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<a href="./Home">MLton 20201002</a>
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<h1>Restore</h1>
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<div class="paragraph"><p><a href="Restore">Restore</a> is a rewrite pass for the <a href="SSA">SSA</a> and <a href="SSA2">SSA2</a>
<a href="IntermediateLanguage">IntermediateLanguage</a>s, invoked from <a href="KnownCase">KnownCase</a> and
<a href="LocalRef">LocalRef</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 restores the SSA condition for a violating <a href="SSA">SSA</a> or
<a href="SSA2">SSA2</a> program; the program must satisfy:</p></div>
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<div class="content">
<div class="paragraph"><p>Every path from the root to a use of a variable (excluding globals)
passes through a def of that variable.</p></div>
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<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/restore.sig"><span class="monospaced">restore.sig</span></a>
</p>
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<p>
<a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/restore.fun"><span class="monospaced">restore.fun</span></a>
</p>
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<p>
<a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/restore2.sig"><span class="monospaced">restore2.sig</span></a>
</p>
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<p>
<a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/restore2.fun"><span class="monospaced">restore2.fun</span></a>
</p>
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</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>Based primarily on Section 19.1 of <a href="References#Appel98"> Modern Compiler
Implementation in ML</a>.</p></div>
<div class="paragraph"><p>The main deviation is the calculation of liveness of the violating
variables, which is used to predicate the insertion of phi arguments.
This is due to the algorithm&#8217;s bias towards imperative languages, for
which it makes the assumption that all variables are defined in the
start block and all variables are "used" at exit.</p></div>
<div class="paragraph"><p>This is "optimized" for restoration of functions with small numbers of
violating variables&#8201;&#8212;&#8201;use bool vectors to represent sets of violating
variables.</p></div>
<div class="paragraph"><p>Also, we use a <span class="monospaced">Promise.t</span> to suspend part of the dominance frontier
computation.</p></div>
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