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MLton
20241230
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<h1>LocalRef</h1>
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<div class="paragraph">
<p><a href="#">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>
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<h2 id="_description">Description</h2>
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<p>This pass optimizes <code>ref</code> cells local to a <a href="SSA">SSA</a> function:</p>
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<p>global <code>ref</code>-s only used in one function are moved to the function</p>
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<p><code>ref</code>-s only created, read from, and written to (i.e., don&#8217;t escape)
are converted into function local variables</p>
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<p>Uses <a href="Multi">Multi</a> and <a href="Restore">Restore</a>.</p>
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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/local-ref.fun"><code>local-ref.fun</code></a></p>
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<h2 id="_details_and_notes">Details and Notes</h2>
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<p>Moving a global <code>ref</code> requires the <a href="Multi">Multi</a> analysis, because a
global <code>ref</code> can only be moved into a function that is executed at
most once.</p>
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<p>Conversion of non-escaping <code>ref</code>-s is structured in three phases:</p>
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<p>analysis&#8201;&#8212;&#8201;a variable <code>r = Ref_ref x</code> escapes if</p>
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<p><code>r</code> is used in any context besides <code>Ref_assign (r, _)</code> or <code>Ref_deref r</code></p>
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<p>all uses <code>r</code> reachable from a (direct or indirect) call to <code>Thread_copyCurrent</code> are of the same flavor (either <code>Ref_assign</code> or <code>Ref_deref</code>); this also requires the <a href="Multi">Multi</a> analysis.</p>
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<p>transformation</p>
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<p>rewrites <code>r = Ref_ref x</code> to <code>r = x</code></p>
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<p>rewrites <code>_ = Ref_assign (r, y)</code> to <code>r = y</code></p>
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<p>rewrites <code>z = Ref_deref r</code> to <code>z = r</code></p>
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<p>Note that the resulting program violates the SSA condition.</p>
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<p><a href="Restore">Restore</a>&#8201;&#8212;&#8201;restore the SSA condition.</p>
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Last updated Thu Oct 21 15:53:06 2021 -0400 by Matthew Fluet.
<a href="https://github.com/MLton/mlton/commits/master/doc/guide/src/LocalRef.adoc">Log</a>
<a href="https://github.com/MLton/mlton/edit/master/doc/guide/src/LocalRef.adoc">Edit</a>
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