<!DOCTYPE html> <html lang="en"> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <meta name="generator" content="AsciiDoc 8.6.8"> <title>KnownCase</title> <link rel="stylesheet" href="./asciidoc.css" type="text/css"> <link rel="stylesheet" href="./pygments.css" type="text/css"> <script type="text/javascript" src="./asciidoc.js"></script> <script type="text/javascript"> /*<![CDATA[*/ asciidoc.install(); /*]]>*/ </script> <link rel="stylesheet" href="./mlton.css" type="text/css"/> </head> <body class="article"> <div id="banner"> <div id="banner-home"> <a href="./Home">MLton 20130715</a> </div> </div> <div id="header"> <h1>KnownCase</h1> </div> <div id="content"> <div id="preamble"> <div class="sectionbody"> <div class="paragraph"><p><a href="KnownCase">KnownCase</a> is an optimization pass for the <a href="SSA">SSA</a> <a href="IntermediateLanguage">IntermediateLanguage</a>, invoked from <a href="SSASimplify">SSASimplify</a>.</p></div> </div> </div> <div class="sect1"> <h2 id="_description">Description</h2> <div class="sectionbody"> <div class="paragraph"><p>This pass duplicates and simplifies <span class="monospaced">Case</span> transfers when the constructor of the scrutinee is known.</p></div> <div class="paragraph"><p>Uses <a href="Restore">Restore</a>.</p></div> <div class="paragraph"><p>For example, the program</p></div> <div class="listingblock"> <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="k">rec</span><span class="w"> </span><span class="n">last</span><span class="w"> </span><span class="p">=</span><span class="w"></span> <span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="p">[]</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="mi">0</span><span class="w"></span> <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="p">[</span><span class="n">x</span><span class="p">]</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">x</span><span class="w"></span> <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="p">_</span><span class="w"> </span><span class="n">::</span><span class="w"> </span><span class="n">l</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">last</span><span class="w"> </span><span class="n">l</span><span class="w"></span> <span class="k">val</span><span class="w"> </span><span class="p">_</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="n">+</span><span class="w"> </span><span class="n">last</span><span class="w"> </span><span class="p">[</span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">,</span><span class="w"> </span><span class="mi">5</span><span class="p">,</span><span class="w"> </span><span class="mi">6</span><span class="p">,</span><span class="w"> </span><span class="mi">7</span><span class="p">]</span><span class="w"></span> </pre></div></div></div> <div class="paragraph"><p>gives rise to the <a href="SSA">SSA</a> function</p></div> <div class="listingblock"> <div class="content monospaced"> <pre>fun last_0 (x_142) = loopS_1 () loopS_1 () loop_11 (x_142) loop_11 (x_143) case x_143 of nil_1 => L_73 | ::_0 => L_74 L_73 () return global_5 L_74 (x_145, x_144) case x_145 of nil_1 => L_75 | _ => L_76 L_75 () return x_144 L_76 () loop_11 (x_145)</pre> </div></div> <div class="paragraph"><p>which is simplified to</p></div> <div class="listingblock"> <div class="content monospaced"> <pre>fun last_0 (x_142) = loopS_1 () loopS_1 () case x_142 of nil_1 => L_73 | ::_0 => L_118 L_73 () return global_5 L_118 (x_230, x_229) L_74 (x_230, x_229, x_142) L_74 (x_145, x_144, x_232) case x_145 of nil_1 => L_75 | ::_0 => L_114 L_75 () return x_144 L_114 (x_227, x_226) L_74 (x_227, x_226, x_145)</pre> </div></div> </div> </div> <div class="sect1"> <h2 id="_implementation">Implementation</h2> <div class="sectionbody"> <div class="ulist"><ul> <li> <p> <a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/known-case.fun"><span class="monospaced">known-case.fun</span></a> </p> </li> </ul></div> </div> </div> <div class="sect1"> <h2 id="_details_and_notes">Details and Notes</h2> <div class="sectionbody"> <div class="paragraph"><p>One interesting aspect of <a href="KnownCase">KnownCase</a>, is that it often has the effect of unrolling list traversals by one iteration, moving the <span class="monospaced">nil</span>/<span class="monospaced">::</span> check to the end of the loop, rather than the beginning.</p></div> </div> </div> </div> <div id="footnotes"><hr></div> <div id="footer"> <div id="footer-text"> </div> <div id="footer-badges"> </div> </div> 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