doc/guide/localhost/SimplifyTypes

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  26 <h1>SimplifyTypes</h1>
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  28 <div id="content">
  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p><a href="SimplifyTypes">SimplifyTypes</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 computes a "cardinality" of each datatype, which is an
  39 abstraction of the number of values of the datatype.</p></div>
  40 <div class="ulist"><ul>
  41 <li>
  42 <p>
  43 <span class="monospaced">Zero</span> means the datatype has no values (except for bottom).
  44 </p>
  45 </li>
  46 <li>
  47 <p>
  48 <span class="monospaced">One</span> means the datatype has one value (except for bottom).
  49 </p>
  50 </li>
  51 <li>
  52 <p>
  53 <span class="monospaced">Many</span> means the datatype has many values.
  54 </p>
  55 </li>
  56 </ul></div>
  57 <div class="paragraph"><p>This pass removes all datatypes whose cardinality is <span class="monospaced">Zero</span> or <span class="monospaced">One</span>
  58 and removes:</p></div>
  59 <div class="ulist"><ul>
  60 <li>
  61 <p>
  62 components of tuples
  63 </p>
  64 </li>
  65 <li>
  66 <p>
  67 function args
  68 </p>
  69 </li>
  70 <li>
  71 <p>
  72 constructor args
  73 </p>
  74 </li>
  75 </ul></div>
  76 <div class="paragraph"><p>which are such datatypes.</p></div>
  77 <div class="paragraph"><p>This pass marks constructors as one of:</p></div>
  78 <div class="ulist"><ul>
  79 <li>
  80 <p>
  81 <span class="monospaced">Useless</span>: it never appears in a <span class="monospaced">ConApp</span>.
  82 </p>
  83 </li>
  84 <li>
  85 <p>
  86 <span class="monospaced">Transparent</span>: it is the only variant in its datatype and its argument type does not contain any uses of <span class="monospaced">array</span> or <span class="monospaced">vector</span>.
  87 </p>
  88 </li>
  89 <li>
  90 <p>
  91 <span class="monospaced">Useful</span>: otherwise
  92 </p>
  93 </li>
  94 </ul></div>
  95 <div class="paragraph"><p>This pass also removes <span class="monospaced">Useless</span> and <span class="monospaced">Transparent</span> constructors.</p></div>
  96 </div>
  97 </div>
  98 <div class="sect1">
  99 <h2 id="_implementation">Implementation</h2>
 100 <div class="sectionbody">
 101 <div class="ulist"><ul>
 102 <li>
 103 <p>
 104 <a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/simplify-types.fun"><span class="monospaced">simplify-types.fun</span></a>
 105 </p>
 106 </li>
 107 </ul></div>
 108 </div>
 109 </div>
 110 <div class="sect1">
 111 <h2 id="_details_and_notes">Details and Notes</h2>
 112 <div class="sectionbody">
 113 <div class="paragraph"><p>This pass must happen before polymorphic equality is implemented because</p></div>
 114 <div class="ulist"><ul>
 115 <li>
 116 <p>
 117 it will make polymorphic equality faster because some types are simpler
 118 </p>
 119 </li>
 120 <li>
 121 <p>
 122 it removes uses of polymorphic equality that must return true
 123 </p>
 124 </li>
 125 </ul></div>
 126 <div class="paragraph"><p>We must keep track of <span class="monospaced">Transparent</span> constructors whose argument type
 127 uses <span class="monospaced">array</span> because of datatypes like the following:</p></div>
 128 <div class="listingblock">
 129 <div class="content"><div class="highlight"><pre><span class="k">datatype</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="n">array</span><span class="w"></span>
 130 </pre></div></div></div>
 131 <div class="paragraph"><p>Such a datatype has <span class="monospaced">Cardinality.Many</span>, but we cannot eliminate the
 132 datatype and replace the lhs by the rhs, i.e. we must keep the
 133 circularity around.</p></div>
 134 <div class="paragraph"><p>Must do similar things for <span class="monospaced">vectors</span>.</p></div>
 135 <div class="paragraph"><p>Also, to eliminate as many <span class="monospaced">Transparent</span> constructors as possible, for
 136 something like the following,</p></div>
 137 <div class="listingblock">
 138 <div class="content"><div class="highlight"><pre><span class="k">datatype</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">u</span><span class="w"> </span><span class="n">array</span><span class="w"></span>
 139 <span class="w">     </span><span class="k">and</span><span class="w"> </span><span class="n">u</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">U</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="n">vector</span><span class="w"></span>
 140 </pre></div></div></div>
 141 <div class="paragraph"><p>we (arbitrarily) expand one of the datatypes first.  The result will
 142 be something like</p></div>
 143 <div class="listingblock">
 144 <div class="content"><div class="highlight"><pre><span class="k">datatype</span><span class="w"> </span><span class="n">u</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">U</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">u</span><span class="w"> </span><span class="n">array</span><span class="w"> </span><span class="n">array</span><span class="w"></span>
 145 </pre></div></div></div>
 146 <div class="paragraph"><p>where all uses of <span class="monospaced">t</span> are replaced by <span class="monospaced">u array</span>.</p></div>
 147 </div>
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