doc/guide/localhost/XMLShrink

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  22 <a href="./Home">MLton 20180207</a>
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  26 <h1>XMLShrink</h1>
  27 </div>
  28 <div id="content">
  29 <div id="preamble">
  30 <div class="sectionbody">
  31 <div class="paragraph"><p>XMLShrink is an optimization pass for the <a href="XML">XML</a>
  32 <a href="IntermediateLanguage">IntermediateLanguage</a>, invoked from <a href="XMLSimplify">XMLSimplify</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 performs optimizations based on a reduction system.</p></div>
  39 </div>
  40 </div>
  41 <div class="sect1">
  42 <h2 id="_implementation">Implementation</h2>
  43 <div class="sectionbody">
  44 <div class="ulist"><ul>
  45 <li>
  46 <p>
  47 <a href="https://github.com/MLton/mlton/blob/master/mlton/xml/shrink.sig"><span class="monospaced">shrink.sig</span></a>
  48 </p>
  49 </li>
  50 <li>
  51 <p>
  52 <a href="https://github.com/MLton/mlton/blob/master/mlton/xml/shrink.fun"><span class="monospaced">shrink.fun</span></a>
  53 </p>
  54 </li>
  55 </ul></div>
  56 </div>
  57 </div>
  58 <div class="sect1">
  59 <h2 id="_details_and_notes">Details and Notes</h2>
  60 <div class="sectionbody">
  61 <div class="paragraph"><p>The simplifier is based on <a href="References#AppelJim97"> Shrinking Lambda
  62 Expressions in Linear Time</a>.</p></div>
  63 <div class="paragraph"><p>The source program may contain functions that are only called once, or
  64 not even called at all.  Match compilation introduces many such
  65 functions.  In order to reduce the program size, speed up later
  66 phases, and improve the flow analysis, a source to source simplifier
  67 is run on <a href="XML">XML</a> after type inference and match compilation.</p></div>
  68 <div class="paragraph"><p>The simplifier implements the reductions shown below.  The reductions
  69 eliminate unnecessary declarations (see the side constraint in the
  70 figure), applications where the function is immediate, and case
  71 statements where the test is immediate.  Declarations can be
  72 eliminated only when the expression is nonexpansive (see Section 4.7
  73 of the <a href="DefinitionOfStandardML"> Definition</a>), which is a syntactic
  74 condition that ensures that the expression has no effects
  75 (assignments, raises, or nontermination).  The reductions on case
  76 statements do not show the other irrelevant cases that may exist.  The
  77 reductions were chosen so that they were strongly normalizing and so
  78 that they never increased tree size.</p></div>
  79 <div class="ulist"><ul>
  80 <li>
  81 <p>
  82 </p>
  83 <div class="openblock">
  84 <div class="content">
  85 <div class="listingblock">
  86 <div class="content"><div class="highlight"><pre><span class="k">let</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">e1</span><span class="w"> </span><span class="k">in</span><span class="w"> </span><span class="n">e2</span><span class="w"></span>
  87 </pre></div></div></div>
  88 <div class="paragraph"><p>reduces to</p></div>
  89 <div class="listingblock">
  90 <div class="content"><div class="highlight"><pre><span class="n">e2</span><span class="w"> </span><span class="p">[</span><span class="n">x</span><span class="w"> </span><span class="p">-&gt;</span><span class="w"> </span><span class="n">e1</span><span class="p">]</span><span class="w"></span>
  91 </pre></div></div></div>
  92 <div class="paragraph"><p>if <span class="monospaced">e1</span> is a constant or variable or if <span class="monospaced">e1</span> is nonexpansive and <span class="monospaced">x</span> occurs zero or one time in <span class="monospaced">e2</span></p></div>
  93 </div></div>
  94 </li>
  95 <li>
  96 <p>
  97 </p>
  98 <div class="openblock">
  99 <div class="content">
 100 <div class="listingblock">
 101 <div class="content"><div class="highlight"><pre><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">e1</span><span class="p">)</span><span class="w"> </span><span class="n">e2</span><span class="w"></span>
 102 </pre></div></div></div>
 103 <div class="paragraph"><p>reduces to</p></div>
 104 <div class="listingblock">
 105 <div class="content"><div class="highlight"><pre><span class="k">let</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">e2</span><span class="w"> </span><span class="k">in</span><span class="w"> </span><span class="n">e1</span><span class="w"></span>
 106 </pre></div></div></div>
 107 </div></div>
 108 </li>
 109 <li>
 110 <p>
 111 </p>
 112 <div class="openblock">
 113 <div class="content">
 114 <div class="listingblock">
 115 <div class="content"><div class="highlight"><pre><span class="n">e1</span><span class="w"> </span><span class="k">handle</span><span class="w"> </span><span class="n">e2</span><span class="w"></span>
 116 </pre></div></div></div>
 117 <div class="paragraph"><p>reduces to</p></div>
 118 <div class="listingblock">
 119 <div class="content"><div class="highlight"><pre><span class="n">e1</span><span class="w"></span>
 120 </pre></div></div></div>
 121 <div class="paragraph"><p>if <span class="monospaced">e1</span> is nonexpansive</p></div>
 122 </div></div>
 123 </li>
 124 <li>
 125 <p>
 126 </p>
 127 <div class="openblock">
 128 <div class="content">
 129 <div class="listingblock">
 130 <div class="content"><div class="highlight"><pre><span class="k">case</span><span class="w"> </span><span class="k">let</span><span class="w"> </span><span class="n">d</span><span class="w"> </span><span class="k">in</span><span class="w"> </span><span class="n">e</span><span class="w"> </span><span class="k">end</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">p1</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">e1</span><span class="w"> </span><span class="p">...</span><span class="w"></span>
 131 </pre></div></div></div>
 132 <div class="paragraph"><p>reduces to</p></div>
 133 <div class="listingblock">
 134 <div class="content"><div class="highlight"><pre><span class="k">let</span><span class="w"> </span><span class="n">d</span><span class="w"> </span><span class="k">in</span><span class="w"> </span><span class="k">case</span><span class="w"> </span><span class="n">e</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">p1</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">e1</span><span class="w"> </span><span class="p">...</span><span class="w"> </span><span class="k">end</span><span class="w"></span>
 135 </pre></div></div></div>
 136 </div></div>
 137 </li>
 138 <li>
 139 <p>
 140 </p>
 141 <div class="openblock">
 142 <div class="content">
 143 <div class="listingblock">
 144 <div class="content"><div class="highlight"><pre><span class="k">case</span><span class="w"> </span><span class="n">C</span><span class="w"> </span><span class="n">e1</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="n">C</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=&gt;</span><span class="w"> </span><span class="n">e2</span><span class="w"></span>
 145 </pre></div></div></div>
 146 <div class="paragraph"><p>reduces to</p></div>
 147 <div class="listingblock">
 148 <div class="content"><div class="highlight"><pre><span class="k">let</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">e1</span><span class="w"> </span><span class="k">in</span><span class="w"> </span><span class="n">e2</span><span class="w"></span>
 149 </pre></div></div></div>
 150 </div></div>
 151 </li>
 152 </ul></div>
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