doc/guide/localhost/Useless

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  26 <h1>Useless</h1>
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  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p><a href="Useless">Useless</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>This pass:</p></div>
  39 <div class="ulist"><ul>
  40 <li>
  41 <p>
  42 removes components of tuples that are constants (use unification)
  43 </p>
  44 </li>
  45 <li>
  46 <p>
  47 removes function arguments that are constants
  48 </p>
  49 </li>
  50 <li>
  51 <p>
  52 builds some kind of dependence graph where
  53 </p>
  54 <div class="ulist"><ul>
  55 <li>
  56 <p>
  57 a value of ground type is useful if it is an arg to a primitive
  58 </p>
  59 </li>
  60 <li>
  61 <p>
  62 a tuple is useful if it contains a useful component
  63 </p>
  64 </li>
  65 <li>
  66 <p>
  67 a constructor is useful if it contains a useful component or is used in a <span class="monospaced">Case</span> transfer
  68 </p>
  69 </li>
  70 </ul></div>
  71 </li>
  72 </ul></div>
  73 <div class="paragraph"><p>If a useful tuple is coerced to another useful tuple, then all of
  74 their components must agree (exactly).  It is trivial to convert a
  75 useful value to a useless one.</p></div>
  76 </div>
  77 </div>
  78 <div class="sect1">
  79 <h2 id="_implementation">Implementation</h2>
  80 <div class="sectionbody">
  81 <div class="ulist"><ul>
  82 <li>
  83 <p>
  84 <a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/useless.fun"><span class="monospaced">useless.fun</span></a>
  85 </p>
  86 </li>
  87 </ul></div>
  88 </div>
  89 </div>
  90 <div class="sect1">
  91 <h2 id="_details_and_notes">Details and Notes</h2>
  92 <div class="sectionbody">
  93 <div class="paragraph"><p>It is also trivial to convert a useful tuple to one of its useful
  94 components&#8201;&#8212;&#8201;but this seems hard.</p></div>
  95 <div class="paragraph"><p>Suppose that you have a <span class="monospaced">ref</span>/<span class="monospaced">array</span>/<span class="monospaced">vector</span> that is useful, but the
  96 components aren&#8217;t&#8201;&#8212;&#8201;then the components are converted to type <span class="monospaced">unit</span>,
  97 and any primitive args must be as well.</p></div>
  98 <div class="paragraph"><p>Unify all handler arguments so that <span class="monospaced">raise</span>/<span class="monospaced">handle</span> has a consistent
  99 calling convention.</p></div>
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