HCoop / hcoop / debian / mlton.git / blame
| Commit | Line | Data |
|---|---|---|
| 7f918cf1 CE |
1 | <!DOCTYPE html>\r |
| 2 | <html lang="en">\r | |
| 3 | <head>\r | |
| 4 | <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">\r | |
| 5 | <meta name="generator" content="AsciiDoc 8.6.9">\r | |
| 6 | <title>CommonSubexp</title>\r | |
| 7 | <link rel="stylesheet" href="./asciidoc.css" type="text/css">\r | |
| 8 | <link rel="stylesheet" href="./pygments.css" type="text/css">\r | |
| 9 | \r | |
| 10 | \r | |
| 11 | <script type="text/javascript" src="./asciidoc.js"></script>\r | |
| 12 | <script type="text/javascript">\r | |
| 13 | /*<![CDATA[*/\r | |
| 14 | asciidoc.install();\r | |
| 15 | /*]]>*/\r | |
| 16 | </script>\r | |
| 17 | <link rel="stylesheet" href="./mlton.css" type="text/css">\r | |
| 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>CommonSubexp</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="CommonSubexp">CommonSubexp</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>It eliminates instances of common subexpressions.</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/ssa/common-subexp.fun"><span class="monospaced">common-subexp.fun</span></a>\r | |
| 48 | </p>\r | |
| 49 | </li>\r | |
| 50 | </ul></div>\r | |
| 51 | </div>\r | |
| 52 | </div>\r | |
| 53 | <div class="sect1">\r | |
| 54 | <h2 id="_details_and_notes">Details and Notes</h2>\r | |
| 55 | <div class="sectionbody">\r | |
| 56 | <div class="paragraph"><p>In addition to getting the usual sorts of things like</p></div>\r | |
| 57 | <div class="ulist"><ul>\r | |
| 58 | <li>\r | |
| 59 | <p>\r | |
| 60 | </p>\r | |
| 61 | <div class="listingblock">\r | |
| 62 | <div class="content monospaced">\r | |
| 63 | <pre>(w + 0wx1) + (w + 0wx1)</pre>\r | |
| 64 | </div></div>\r | |
| 65 | <div class="paragraph"><p>rewritten to</p></div>\r | |
| 66 | <div class="listingblock">\r | |
| 67 | <div class="content monospaced">\r | |
| 68 | <pre>let val w' = w + 0wx1 in w' + w' end</pre>\r | |
| 69 | </div></div>\r | |
| 70 | </li>\r | |
| 71 | </ul></div>\r | |
| 72 | <div class="paragraph"><p>it also gets things like</p></div>\r | |
| 73 | <div class="ulist"><ul>\r | |
| 74 | <li>\r | |
| 75 | <p>\r | |
| 76 | </p>\r | |
| 77 | <div class="listingblock">\r | |
| 78 | <div class="content monospaced">\r | |
| 79 | <pre>val a = Array_uninit n\r | |
| 80 | val b = Array_length a</pre>\r | |
| 81 | </div></div>\r | |
| 82 | <div class="paragraph"><p>rewritten to</p></div>\r | |
| 83 | <div class="listingblock">\r | |
| 84 | <div class="content monospaced">\r | |
| 85 | <pre>val a = Array_uninit n\r | |
| 86 | val b = n</pre>\r | |
| 87 | </div></div>\r | |
| 88 | </li>\r | |
| 89 | </ul></div>\r | |
| 90 | <div class="paragraph"><p><span class="monospaced">Arith</span> transfers are handled specially. The <em>result</em> of an <span class="monospaced">Arith</span>\r | |
| 91 | transfer can be used in <em>common</em> <span class="monospaced">Arith</span> transfers that it dominates:</p></div>\r | |
| 92 | <div class="ulist"><ul>\r | |
| 93 | <li>\r | |
| 94 | <p>\r | |
| 95 | </p>\r | |
| 96 | <div class="listingblock">\r | |
| 97 | <div class="content monospaced">\r | |
| 98 | <pre>val l = (n + m) + (n + m)\r | |
| 99 | \r | |
| 100 | val k = (l + n) + ((l + m) handle Overflow => ((l + m)\r | |
| 101 | handle Overflow => l + n))</pre>\r | |
| 102 | </div></div>\r | |
| 103 | <div class="paragraph"><p>is rewritten so that <span class="monospaced">(n + m)</span> is computed exactly once, as are\r | |
| 104 | <span class="monospaced">(l + n)</span> and <span class="monospaced">(l + m)</span>.</p></div>\r | |
| 105 | </li>\r | |
| 106 | </ul></div>\r | |
| 107 | </div>\r | |
| 108 | </div>\r | |
| 109 | </div>\r | |
| 110 | <div id="footnotes"><hr></div>\r | |
| 111 | <div id="footer">\r | |
| 112 | <div id="footer-text">\r | |
| 113 | </div>\r | |
| 114 | <div id="footer-badges">\r | |
| 115 | </div>\r | |
| 116 | </div>\r | |
| 117 | </body>\r | |
| 118 | </html>\r |