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