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<!DOCTYPE html> <html xmlns='http://www.w3.org/1999/xhtml' xmlns:b='http://www.google.com/2005/gml/b' xmlns:data='http://www.google.com/2005/gml/data' xmlns:expr='http://www.google.com/2005/gml/expr'> <head> <link href='https://www.blogger.com/static/v1/widgets/55013136-widget_css_bundle.css' rel='stylesheet' type='text/css'/> <meta content='text/html; charset=UTF-8' http-equiv='Content-Type'/> <meta content='blogger' name='generator'/> <link href='https://gafter.blogspot.com/favicon.ico' rel='icon' type='image/x-icon'/> <link href='http://gafter.blogspot.com/2019/08/' rel='canonical'/> <link rel="alternate" type="application/atom+xml" title="Neal Gafter's blog - Atom" href="https://gafter.blogspot.com/feeds/posts/default" /> <link rel="alternate" type="application/rss+xml" title="Neal Gafter's blog - RSS" href="https://gafter.blogspot.com/feeds/posts/default?alt=rss" /> <link rel="service.post" type="application/atom+xml" title="Neal Gafter's blog - Atom" href="https://www.blogger.com/feeds/7803021/posts/default" />  <meta content='http://gafter.blogspot.com/2019/08/' property='og:url'/> <meta content='Neal Gafter's blog' property='og:title'/> <meta content='Thoughts about Programming Languages, Science and Philosophy.' property='og:description'/> <title>Neal Gafter's blog: August 2019</title> <style id='page-skin-1' type='text/css'></style> <link href='https://www.blogger.com/dyn-css/authorization.css?targetBlogID=7803021&zx=47ee029f-c22c-4119-b7c8-21e805d2c7c9' media='none' onload='if(media!='all')media='all'' rel='stylesheet'/><noscript><link href='https://www.blogger.com/dyn-css/authorization.css?targetBlogID=7803021&zx=47ee029f-c22c-4119-b7c8-21e805d2c7c9' rel='stylesheet'/></noscript> <meta name='google-adsense-platform-account' content='ca-host-pub-1556223355139109'/> <meta name='google-adsense-platform-domain' content='blogspot.com'/> </head> <body> <div class='navbar section' id='navbar'><div class='widget Navbar' data-version='1' id='Navbar1'><script type="text/javascript"> function setAttributeOnload(object, attribute, val) { if(window.addEventListener) { window.addEventListener('load', function(){ object[attribute] = val; }, false); } else { window.attachEvent('onload', function(){ object[attribute] = val; }); } } </script> <div id="navbar-iframe-container"></div> <script type="text/javascript" src="https://apis.google.com/js/platform.js"></script> <script type="text/javascript"> gapi.load("gapi.iframes:gapi.iframes.style.bubble", function() { if (gapi.iframes && gapi.iframes.getContext) { gapi.iframes.getContext().openChild({ url: 'https://www.blogger.com/navbar/7803021?origin\x3dhttps://gafter.blogspot.com', where: document.getElementById("navbar-iframe-container"), id: "navbar-iframe" }); } }); </script><script type="text/javascript"> (function() { var script = document.createElement('script'); script.type = 'text/javascript'; script.src = '//pagead2.googlesyndication.com/pagead/js/google_top_exp.js'; var head = document.getElementsByTagName('head')[0]; if (head) { head.appendChild(script); }})(); </script> </div></div> <div id='outer-wrapper'><div id='wrap2'>  <span id='skiplinks' style='display:none;'> <a href='#main'>skip to main </a> | <a href='#sidebar'>skip to sidebar</a> </span> <div id='header-wrapper'> <div class='header section' id='header'><div class='widget Header' data-version='1' id='Header1'> <div id='header-inner'> <div class='titlewrapper'> <h1 class='title'> <a href='https://gafter.blogspot.com/'> Neal Gafter's blog </a> </h1> </div> <div class='descriptionwrapper'> <p class='description'><span>Thoughts about Programming Languages, Science and Philosophy.</span></p> </div> </div> </div></div> </div> <div id='content-wrapper'> <div id='main-wrapper'> <div class='main section' id='main'><div class='widget Blog' data-version='1' id='Blog1'> <div class='blog-posts hfeed'> <div class="date-outer"> <h2 class='date-header'><span>Wednesday, August 14, 2019</span></h2> <div class="date-posts"> <div class='post-outer'> <div class='post'> <a name='3901483451715988807'></a> <div class='post-header-line-1'></div> <div class='post-body'> <p><h1>Accidentally Quadratic Constant Folding</h1><br /> I just fixed a bug in the implementation of constant folding in the C# and VB.Net compilers. They used to require <i>O(n^2)</i> space. <a href="https://github.com/dotnet/roslyn/pull/37915">Now they require <i>O(n)</i> space</a>.<br /> <br /> The naive approach to implementing string concatenation for an expression such as <code>"S1" + "S2" + "S3"</code> is to recursively visit the left-hand and right-hand operands and compute their constant string values, and then concatenate these strings to produce the constant string for the concatenation. It seems obviously correct and the only reasonable way to do it. But it has a serious problem.