From: scs@eskimo.com (Steve Summit)
Newsgroups: comp.lang.c, comp.lang.c.moderated
Subject: comp.lang.c Answers to Frequently Asked Questions (FAQ list)
URL: http://www.eskimo.com/~scs/C-faq/top.html

20.13:	What's the best way of making my program efficient?

A:	By picking good algorithms, implementing them carefully, and
	making sure that your program isn't doing any extra work.  For
	example, the most microoptimized character-copying loop in the
	world will be beat by code which avoids having to copy
	characters at all.

	When worrying about efficiency, it's important to keep several
	things in perspective.  First of all, although efficiency is an
	enormously popular topic, it is not always as important as
	people tend to think it is.  Most of the code in most programs
	is not time-critical.  When code is not time-critical, it is
	usually more important that it be written clearly and portably
	than that it be written maximally efficiently.  (Remember that
	computers are very, very fast, and that seemingly "inefficient"
	code may be quite efficiently compilable, and run without
	apparent delay.)

	It is notoriously difficult to predict what the "hot spots" in a
	program will be.  When efficiency is a concern, it is important
	to use profiling software to determine which parts of the
	program deserve attention.  Often, actual computation time is
	swamped by peripheral tasks such as I/O and memory allocation,
	which can be sped up by using buffering and caching techniques.

	Even for code that *is* time-critical, one of the least
	effective optimization techniques is to fuss with the coding
	details.  Many of the "efficient coding tricks" which are
	frequently suggested (e.g. substituting shift operators for
	multiplication by powers of two) are performed automatically by
	even simpleminded compilers.  Heavyhanded optimization attempts
	can make code so bulky that performance is actually degraded,
	and are rarely portable (i.e. they may speed things up on one
	machine but slow them down on another).  In any case, tweaking
	the coding usually results in at best linear performance
	improvements; the big payoffs are in better algorithms.

	For more discussion of efficiency tradeoffs, as well as good
	advice on how to improve efficiency when it is important, see
	chapter 7 of Kernighan and Plauger's _The Elements of
	Programming Style_, and Jon Bentley's _Writing Efficient
	Programs_.