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<p>The term <b>dead code</b> has multiple definitions. Some use the term to refer to code (i.e. instructions in memory) which can never be executed at run-time.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>

In some areas of <a href="Computer_programming" title="Computer programming">computer programming</a>, <b>dead code</b> is a section in the <a href="Source_code" title="Source code">source code</a> of a program which is executed but whose result is never used in any other computation.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> The execution of dead code wastes computation time and memory.

</p><p>While the result of a dead computation may never be used, it may raise <a href="Exception_handling" title="Exception handling">exceptions</a> or affect some global state, thus removal of such code may change the output of the program and introduce unintended <a href="Software_bug" title="Software bug">bugs</a>. Compiler optimizations are typically conservative in their approach to dead-code removal if there is any ambiguity as to whether removal of the dead code will affect the program output. The programmer may aid the compiler in this matter by making additional use of <a href="Method_(computer_programming)#Static_methods" title="Method (computer programming)">static</a> and/or <a href="Inline_function" class="mw-redirect" title="Inline function">inline</a> functions and enabling the use of <a href="Link-time_optimization" class="mw-redirect" title="Link-time optimization">link-time optimization</a>.

</p>

<div class="mw-heading mw-heading2"><h2 id="Example">Example</h2></div>

<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">iX</span><span class="o">*</span><span class="n">iY</span><span class="p">;</span>

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<p>In the above example, although the division of <style data-mw-deduplicate="TemplateStyles:r886049734">

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</style><span class="monospaced">iX</span> by <span class="monospaced">iY</span> is computed and never used, it will throw an exception when a division by zero occurs. Therefore, the removal of the dead code may change the output of the program.

</p>

<div class="mw-heading mw-heading2"><h2 id="Analysis">Analysis</h2></div>

<p><a href="Dead-code_elimination" title="Dead-code elimination">Dead-code elimination</a> is a form of <a href="Compiler_optimization" class="mw-redirect" title="Compiler optimization">compiler optimization</a> in which dead code is removed from a program. Dead code analysis can be performed using <a href="Live-variable_analysis" title="Live-variable analysis">live-variable analysis</a>, a form of <a href="Static_program_analysis" title="Static program analysis">static-code analysis</a> and <a href="Data-flow_analysis" title="Data-flow analysis">data-flow analysis</a>. This is in contrast to unreachable code analysis which is based on <a href="Control_flow_analysis" class="mw-redirect" title="Control flow analysis">control-flow analysis</a>.

</p><p>The dead-code elimination technique is in the same class of optimizations as <a href="Unreachable_code" title="Unreachable code">unreachable code</a> elimination and <a href="Redundant_code" title="Redundant code">redundant code</a> elimination.

</p><p>In large programming projects, it is sometimes difficult to recognize and eliminate dead code, particularly when entire modules become dead. Test scaffolding can make it appear that the code is still live, and at times, contract language can require delivery of the code even when the code is no longer relevant.<sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup>

</p><p>Some <a href="Integrated_development_environment" title="Integrated development environment">IDEs</a> (such as Xcode, Visual Studio 2010<sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> and Eclipse Galileo<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup>) have the ability to locate dead code during the compiling stage.

</p><p>While most optimization techniques seek to remove dead code in an implementation, in extreme forms of optimization for size it may sometimes be desirable to deliberately introduce and carefully shape seemingly dead code, when it allows to fold otherwise unrelated code sections together (and thereby reduce their combined size) so that the extra code will effectively not harm the first path of execution through the code but is used to carry out the actions necessary for the alternative paths of execution, for which other sections of the code may become dead code. On a more functional level, this can be seen as both, artificially introduction of harmless/useful side-effects and reduction of the redundancy of the code, but it can also be used down to opcode level in order to allow the usage of shorter instructions, which would not be possible when folding code sequences without the concerted introduction of side-effects caused by the dead code.