<br /> <br /> The concatenation operation is left associative, so for a long sequence of <i>n</i> concatenations, each of size <i>k</i>, we would produce intermediate results of size <i>k</i>, <i>2k</i>, <i>3k</i>, and so on up to <i>nk</i>. The total amount of space needed to represent this collection of strings is <i>O(k n^2)</i>. For moderately-sized values of <i>n</i>, this would cause the compilers to run out of memory and crash. This is not merely a theoretical problem, as we had been getting customer reports of this problem about quarterly for the past 7-8 years.<br /> <br /> Fixing the problem was fairly straightforward, using a technique I learned from the Cedar/Mesa compiler in the early '80s. Rather than representing string constants in the compilers using strings, they are now represented using a data structure called <a href="https://en.wikipedia.org/wiki/Rope_(data_structure)">Ropes</a>. Concatenating two ropes requires only a small constant amount of memory.<br /> <br /> Any compiler for a language that supports constant folding of string concatenation will run into this problem unless there is a similarly low-space representation of two concatenated strings. This is a technique that should be in the bag of tricks of any compiler writer.<br /> </p> <div style='clear: both;'></div> </div> <div class='post-footer'> <p class='post-footer-line post-footer-line-1'><span class='post-comment-link'> <a class='comment-link' href='https://www.blogger.com/comment/fullpage/post/7803021/3901483451715988807' onclick=''>3 comments</a> </span> <span class='post-icons'> </span> <span class='post-backlinks post-comment-link'> </span> </p> <p class='post-footer-line post-footer-line-2'></p> <p class='post-footer-line post-footer-line-3'></p> </div> </div> </div> </div></div> </div> <div class='blog-pager' id='blog-pager'> <span id='blog-pager-newer-link'> <a class='blog-pager-newer-link' href='https://gafter.blogspot.com/search?updated-max=2023-08-09T19:21:00-07:00&max-results=2&reverse-paginate=true' id='Blog1_blog-pager-newer-link' title='Newer Posts'>Newer Posts</a> </span> <span id='blog-pager-older-link'> <a class='blog-pager-older-link' 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class='profile-datablock'> <dt class='profile-data'> <a class='profile-name-link g-profile' href='https://www.blogger.com/profile/08579466817032124881' rel='author' style='background-image: url(//www.blogger.com/img/logo-16.png);'> Neal Gafter </a> </dt> <dd class='profile-textblock'>Neal Gafter is a Computer Programming Language Designer and Implementer, Amateur Scientist and Philosopher. He works on the Rel compiler at Relational.AI. He previously worked for Microsoft on C#, for Google on Calendar, and for Sun Microsystems on Java. Neal was granted an OpenJDK Community Innovators' Challenge award for his design and implementation of lambda expressions for Java. He is coauthor of <em>Java Puzzlers: Traps, Pitfalls, and Corner Cases</em> (Addison Wesley, 2005). He was a member of the C++ Standards Committee and led the development of C and C++ compilers at Sun Microsystems, Microtec Research, and Texas Instruments. He holds a Ph.D. in computer science from the University of Rochester.</dd> </dl> <a class='profile-link' href='https://www.blogger.com/profile/08579466817032124881' rel='author'>View my complete profile</a> <div class='clear'></div> </div> </div></div> </div>  <div class='clear'> </div> </div>  <div id='footer-wrapper'> <div class='footer no-items section' id='footer'></div> </div> </div></div>  <script src="//www.google-analytics.com/urchin.js" type="text/javascript"> </script> <script type='text/javascript'> _uacct = "UA-605497-1"; urchinTracker(); </script> <script type="text/javascript" src="https://www.blogger.com/static/v1/widgets/2538535903-widgets.js"></script> <script type='text/javascript'> window['__wavt'] = 'AOuZoY7iutdOWedtGMKZuAe1PPd00HY4Kg:1744353989585';_WidgetManager._Init('//www.blogger.com/rearrange?blogID\x3d7803021','//gafter.blogspot.com/2019/08/','7803021'); _WidgetManager._SetDataContext([{'name': 'blog', 'data': {'blogId': '7803021', 'title': 'Neal Gafter\x27s blog', 'url': 'https://gafter.blogspot.com/2019/08/', 'canonicalUrl': 'http://gafter.blogspot.com/2019/08/', 'homepageUrl': 'https://gafter.blogspot.com/', 'searchUrl': 'https://gafter.blogspot.com/search', 'canonicalHomepageUrl': 'http://gafter.blogspot.com/', 'blogspotFaviconUrl': 'https://gafter.blogspot.com/favicon.ico', 'bloggerUrl': 'https://www.blogger.com', 'hasCustomDomain': false, 'httpsEnabled': true, 'enabledCommentProfileImages': true, 'gPlusViewType': 'FILTERED_POSTMOD', 'adultContent': false, 'analyticsAccountNumber': '', 'encoding': 'UTF-8', 'locale': 'en', 'localeUnderscoreDelimited': 'en', 'languageDirection': 'ltr', 'isPrivate': false, 'isMobile': false, 'isMobileRequest': false, 'mobileClass': '', 'isPrivateBlog': false, 'isDynamicViewsAvailable': false, 'feedLinks': '\x3clink rel\x3d\x22alternate\x22 type\x3d\x22application/atom+xml\x22 title\x3d\x22Neal Gafter\x26#39;s blog - 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