doc/guide/localhost/LocalRef

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  22 <a href="./Home">MLton 20180207</a>
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  26 <h1>LocalRef</h1>
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  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p><a href="LocalRef">LocalRef</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>
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  34 </div>
  35 <div class="sect1">
  36 <h2 id="_description">Description</h2>
  37 <div class="sectionbody">
  38 <div class="paragraph"><p>This pass optimizes <span class="monospaced">ref</span> cells local to a <a href="SSA">SSA</a> function:</p></div>
  39 <div class="ulist"><ul>
  40 <li>
  41 <p>
  42 global <span class="monospaced">ref</span>-s only used in one function are moved to the function
  43 </p>
  44 </li>
  45 <li>
  46 <p>
  47 <span class="monospaced">ref</span>-s only created, read from, and written to (i.e., don&#8217;t escape)
  48 are converted into function local variables
  49 </p>
  50 </li>
  51 </ul></div>
  52 <div class="paragraph"><p>Uses <a href="Multi">Multi</a> and <a href="Restore">Restore</a>.</p></div>
  53 </div>
  54 </div>
  55 <div class="sect1">
  56 <h2 id="_implementation">Implementation</h2>
  57 <div class="sectionbody">
  58 <div class="ulist"><ul>
  59 <li>
  60 <p>
  61 <a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/local-ref.fun"><span class="monospaced">local-ref.fun</span></a>
  62 </p>
  63 </li>
  64 </ul></div>
  65 </div>
  66 </div>
  67 <div class="sect1">
  68 <h2 id="_details_and_notes">Details and Notes</h2>
  69 <div class="sectionbody">
  70 <div class="paragraph"><p>Moving a global <span class="monospaced">ref</span> requires the <a href="Multi">Multi</a> analysis, because a
  71 global <span class="monospaced">ref</span> can only be moved into a function that is executed at
  72 most once.</p></div>
  73 <div class="paragraph"><p>Conversion of non-escaping <span class="monospaced">ref</span>-s is structured in three phases:</p></div>
  74 <div class="ulist"><ul>
  75 <li>
  76 <p>
  77 analysis&#8201;&#8212;&#8201;a variable <span class="monospaced">r = Ref_ref x</span> escapes if
  78 </p>
  79 <div class="ulist"><ul>
  80 <li>
  81 <p>
  82 <span class="monospaced">r</span> is used in any context besides <span class="monospaced">Ref_assign (r, _)</span> or <span class="monospaced">Ref_deref r</span>
  83 </p>
  84 </li>
  85 <li>
  86 <p>
  87 all uses <span class="monospaced">r</span> reachable from a (direct or indirect) call to <span class="monospaced">Thread_copyCurrent</span> are of the same flavor (either <span class="monospaced">Ref_assign</span> or <span class="monospaced">Ref_deref</span>); this also requires the <a href="Multi">Multi</a> analysis.
  88 </p>
  89 </li>
  90 </ul></div>
  91 </li>
  92 <li>
  93 <p>
  94 transformation
  95 </p>
  96 <div class="openblock">
  97 <div class="content">
  98 <div class="ulist"><ul>
  99 <li>
 100 <p>
 101 rewrites <span class="monospaced">r = Ref_ref x</span> to <span class="monospaced">r = x</span>
 102 </p>
 103 </li>
 104 <li>
 105 <p>
 106 rewrites <span class="monospaced">_ = Ref_assign (r, y)</span> to <span class="monospaced">r = y</span>
 107 </p>
 108 </li>
 109 <li>
 110 <p>
 111 rewrites <span class="monospaced">z = Ref_deref r</span> to <span class="monospaced">z = r</span>
 112 </p>
 113 </li>
 114 </ul></div>
 115 </div></div>
 116 <div class="paragraph"><p>Note that the resulting program violates the SSA condition.</p></div>
 117 </li>
 118 <li>
 119 <p>
 120 <a href="Restore">Restore</a>&#8201;&#8212;&#8201;restore the SSA condition.
 121 </p>
 122 </li>
 123 </ul></div>
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