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<a href="./Home">MLton 20130715</a>
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<h1>Useless</h1>
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<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p><a href="Useless">Useless</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>This pass:</p></div>
<div class="ulist"><ul>
<li>
<p>
removes components of tuples that are constants (use unification)
</p>
</li>
<li>
<p>
removes function arguments that are constants
</p>
</li>
<li>
<p>
builds some kind of dependence graph where
</p>
<div class="ulist"><ul>
<li>
<p>
a value of ground type is useful if it is an arg to a primitive
</p>
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<li>
<p>
a tuple is useful if it contains a useful component
</p>
</li>
<li>
<p>
a constructor is useful if it contains a useful component or is used in a <span class="monospaced">Case</span> transfer
</p>
</li>
</ul></div>
</li>
</ul></div>
<div class="paragraph"><p>If a useful tuple is coerced to another useful tuple, then all of
their components must agree (exactly).  It is trivial to convert a
useful value to a useless one.</p></div>
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<div class="sect1">
<h2 id="_implementation">Implementation</h2>
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<div class="ulist"><ul>
<li>
<p>
<a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/useless.fun"><span class="monospaced">useless.fun</span></a>
</p>
</li>
</ul></div>
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<div class="sect1">
<h2 id="_details_and_notes">Details and Notes</h2>
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<div class="paragraph"><p>It is also trivial to convert a useful tuple to one of its useful
components&#8201;&#8212;&#8201;but this seems hard.</p></div>
<div class="paragraph"><p>Suppose that you have a <span class="monospaced">ref</span>/<span class="monospaced">array</span>/<span class="monospaced">vector</span> that is useful, but the
components aren&#8217;t&#8201;&#8212;&#8201;then the components are converted to type <span class="monospaced">unit</span>,
and any primitive args must be as well.</p></div>
<div class="paragraph"><p>Unify all handler arguments so that <span class="monospaced">raise</span>/<span class="monospaced">handle</span> has a consistent
calling convention.</p></div>
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