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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!-- but may not be published or retransmitted without explicit permission.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 21:47:46 2005 by faqproc version 2.7 --><!-- from source file ansi.sgml dated Wed Aug 11 00:43:45 2004 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/ansi/noalias.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:56 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 11.28</title><link href="extidsignif.html" rev=precedes><link href="preansi.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="extidsignif.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="preansi.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 11.28</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>What was <TT>noalias</TT> and what ever happened to it?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font><TT>noalias</TT> was another type qualifier,in the same syntactic class as <TT>const</TT> and <TT>volatile</TT>,which was intended to assert thatanobjectwas notpointed to (``aliased'') by other pointers.The primary application,which is an important one,would have been for the formal parameters of functionsdesigned to perform computations on large arrays.A compiler cannot usually take advantage of vectorizationor other parallelizationhardware(on supercomputers which have it)unless it can ensurethat the source and destination arrays do not overlap.</p><p>The <TT>noalias</TT> keyword was not backed up by any ``prior art,''and it was introduced late in the review and approval process.It wassurprisinglydifficult to define precisely and explain coherently,and sparked widespread, acrimonious debate,including a scathing pan by Dennis Ritchie.It had far-ranging implications,particularly for several standard library interfaces,for which easy fixes were not readily apparent.</p><p>Because of the criticism and the difficulty of defining<TT>noalias</TT> well,the Committeedeclined to adopt it,in spite of its superficialattractions.(When writing a standard,features cannot be introduced halfway;their full integration,and all implications,must be understood.)The need for an explicit mechanismto support parallel implementation ofnon-overlapping operations remainsunfilled(althoughsome work is being doneonthe problem).</p><p>References:ISO Sec. 6.9.6<br></p><!-- aend --><p><hr><a href="extidsignif.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="preansi.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/ansi/noalias.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:56 GMT --></html>⌨️ 快捷键说明
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