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<a href="./Home">MLton 20201002</a>
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<h1>CommonSubexp</h1>
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<div class="paragraph"><p><a href="CommonSubexp">CommonSubexp</a> is an optimization pass for the <a href="SSA">SSA</a>
<a href="IntermediateLanguage">IntermediateLanguage</a>, invoked from <a href="SSASimplify">SSASimplify</a>.</p></div>
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<div class="sect1">
<h2 id="_description">Description</h2>
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<div class="paragraph"><p>It eliminates instances of common subexpressions.</p></div>
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<h2 id="_implementation">Implementation</h2>
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<p>
<a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/common-subexp.fun"><span class="monospaced">common-subexp.fun</span></a>
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<h2 id="_details_and_notes">Details and Notes</h2>
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<div class="paragraph"><p>In addition to getting the usual sorts of things like</p></div>
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<div class="content monospaced">
<pre>(w + 0wx1) + (w + 0wx1)</pre>
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<div class="paragraph"><p>rewritten to</p></div>
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<div class="content monospaced">
<pre>let val w' = w + 0wx1 in w' + w' end</pre>
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<div class="paragraph"><p>it also gets things like</p></div>
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<div class="content monospaced">
<pre>val a = Array_uninit n
val b = Array_length a</pre>
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<div class="paragraph"><p>rewritten to</p></div>
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<div class="content monospaced">
<pre>val a = Array_uninit n
val b = n</pre>
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<div class="paragraph"><p><span class="monospaced">Arith</span> transfers are handled specially.  The <em>result</em> of an <span class="monospaced">Arith</span>
transfer can be used in <em>common</em> <span class="monospaced">Arith</span> transfers that it dominates:</p></div>
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<div class="content monospaced">
<pre>val l = (n + m) + (n + m)

val k = (l + n) + ((l + m) handle Overflow =&gt; ((l + m)
                                               handle Overflow =&gt; l + n))</pre>
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<div class="paragraph"><p>is rewritten so that <span class="monospaced">(n + m)</span> is computed exactly once, as are
<span class="monospaced">(l + n)</span> and <span class="monospaced">(l + m)</span>.</p></div>
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