HCoop / hcoop / debian / mlton.git / blame
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| 5 | <meta name="generator" content="AsciiDoc 8.6.9">\r | |
| 6 | <title>SimplifyTypes</title>\r | |
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| 18 | </head>\r | |
| 19 | <body class="article">\r | |
| 20 | <div id="banner">\r | |
| 21 | <div id="banner-home">\r | |
| 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 | |
| 151 | <div id="footer">\r | |
| 152 | <div id="footer-text">\r | |
| 153 | </div>\r | |
| 154 | <div id="footer-badges">\r | |
| 155 | </div>\r | |
| 156 | </div>\r | |
| 157 | </body>\r | |
| 158 | </html>\r |