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