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Import Upstream version 20180207
[hcoop/debian/mlton.git] / doc / guide / localhost / CombineConversions
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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>CombineConversions</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="CombineConversions">CombineConversions</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 looks for and simplifies nested calls to (signed)\r
39extension/truncation.</p></div>\r
40</div>\r
41</div>\r
42<div class="sect1">\r
43<h2 id="_implementation">Implementation</h2>\r
44<div class="sectionbody">\r
45<div class="ulist"><ul>\r
46<li>\r
47<p>\r
48<a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/combine-conversions.fun"><span class="monospaced">combine-conversions.fun</span></a>\r
49</p>\r
50</li>\r
51</ul></div>\r
52</div>\r
53</div>\r
54<div class="sect1">\r
55<h2 id="_details_and_notes">Details and Notes</h2>\r
56<div class="sectionbody">\r
57<div class="paragraph"><p>It processes each block in dfs order (visiting definitions before uses):</p></div>\r
58<div class="ulist"><ul>\r
59<li>\r
60<p>\r
61If the statement is not a <span class="monospaced">PrimApp</span> with <span class="monospaced">Word_extdToWord</span>, skip it.\r
62</p>\r
63</li>\r
64<li>\r
65<p>\r
66After processing a conversion, it tags the <span class="monospaced">Var</span> for subsequent use.\r
67</p>\r
68</li>\r
69<li>\r
70<p>\r
71When inspecting a conversion, check if the <span class="monospaced">Var</span> operand is also the\r
72result of a conversion. If it is, try to combine the two operations.\r
73Repeatedly simplify until hitting either a non-conversion <span class="monospaced">Var</span> or a\r
74case where the conversion cannot be simplified.\r
75</p>\r
76</li>\r
77</ul></div>\r
78<div class="paragraph"><p>The optimization rules are very simple:</p></div>\r
79<div class="listingblock">\r
80<div class="content monospaced">\r
81<pre>x1 = ...\r
82x2 = Word_extdToWord (W1, W2, {signed=s1}) x1\r
83x3 = Word_extdToWord (W2, W3, {signed=s2}) x2</pre>\r
84</div></div>\r
85<div class="ulist"><ul>\r
86<li>\r
87<p>\r
88If <span class="monospaced">W1 = W2</span>, then there is no conversions before <span class="monospaced">x_1</span>.\r
89</p>\r
90<div class="paragraph"><p>This is guaranteed because <span class="monospaced">W2 = W3</span> will always trigger optimization.</p></div>\r
91</li>\r
92<li>\r
93<p>\r
94Case <span class="monospaced">W1 &lt;= W3 &lt;= W2</span>:\r
95</p>\r
96<div class="listingblock">\r
97<div class="content monospaced">\r
98<pre>x3 = Word_extdToWord (W1, W3, {signed=s1}) x1</pre>\r
99</div></div>\r
100</li>\r
101<li>\r
102<p>\r
103Case <span class="monospaced">W1 &lt; W2 &lt; W3 AND ((NOT s1) OR s2)</span>:\r
104</p>\r
105<div class="listingblock">\r
106<div class="content monospaced">\r
107<pre>x3 = Word_extdToWord (W1, W3, {signed=s1}) x1</pre>\r
108</div></div>\r
109</li>\r
110<li>\r
111<p>\r
112Case <span class="monospaced">W1 = W2 &lt; W3</span>:\r
113</p>\r
114<div class="paragraph"><p>unoptimized, because there are no conversions past <span class="monospaced">W1</span> and <span class="monospaced">x2 = x1</span></p></div>\r
115</li>\r
116<li>\r
117<p>\r
118Case <span class="monospaced">W3 &lt;= W2 &lt;= W1 OR W3 &lt;= W1 &lt;= W2</span>:\r
119</p>\r
120<div class="listingblock">\r
121<div class="content monospaced">\r
122<pre>x_3 = Word_extdToWord (W1, W3, {signed=_}) x1</pre>\r
123</div></div>\r
124<div class="paragraph"><p>because <span class="monospaced">W3 &lt;= W1 &amp;&amp; W3 &lt;= W2</span>, just clip <span class="monospaced">x1</span></p></div>\r
125</li>\r
126<li>\r
127<p>\r
128Case <span class="monospaced">W2 &lt; W1 &lt;= W3 OR W2 &lt; W3 &lt;= W1</span>:\r
129</p>\r
130<div class="paragraph"><p>unoptimized, because <span class="monospaced">W2 &lt; W1 &amp;&amp; W2 &lt; W3</span>, has truncation effect</p></div>\r
131</li>\r
132<li>\r
133<p>\r
134Case <span class="monospaced">W1 &lt; W2 &lt; W3 AND (s1 AND (NOT s2))</span>:\r
135</p>\r
136<div class="paragraph"><p>unoptimized, because each conversion affects the result separately</p></div>\r
137</li>\r
138</ul></div>\r
139</div>\r
140</div>\r
141</div>\r
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