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LocalRef is an optimization pass for the <a href="SSA">SSA</a> <a href="IntermediateLanguage">IntermediateLanguage</a>, invoked from <a href="SSASimplify">SSASimplify</a>. <h2 id="head-55f8ebc805e65b5b71ddafdae390e3be2bcd69af">Description</h2>
<p>
This pas optimizes <tt>ref</tt> cells local to a <a href="SSA">SSA</a> function:
</p>
<ul>
<li>
<p>
global <tt>ref</tt>s only used in one function are moved to the function
</p>
</li>
<li class="gap">
<p>
<tt>ref</tt>s only created, read from, and written to (i.e., don't escape) are converted into function local variables
</p>
</li>
</ul>
<p>
Uses <a href="Multi">Multi</a> and <a href="Restore">Restore</a>.
</p>
<h2 id="head-8781d615fd77be9578225c40ac67b9471394cced">Implementation</h2>
<a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/mlton/ssa/local-ref.sig?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">local-ref.sig</a> <a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/mlton/ssa/local-ref.fun?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">local-ref.fun</a> <h2 id="head-35ec00231a68203708e39f0e2cc10b50c6bf62de">Details and Notes</h2>
<p>
Moving a global <tt>ref</tt> requires the <a href="Multi">Multi</a> analysis, because a global <tt>ref</tt> can only be moved into a function that is executed at most once.
</p>
<p>
Conversion of non-escaping <tt>ref</tt>s is structured in three phases:
</p>
<ul>
<li>
<p>
analysis -- a variable <tt>r = Ref_ref x</tt> escapes if
</p>
</li>
<ul>
<li>
<p>
<tt>r</tt> is used in any context besides <tt>Ref_assign (r, _)</tt> or <tt>Ref_deref r</tt>
</p>
</li>
<li>
<p>
all uses <tt>r</tt> reachable from a (direct or indirect) call to <tt>Thread_copyCurrent</tt> are of the same flavor (either <tt>Ref_assign</tt> or <tt>Ref_deref</tt>); this also requires the <a href="Multi">Multi</a> analysis.
</p>
</li>
</ul>
<li>
<p>
transformation
</p>
</li>
<ul>
<li>
<p>
rewrites <tt>r = Ref_ref x</tt> to <tt>r = x</tt>
</p>
</li>
<li>
<p>
rewrites <tt>_ = Ref_assign (r, y)</tt> to <tt>r = y</tt>
</p>
</li>
<li>
<p>
rewrites <tt>z = Ref_deref r</tt> to <tt>z = r</tt> Note that the resulting program violates the SSA condition.
</p>
</li>
</ul>
<li>
<p>
<a href="Restore">Restore</a> -- restore the SSA condition.
</p>
</li>
</ul>
</div>
<p>
<hr>
Last edited on 2006-11-02 17:46:41 by <span title="76.16.241.4"><a href="MatthewFluet">MatthewFluet</a></span>.
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