doc/guide/localhost/CommonSubexp

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  26 <h1>CommonSubexp</h1>
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  28 <div id="content">
  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p><a href="CommonSubexp">CommonSubexp</a> is an optimization pass for the <a href="SSA">SSA</a>
  32 <a href="IntermediateLanguage">IntermediateLanguage</a>, invoked from <a href="SSASimplify">SSASimplify</a>.</p></div>
  33 </div>
  34 </div>
  35 <div class="sect1">
  36 <h2 id="_description">Description</h2>
  37 <div class="sectionbody">
  38 <div class="paragraph"><p>It eliminates instances of common subexpressions.</p></div>
  39 </div>
  40 </div>
  41 <div class="sect1">
  42 <h2 id="_implementation">Implementation</h2>
  43 <div class="sectionbody">
  44 <div class="ulist"><ul>
  45 <li>
  46 <p>
  47 <a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/common-subexp.fun"><span class="monospaced">common-subexp.fun</span></a>
  48 </p>
  49 </li>
  50 </ul></div>
  51 </div>
  52 </div>
  53 <div class="sect1">
  54 <h2 id="_details_and_notes">Details and Notes</h2>
  55 <div class="sectionbody">
  56 <div class="paragraph"><p>In addition to getting the usual sorts of things like</p></div>
  57 <div class="ulist"><ul>
  58 <li>
  59 <p>
  60 </p>
  61 <div class="listingblock">
  62 <div class="content monospaced">
  63 <pre>(w + 0wx1) + (w + 0wx1)</pre>
  64 </div></div>
  65 <div class="paragraph"><p>rewritten to</p></div>
  66 <div class="listingblock">
  67 <div class="content monospaced">
  68 <pre>let val w' = w + 0wx1 in w' + w' end</pre>
  69 </div></div>
  70 </li>
  71 </ul></div>
  72 <div class="paragraph"><p>it also gets things like</p></div>
  73 <div class="ulist"><ul>
  74 <li>
  75 <p>
  76 </p>
  77 <div class="listingblock">
  78 <div class="content monospaced">
  79 <pre>val a = Array_uninit n
  80 val b = Array_length a</pre>
  81 </div></div>
  82 <div class="paragraph"><p>rewritten to</p></div>
  83 <div class="listingblock">
  84 <div class="content monospaced">
  85 <pre>val a = Array_uninit n
  86 val b = n</pre>
  87 </div></div>
  88 </li>
  89 </ul></div>
  90 <div class="paragraph"><p><span class="monospaced">Arith</span> transfers are handled specially.  The <em>result</em> of an <span class="monospaced">Arith</span>
  91 transfer can be used in <em>common</em> <span class="monospaced">Arith</span> transfers that it dominates:</p></div>
  92 <div class="ulist"><ul>
  93 <li>
  94 <p>
  95 </p>
  96 <div class="listingblock">
  97 <div class="content monospaced">
  98 <pre>val l = (n + m) + (n + m)
  99
 100 val k = (l + n) + ((l + m) handle Overflow =&gt; ((l + m)
 101                                                handle Overflow =&gt; l + n))</pre>
 102 </div></div>
 103 <div class="paragraph"><p>is rewritten so that <span class="monospaced">(n + m)</span> is computed exactly once, as are
 104 <span class="monospaced">(l + n)</span> and <span class="monospaced">(l + m)</span>.</p></div>
 105 </li>
 106 </ul></div>
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