If you've chosen the right data structures and organized things well, the algorithms will almost always be self-evident. Data structures, not algorithms, are central to programming.
Note: Pike's rules 1 and 2 restate Tony Hoare's "Premature optimization is the root of all evil". Ken Thompson rephrased Pike's rules 3 and 4 as "When in doubt, use brute force". Rules 3 and 4 are instances of KISS. Rule 5 was stated by Fred Brooks in The Mythical Man-Month and is often shortened to "write stupid code that uses smart objects".

-- Rob Pike

Without good design, good algorithms, and complete understanding of the program's operation, your carefully optimized code will amount to one of mankind's least fruitful creations -- a fast slow program.

-- Michael Abrash

A couple of related general guidelines from On Coding Standards and Code Reviews:

  • Correctness, simplicity and clarity come first. Avoid unnecessary cleverness. If you must rely on cleverness, encapsulate and comment it.
  • Don't optimize prematurely. Benchmark before you optimize. Comment why you are optimizing.

On Interfaces and APIs cautions that library interfaces are very difficult to change once they become widely used - a fundamentally inefficient interface cannot be easily fixed later by optimizing. So it is not "premature optimization" to consider efficiency when designing public library interfaces.

Having got all that off my chest, personally I love diddling the code, as indicated by these two fun challenges:

These endeavours convinced me of don't assume measure and especially find a better algorithm!

Updated: Added Donald Knuth premature optimization quote and Alexandrescu/Sutter premature pessimization quotes and Rob Pike quotes. Mentioned efficiency of interfaces.

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Re^2: declaring lexical variables in shortest scope: performance?
by bliako (Prior) on Apr 01, 2020 at 08:13 UTC

    fine with all these but if there is a long loop, then every little flop counts.

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