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22 | <a href="./Home">MLton 20180207</a>\r | |
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25 | <div id="header">\r | |
26 | <h1>Useless</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="Useless">Useless</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:</p></div>\r | |
39 | <div class="ulist"><ul>\r | |
40 | <li>\r | |
41 | <p>\r | |
42 | removes components of tuples that are constants (use unification)\r | |
43 | </p>\r | |
44 | </li>\r | |
45 | <li>\r | |
46 | <p>\r | |
47 | removes function arguments that are constants\r | |
48 | </p>\r | |
49 | </li>\r | |
50 | <li>\r | |
51 | <p>\r | |
52 | builds some kind of dependence graph where\r | |
53 | </p>\r | |
54 | <div class="ulist"><ul>\r | |
55 | <li>\r | |
56 | <p>\r | |
57 | a value of ground type is useful if it is an arg to a primitive\r | |
58 | </p>\r | |
59 | </li>\r | |
60 | <li>\r | |
61 | <p>\r | |
62 | a tuple is useful if it contains a useful component\r | |
63 | </p>\r | |
64 | </li>\r | |
65 | <li>\r | |
66 | <p>\r | |
67 | a constructor is useful if it contains a useful component or is used in a <span class="monospaced">Case</span> transfer\r | |
68 | </p>\r | |
69 | </li>\r | |
70 | </ul></div>\r | |
71 | </li>\r | |
72 | </ul></div>\r | |
73 | <div class="paragraph"><p>If a useful tuple is coerced to another useful tuple, then all of\r | |
74 | their components must agree (exactly). It is trivial to convert a\r | |
75 | useful value to a useless one.</p></div>\r | |
76 | </div>\r | |
77 | </div>\r | |
78 | <div class="sect1">\r | |
79 | <h2 id="_implementation">Implementation</h2>\r | |
80 | <div class="sectionbody">\r | |
81 | <div class="ulist"><ul>\r | |
82 | <li>\r | |
83 | <p>\r | |
84 | <a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/useless.fun"><span class="monospaced">useless.fun</span></a>\r | |
85 | </p>\r | |
86 | </li>\r | |
87 | </ul></div>\r | |
88 | </div>\r | |
89 | </div>\r | |
90 | <div class="sect1">\r | |
91 | <h2 id="_details_and_notes">Details and Notes</h2>\r | |
92 | <div class="sectionbody">\r | |
93 | <div class="paragraph"><p>It is also trivial to convert a useful tuple to one of its useful\r | |
94 | components — but this seems hard.</p></div>\r | |
95 | <div class="paragraph"><p>Suppose that you have a <span class="monospaced">ref</span>/<span class="monospaced">array</span>/<span class="monospaced">vector</span> that is useful, but the\r | |
96 | components aren’t — then the components are converted to type <span class="monospaced">unit</span>,\r | |
97 | and any primitive args must be as well.</p></div>\r | |
98 | <div class="paragraph"><p>Unify all handler arguments so that <span class="monospaced">raise</span>/<span class="monospaced">handle</span> has a consistent\r | |
99 | calling convention.</p></div>\r | |
100 | </div>\r | |
101 | </div>\r | |
102 | </div>\r | |
103 | <div id="footnotes"><hr></div>\r | |
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