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<h1>DeepFlatten</h1>
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<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p><a href="DeepFlatten">DeepFlatten</a> is an optimization pass for the <a href="SSA2">SSA2</a>
<a href="IntermediateLanguage">IntermediateLanguage</a>, invoked from <a href="SSA2Simplify">SSA2Simplify</a>.</p></div>
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<div class="sect1">
<h2 id="_description">Description</h2>
<div class="sectionbody">
<div class="paragraph"><p>This pass flattens into mutable fields of objects and into vectors.</p></div>
<div class="paragraph"><p>For example, an <span class="monospaced">(int * int) ref</span> is represented by a 2 word
object, and an <span class="monospaced">(int * int) array</span> contains pairs of <span class="monospaced">int</span>-s,
rather than pointers to pairs of <span class="monospaced">int</span>-s.</p></div>
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<div class="sect1">
<h2 id="_implementation">Implementation</h2>
<div class="sectionbody">
<div class="ulist"><ul>
<li>
<p>
<a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/deep-flatten.fun"><span class="monospaced">deep-flatten.fun</span></a>
</p>
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<div class="sect1">
<h2 id="_details_and_notes">Details and Notes</h2>
<div class="sectionbody">
<div class="paragraph"><p>There are some performance issues with the deep flatten pass, where it
consumes an excessive amount of memory.</p></div>
<div class="ulist"><ul>
<li>
<p>
<a href="http://www.mlton.org/pipermail/mlton/2005-April/026990.html">http://www.mlton.org/pipermail/mlton/2005-April/026990.html</a>
</p>
</li>
<li>
<p>
<a href="http://www.mlton.org/pipermail/mlton-user/2010-June/001626.html">http://www.mlton.org/pipermail/mlton-user/2010-June/001626.html</a>
</p>
</li>
<li>
<p>
<a href="http://www.mlton.org/pipermail/mlton/2010-December/030876.html">http://www.mlton.org/pipermail/mlton/2010-December/030876.html</a>
</p>
</li>
</ul></div>
<div class="paragraph"><p>A number of applications require compilation with
<span class="monospaced">-drop-pass deepFlatten</span> to avoid exceeding available memory. It is
often asked whether the deep flatten pass usually has a significant
impact on performance. The standard benchmark suite was run with and
without the deep flatten pass enabled when the pass was first
introduced:</p></div>
<div class="ulist"><ul>
<li>
<p>
<a href="http://www.mlton.org/pipermail/mlton/2004-August/025760.html">http://www.mlton.org/pipermail/mlton/2004-August/025760.html</a>
</p>
</li>
</ul></div>
<div class="paragraph"><p>The conclusion is that it does not have a significant impact.
However, these are micro benchmarks; other applications may derive
greater benefit from the pass.</p></div>
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