doc/guide/localhost/Shrink

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  26 <h1>Shrink</h1>
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  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p><a href="Shrink">Shrink</a> is a rewrite pass for the <a href="SSA">SSA</a> and <a href="SSA2">SSA2</a>
  32 <a href="IntermediateLanguage">IntermediateLanguage</a>s, invoked from every optimization pass (see
  33 <a href="SSASimplify">SSASimplify</a> and <a href="SSA2Simplify">SSA2Simplify</a>).</p></div>
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  36 <div class="sect1">
  37 <h2 id="_description">Description</h2>
  38 <div class="sectionbody">
  39 <div class="paragraph"><p>This pass implements a whole family of compile-time reductions, like:</p></div>
  40 <div class="ulist"><ul>
  41 <li>
  42 <p>
  43 <span class="monospaced">#1(a, b)</span> &#8658; <span class="monospaced">a</span>
  44 </p>
  45 </li>
  46 <li>
  47 <p>
  48 <span class="monospaced">case C x of C y =&gt; e</span> &#8658; <span class="monospaced">let y = x in e</span>
  49 </p>
  50 </li>
  51 <li>
  52 <p>
  53 constant folding, copy propagation
  54 </p>
  55 </li>
  56 <li>
  57 <p>
  58 eta blocks
  59 </p>
  60 </li>
  61 <li>
  62 <p>
  63 tuple reconstruction elimination
  64 </p>
  65 </li>
  66 </ul></div>
  67 </div>
  68 </div>
  69 <div class="sect1">
  70 <h2 id="_implementation">Implementation</h2>
  71 <div class="sectionbody">
  72 <div class="ulist"><ul>
  73 <li>
  74 <p>
  75 <a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/shrink.sig"><span class="monospaced">shrink.sig</span></a>
  76 </p>
  77 </li>
  78 <li>
  79 <p>
  80 <a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/shrink.fun"><span class="monospaced">shrink.fun</span></a>
  81 </p>
  82 </li>
  83 <li>
  84 <p>
  85 <a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/shrink2.sig"><span class="monospaced">shrink2.sig</span></a>
  86 </p>
  87 </li>
  88 <li>
  89 <p>
  90 <a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/shrink2.fun"><span class="monospaced">shrink2.fun</span></a>
  91 </p>
  92 </li>
  93 </ul></div>
  94 </div>
  95 </div>
  96 <div class="sect1">
  97 <h2 id="_details_and_notes">Details and Notes</h2>
  98 <div class="sectionbody">
  99 <div class="paragraph"><p>The <a href="Shrink">Shrink</a> pass is run after every <a href="SSA">SSA</a> and <a href="SSA2">SSA2</a>
 100 optimization pass.</p></div>
 101 <div class="paragraph"><p>The <a href="Shrink">Shrink</a> implementation also includes functions to eliminate
 102 unreachable blocks from a <a href="SSA">SSA</a> or <a href="SSA2">SSA2</a> program or function.
 103 The <a href="Shrink">Shrink</a> pass does not guarantee to eliminate all unreachable
 104 blocks.  Doing so would unduly complicate the implementation, and it
 105 is almost always the case that all unreachable blocks are eliminated.
 106 However, a small number of optimization passes require that the input
 107 have no unreachable blocks (essentially, when the analysis works on
 108 the control flow graph and the rewrite iterates on the vector of
 109 blocks).  These passes explicitly call <span class="monospaced">eliminateDeadBlocks</span>.</p></div>
 110 <div class="paragraph"><p>The <a href="Shrink">Shrink</a> pass has a special case to turn a non-tail call where
 111 the continuation and handler only do <span class="monospaced">Profile</span> statements into a tail
 112 call where the <span class="monospaced">Profile</span> statements precede the tail call.</p></div>
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