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| 22 | <a href="./Home">MLton 20180207</a>\r |
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| 25 | <div id="header">\r |
| 26 | <h1>KnownCase</h1>\r |
| 27 | </div>\r |
| 28 | <div id="content">\r |
| 29 | <div id="preamble">\r |
| 30 | <div class="sectionbody">\r |
| 31 | <div class="paragraph"><p><a href="KnownCase">KnownCase</a> is an optimization pass for the <a href="SSA">SSA</a>\r |
| 32 | <a href="IntermediateLanguage">IntermediateLanguage</a>, invoked from <a href="SSASimplify">SSASimplify</a>.</p></div>\r |
| 33 | </div>\r |
| 34 | </div>\r |
| 35 | <div class="sect1">\r |
| 36 | <h2 id="_description">Description</h2>\r |
| 37 | <div class="sectionbody">\r |
| 38 | <div class="paragraph"><p>This pass duplicates and simplifies <span class="monospaced">Case</span> transfers when the\r |
| 39 | constructor of the scrutinee is known.</p></div>\r |
| 40 | <div class="paragraph"><p>Uses <a href="Restore">Restore</a>.</p></div>\r |
| 41 | <div class="paragraph"><p>For example, the program</p></div>\r |
| 42 | <div class="listingblock">\r |
| 43 | <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>\r |
| 44 | <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>\r |
| 45 | <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>\r |
| 46 | <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>\r |
| 47 | \r |
| 48 | <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>\r |
| 49 | </pre></div></div></div>\r |
| 50 | <div class="paragraph"><p>gives rise to the <a href="SSA">SSA</a> function</p></div>\r |
| 51 | <div class="listingblock">\r |
| 52 | <div class="content monospaced">\r |
| 53 | <pre>fun last_0 (x_142) = loopS_1 ()\r |
| 54 | loopS_1 ()\r |
| 55 | loop_11 (x_142)\r |
| 56 | loop_11 (x_143)\r |
| 57 | case x_143 of\r |
| 58 | nil_1 => L_73 | ::_0 => L_74\r |
| 59 | L_73 ()\r |
| 60 | return global_5\r |
| 61 | L_74 (x_145, x_144)\r |
| 62 | case x_145 of\r |
| 63 | nil_1 => L_75 | _ => L_76\r |
| 64 | L_75 ()\r |
| 65 | return x_144\r |
| 66 | L_76 ()\r |
| 67 | loop_11 (x_145)</pre>\r |
| 68 | </div></div>\r |
| 69 | <div class="paragraph"><p>which is simplified to</p></div>\r |
| 70 | <div class="listingblock">\r |
| 71 | <div class="content monospaced">\r |
| 72 | <pre>fun last_0 (x_142) = loopS_1 ()\r |
| 73 | loopS_1 ()\r |
| 74 | case x_142 of\r |
| 75 | nil_1 => L_73 | ::_0 => L_118\r |
| 76 | L_73 ()\r |
| 77 | return global_5\r |
| 78 | L_118 (x_230, x_229)\r |
| 79 | L_74 (x_230, x_229, x_142)\r |
| 80 | L_74 (x_145, x_144, x_232)\r |
| 81 | case x_145 of\r |
| 82 | nil_1 => L_75 | ::_0 => L_114\r |
| 83 | L_75 ()\r |
| 84 | return x_144\r |
| 85 | L_114 (x_227, x_226)\r |
| 86 | L_74 (x_227, x_226, x_145)</pre>\r |
| 87 | </div></div>\r |
| 88 | </div>\r |
| 89 | </div>\r |
| 90 | <div class="sect1">\r |
| 91 | <h2 id="_implementation">Implementation</h2>\r |
| 92 | <div class="sectionbody">\r |
| 93 | <div class="ulist"><ul>\r |
| 94 | <li>\r |
| 95 | <p>\r |
| 96 | <a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/known-case.fun"><span class="monospaced">known-case.fun</span></a>\r |
| 97 | </p>\r |
| 98 | </li>\r |
| 99 | </ul></div>\r |
| 100 | </div>\r |
| 101 | </div>\r |
| 102 | <div class="sect1">\r |
| 103 | <h2 id="_details_and_notes">Details and Notes</h2>\r |
| 104 | <div class="sectionbody">\r |
| 105 | <div class="paragraph"><p>One interesting aspect of <a href="KnownCase">KnownCase</a>, is that it often has the\r |
| 106 | effect of unrolling list traversals by one iteration, moving the\r |
| 107 | <span class="monospaced">nil</span>/<span class="monospaced">::</span> check to the end of the loop, rather than the beginning.</p></div>\r |
| 108 | </div>\r |
| 109 | </div>\r |
| 110 | </div>\r |
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