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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN"><!-- This collection of hypertext pages is Copyright 1995-2005 by Steve Summit. --><!-- Content from the book "C Programming FAQs: Frequently Asked Questions" --><!-- (Addison-Wesley, 1995, ISBN 0-201-84519-9) is made available here by --><!-- permission of the author and the publisher as a service to the community. --><!-- It is intended to complement the use of the published text --><!-- and is protected by international copyright laws. --><!-- The on-line content may be accessed freely for personal use --><!-- but may not be published or retransmitted without explicit permission. --><!-- --><!-- this page built Sat Dec 24 21:47:47 2005 by faqproc version 2.7 --><!-- from source file misceff.sgml dated Sat Feb 7 19:26:17 2004 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/misc/shifts.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:05 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 20.15</title><link href="eff2.html" rev=precedes><link href="unsshift.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="eff2.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="unsshift.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../index-2.html"><img src="../images/buttontop.gif" alt="top/contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><hr><p><!-- qbegin --><h1>comp.lang.c FAQ list<font color=blue>·</font><!-- qtag -->Question 20.15</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>I've been replacing multiplications and divisionswith shift operators,because shifting is more efficient.</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>This is an excellent example of a potentially riskyand usually unnecessary optimization.Any compiler worthy of the name can replace aconstant, power-of-twomultiplication with a left shift,or asimilardivision of an unsigned quantity with a right shift.(Ritchie's original PDP-11 compiler,though it ran in less than 64K of memoryand omitted several features now considered mandatory,performed both of these optimizations,without eventurning on its optional optimization pass.)Furthermore,a compiler will make these optimizations only when they're correct;many programmers overlook the factthat shifting a negative valueto therightis <em>not</em> equivalent to division.(Therefore,when you need to make sure that these optimizations are performed,you may have to declare relevant variables as <TT>unsigned</TT>.)</p><!-- aend --><p><hr><a href="eff2.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="unsshift.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../questions.html"><img src="../images/buttontop.gif" alt="contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><br><!-- lastfooter --><a href="../about.html">about this FAQ list</a> <a href="../eskimo.html">about eskimo</a> <a href="../search.html">search</a> <a href="../feedback.html">feedback</a> <a href="copyright.html">copyright</a><p>Hosted by<a href="http://www.eskimo.com/"><img src="../../www.eskimo.com/img/link/eskitiny.gif" alt="Eskimo North"></a></body><!-- Mirrored from c-faq.com/misc/shifts.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:05 GMT --></html>⌨️ 快捷键说明
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