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<ul><li><a href="Dead-code_elimination" title="Dead-code elimination">Dead-code elimination</a></li>

<li><a href="Redundant_code" title="Redundant code">Redundant code</a></li>

<li><a href="Unreachable_code" title="Unreachable code">Unreachable code</a></li>

<li><a href="Oxbow_code" title="Oxbow code">Oxbow code</a></li>

<li><i>0xDEADC0DE</i> is a <a href="Magic_number_(programming)#Debug_values" title="Magic number (programming)">magic number</a> written in <a href="Hexspeak" title="Hexspeak">Hexspeak</a> used as a marker in <a href="OpenWrt" title="OpenWrt">OpenWRT</a> firmware</li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20120310002543/http://www.cs.bu.edu/~hwxi/academic/papers/padl99.pdf">"Hongwei Xi, Dead Code Elimination through Dependent Types"</a> <span class="cs1-format">(PDF)</span>. Archived from <a rel="nofollow" class="external text" href="https://www.cs.bu.edu/~hwxi/academic/papers/padl99.pdf">the original</a> <span class="cs1-format">(PDF)</span> on 2012-03-10<span class="reference-accessdate">. Retrieved <span class="nowrap">2020-05-06</span></span>.</cite></span>

</li>

<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external autonumber" href="http://www.do178site.com/do178b_questions.php">[1]</a> <a rel="nofollow" class="external text" href="https://web.archive.org/web/20200520200130/http://www.do178site.com/do178b_questions.php">Archived</a> 2020-05-20 at the <a href="Wayback_Machine" title="Wayback Machine">Wayback Machine</a> DO-178B</span>

</li>

<li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><a href="DO-178B" title="DO-178B">DO-178B</a> Wikipedia/DO-178B.</span>

</li>

<li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external text" href="http://doi.acm.org/10.1145/349214.349233">Debray, S. K., Evans, W., Muth, R., and De Sutter, B. 2000. Compiler techniques for code compaction. ACM Trans. Program. Lang. Syst. 22, 2 (Mar. 2000), 378–415.</a></span>

</li>

<li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text"><a href="Andrew_Appel" title="Andrew Appel">Appel, A. W.</a> 1998 Modern Compiler Implementation in Java. Cambridge University Press.</span>

</li>

<li id="cite_note-6"><span class="mw-cite-backlink"><b><a href="#cite_ref-6">^</a></b></span> <span class="reference-text"><a href="Douglas_W._Jones" title="Douglas W. Jones">Douglas W. Jones</a> <a rel="nofollow" class="external text" href="http://catless.com/Risks/8.19.html#subj2">Dead Code Maintenance, Risks 8.19 (Feb. 1, 1989)</a> <a rel="nofollow" class="external text" href="https://web.archive.org/web/20110708124114/http://catless.com/Risks/8.19.html#subj2">Archived</a> 2011-07-08 at the <a href="Wayback_Machine" title="Wayback Machine">Wayback Machine</a></span>

</li>

<li id="cite_note-7"><span class="mw-cite-backlink"><b><a href="#cite_ref-7">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external text" href="http://blogs.msdn.com/b/habibh/archive/2009/07/31/discover-dead-code-in-your-application-using-code-analysis.aspx">Habib Heydarian, Microsoft Corp</a></span>

</li>

<li id="cite_note-8"><span class="mw-cite-backlink"><b><a href="#cite_ref-8">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external text" href="http://help.eclipse.org/galileo/index.jsp?topic=/org.eclipse.jdt.doc.isv/guide/jdt_api_compile.htm">Eclipse Developer Guide</a></span>

</li>

</ol></div></div>

<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

<ul><li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20110722110715/http://java.net/projects/dcd/pages/Home">Dead Code Detector (DCD) simply finds never used code in your Java/JEE applications</a></li>

<li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20110722110615/http://java.net/projects/dcd/pages/Alternatives">Comparisons of some Java Dead Code Detector</a></li>

<li><a rel="nofollow" class="external text" href="http://www.ucdetector.org/">UCDetector</a> Eclipse PlugIn to find dead java code</li></ul></div><div><div class="zim-footer">

This article is issued from <a class="external text" title="Last edited on 2024-08-18" href="https://en.wikipedia.org/wiki/?title=Dead_code&oldid=1240914124">Wikipedia</a>. The text is available under <a class="external text" href="https://creativecommons.org/licenses/by-sa/4.0/deed.en">Creative Commons Attribution-Share Alike 4.0</a> unless otherwise noted. Additional terms may apply for the media files.

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