doc/guide/localhost/CommonBlock

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  22 <a href="./Home">MLton 20180207</a>
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  26 <h1>CommonBlock</h1>
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  28 <div id="content">
  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p><a href="CommonBlock">CommonBlock</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 equivalent blocks in a <a href="SSA">SSA</a> function.  The
  39 equivalence criteria requires blocks to have no arguments or
  40 statements and transfer via <span class="monospaced">Raise</span>, <span class="monospaced">Return</span>, or <span class="monospaced">Goto</span> of a single
  41 global variable.</p></div>
  42 </div>
  43 </div>
  44 <div class="sect1">
  45 <h2 id="_implementation">Implementation</h2>
  46 <div class="sectionbody">
  47 <div class="ulist"><ul>
  48 <li>
  49 <p>
  50 <a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/common-block.fun"><span class="monospaced">common-block.fun</span></a>
  51 </p>
  52 </li>
  53 </ul></div>
  54 </div>
  55 </div>
  56 <div class="sect1">
  57 <h2 id="_details_and_notes">Details and Notes</h2>
  58 <div class="sectionbody">
  59 <div class="ulist"><ul>
  60 <li>
  61 <p>
  62 Rewrites
  63 </p>
  64 <div class="listingblock">
  65 <div class="content monospaced">
  66 <pre>L_X ()
  67   raise (global_Y)</pre>
  68 </div></div>
  69 <div class="paragraph"><p>to</p></div>
  70 <div class="listingblock">
  71 <div class="content monospaced">
  72 <pre>L_X ()
  73   L_Y' ()</pre>
  74 </div></div>
  75 <div class="paragraph"><p>and adds</p></div>
  76 <div class="listingblock">
  77 <div class="content monospaced">
  78 <pre>L_Y' ()
  79   raise (global_Y)</pre>
  80 </div></div>
  81 <div class="paragraph"><p>to the <a href="SSA">SSA</a> function.</p></div>
  82 </li>
  83 <li>
  84 <p>
  85 Rewrites
  86 </p>
  87 <div class="listingblock">
  88 <div class="content monospaced">
  89 <pre>L_X ()
  90   return (global_Y)</pre>
  91 </div></div>
  92 <div class="paragraph"><p>to</p></div>
  93 <div class="listingblock">
  94 <div class="content monospaced">
  95 <pre>L_X ()
  96   L_Y' ()</pre>
  97 </div></div>
  98 <div class="paragraph"><p>and adds</p></div>
  99 <div class="listingblock">
 100 <div class="content monospaced">
 101 <pre>L_Y' ()
 102   return (global_Y)</pre>
 103 </div></div>
 104 <div class="paragraph"><p>to the <a href="SSA">SSA</a> function.</p></div>
 105 </li>
 106 <li>
 107 <p>
 108 Rewrites
 109 </p>
 110 <div class="listingblock">
 111 <div class="content monospaced">
 112 <pre>L_X ()
 113   L_Z (global_Y)</pre>
 114 </div></div>
 115 <div class="paragraph"><p>to</p></div>
 116 <div class="listingblock">
 117 <div class="content monospaced">
 118 <pre>L_X ()
 119   L_Y' ()</pre>
 120 </div></div>
 121 <div class="paragraph"><p>and adds</p></div>
 122 <div class="listingblock">
 123 <div class="content monospaced">
 124 <pre>L_Y' ()
 125   L_Z (global_Y)</pre>
 126 </div></div>
 127 <div class="paragraph"><p>to the <a href="SSA">SSA</a> function.</p></div>
 128 </li>
 129 </ul></div>
 130 <div class="paragraph"><p>The <a href="Shrink">Shrink</a> pass rewrites all uses of <span class="monospaced">L_X</span> to <span class="monospaced">L_Y'</span> and drops <span class="monospaced">L_X</span>.</p></div>
 131 <div class="paragraph"><p>For example, all uncaught <span class="monospaced">Overflow</span> exceptions in a <a href="SSA">SSA</a> function
 132 share the same raising block.</p></div>
 133 </div>
 134 </div>
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