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<a href="./Home">MLton 20201002</a>
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<h1>LocalRef</h1>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p><a href="LocalRef">LocalRef</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>
<div class="sectionbody">
<div class="paragraph"><p>This pass optimizes <span class="monospaced">ref</span> cells local to a <a href="SSA">SSA</a> function:</p></div>
<div class="ulist"><ul>
<li>
<p>
global <span class="monospaced">ref</span>-s only used in one function are moved to the function
</p>
</li>
<li>
<p>
<span class="monospaced">ref</span>-s only created, read from, and written to (i.e., don&#8217;t escape)
are converted into function local variables
</p>
</li>
</ul></div>
<div class="paragraph"><p>Uses <a href="Multi">Multi</a> and <a href="Restore">Restore</a>.</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/local-ref.fun"><span class="monospaced">local-ref.fun</span></a>
</p>
</li>
</ul></div>
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<div class="sect1">
<h2 id="_details_and_notes">Details and Notes</h2>
<div class="sectionbody">
<div class="paragraph"><p>Moving a global <span class="monospaced">ref</span> requires the <a href="Multi">Multi</a> analysis, because a
global <span class="monospaced">ref</span> can only be moved into a function that is executed at
most once.</p></div>
<div class="paragraph"><p>Conversion of non-escaping <span class="monospaced">ref</span>-s is structured in three phases:</p></div>
<div class="ulist"><ul>
<li>
<p>
analysis&#8201;&#8212;&#8201;a variable <span class="monospaced">r = Ref_ref x</span> escapes if
</p>
<div class="ulist"><ul>
<li>
<p>
<span class="monospaced">r</span> is used in any context besides <span class="monospaced">Ref_assign (r, _)</span> or <span class="monospaced">Ref_deref r</span>
</p>
</li>
<li>
<p>
all uses <span class="monospaced">r</span> reachable from a (direct or indirect) call to <span class="monospaced">Thread_copyCurrent</span> are of the same flavor (either <span class="monospaced">Ref_assign</span> or <span class="monospaced">Ref_deref</span>); this also requires the <a href="Multi">Multi</a> analysis.
</p>
</li>
</ul></div>
</li>
<li>
<p>
transformation
</p>
<div class="openblock">
<div class="content">
<div class="ulist"><ul>
<li>
<p>
rewrites <span class="monospaced">r = Ref_ref x</span> to <span class="monospaced">r = x</span>
</p>
</li>
<li>
<p>
rewrites <span class="monospaced">_ = Ref_assign (r, y)</span> to <span class="monospaced">r = y</span>
</p>
</li>
<li>
<p>
rewrites <span class="monospaced">z = Ref_deref r</span> to <span class="monospaced">z = r</span>
</p>
</li>
</ul></div>
</div></div>
<div class="paragraph"><p>Note that the resulting program violates the SSA condition.</p></div>
</li>
<li>
<p>
<a href="Restore">Restore</a>&#8201;&#8212;&#8201;restore the SSA condition.
</p>
</li>
</ul></div>
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