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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>KnownCase</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="KnownCase">KnownCase</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 duplicates and simplifies <span class="monospaced">Case</span> transfers when the\r | |
39 | constructor of the scrutinee is known.</p></div>\r | |
40 | <div class="paragraph"><p>Uses <a href="Restore">Restore</a>.</p></div>\r | |
41 | <div class="paragraph"><p>For example, the program</p></div>\r | |
42 | <div class="listingblock">\r | |
43 | <div class="content"><div class="highlight"><pre><span class="k">val</span><span class="w"> </span><span class="k">rec</span><span class="w"> </span><span class="n">last</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
44 | <span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="p">[]</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="mi">0</span><span class="w"></span>\r | |
45 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="p">[</span><span class="n">x</span><span class="p">]</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">x</span><span class="w"></span>\r | |
46 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="p">_</span><span class="w"> </span><span class="n">::</span><span class="w"> </span><span class="n">l</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">last</span><span class="w"> </span><span class="n">l</span><span class="w"></span>\r | |
47 | \r | |
48 | <span class="k">val</span><span class="w"> </span><span class="p">_</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="n">+</span><span class="w"> </span><span class="n">last</span><span class="w"> </span><span class="p">[</span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">,</span><span class="w"> </span><span class="mi">5</span><span class="p">,</span><span class="w"> </span><span class="mi">6</span><span class="p">,</span><span class="w"> </span><span class="mi">7</span><span class="p">]</span><span class="w"></span>\r | |
49 | </pre></div></div></div>\r | |
50 | <div class="paragraph"><p>gives rise to the <a href="SSA">SSA</a> function</p></div>\r | |
51 | <div class="listingblock">\r | |
52 | <div class="content monospaced">\r | |
53 | <pre>fun last_0 (x_142) = loopS_1 ()\r | |
54 | loopS_1 ()\r | |
55 | loop_11 (x_142)\r | |
56 | loop_11 (x_143)\r | |
57 | case x_143 of\r | |
58 | nil_1 => L_73 | ::_0 => L_74\r | |
59 | L_73 ()\r | |
60 | return global_5\r | |
61 | L_74 (x_145, x_144)\r | |
62 | case x_145 of\r | |
63 | nil_1 => L_75 | _ => L_76\r | |
64 | L_75 ()\r | |
65 | return x_144\r | |
66 | L_76 ()\r | |
67 | loop_11 (x_145)</pre>\r | |
68 | </div></div>\r | |
69 | <div class="paragraph"><p>which is simplified to</p></div>\r | |
70 | <div class="listingblock">\r | |
71 | <div class="content monospaced">\r | |
72 | <pre>fun last_0 (x_142) = loopS_1 ()\r | |
73 | loopS_1 ()\r | |
74 | case x_142 of\r | |
75 | nil_1 => L_73 | ::_0 => L_118\r | |
76 | L_73 ()\r | |
77 | return global_5\r | |
78 | L_118 (x_230, x_229)\r | |
79 | L_74 (x_230, x_229, x_142)\r | |
80 | L_74 (x_145, x_144, x_232)\r | |
81 | case x_145 of\r | |
82 | nil_1 => L_75 | ::_0 => L_114\r | |
83 | L_75 ()\r | |
84 | return x_144\r | |
85 | L_114 (x_227, x_226)\r | |
86 | L_74 (x_227, x_226, x_145)</pre>\r | |
87 | </div></div>\r | |
88 | </div>\r | |
89 | </div>\r | |
90 | <div class="sect1">\r | |
91 | <h2 id="_implementation">Implementation</h2>\r | |
92 | <div class="sectionbody">\r | |
93 | <div class="ulist"><ul>\r | |
94 | <li>\r | |
95 | <p>\r | |
96 | <a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/known-case.fun"><span class="monospaced">known-case.fun</span></a>\r | |
97 | </p>\r | |
98 | </li>\r | |
99 | </ul></div>\r | |
100 | </div>\r | |
101 | </div>\r | |
102 | <div class="sect1">\r | |
103 | <h2 id="_details_and_notes">Details and Notes</h2>\r | |
104 | <div class="sectionbody">\r | |
105 | <div class="paragraph"><p>One interesting aspect of <a href="KnownCase">KnownCase</a>, is that it often has the\r | |
106 | effect of unrolling list traversals by one iteration, moving the\r | |
107 | <span class="monospaced">nil</span>/<span class="monospaced">::</span> check to the end of the loop, rather than the beginning.</p></div>\r | |
108 | </div>\r | |
109 | </div>\r | |
110 | </div>\r | |
111 | <div id="footnotes"><hr></div>\r | |
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