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<a href="./Home">MLton 20130715</a>
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<h1>CommonBlock</h1>
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<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p><a href="CommonBlock">CommonBlock</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>
</div>
</div>
<div class="sect1">
<h2 id="_description">Description</h2>
<div class="sectionbody">
<div class="paragraph"><p>It eliminates equivalent blocks in a <a href="SSA">SSA</a> function. The
equivalence criteria requires blocks to have no arguments or
statements and transfer via <span class="monospaced">Raise</span>, <span class="monospaced">Return</span>, or <span class="monospaced">Goto</span> of a single
global variable.</p></div>
</div>
</div>
<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/common-block.fun"><span class="monospaced">common-block.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="ulist"><ul>
<li>
<p>
Rewrites
</p>
<div class="listingblock">
<div class="content monospaced">
<pre>L_X ()
raise (global_Y)</pre>
</div></div>
<div class="paragraph"><p>to</p></div>
<div class="listingblock">
<div class="content monospaced">
<pre>L_X ()
L_Y' ()</pre>
</div></div>
<div class="paragraph"><p>and adds</p></div>
<div class="listingblock">
<div class="content monospaced">
<pre>L_Y' ()
raise (global_Y)</pre>
</div></div>
<div class="paragraph"><p>to the <a href="SSA">SSA</a> function.</p></div>
</li>
<li>
<p>
Rewrites
</p>
<div class="listingblock">
<div class="content monospaced">
<pre>L_X ()
return (global_Y)</pre>
</div></div>
<div class="paragraph"><p>to</p></div>
<div class="listingblock">
<div class="content monospaced">
<pre>L_X ()
L_Y' ()</pre>
</div></div>
<div class="paragraph"><p>and adds</p></div>
<div class="listingblock">
<div class="content monospaced">
<pre>L_Y' ()
return (global_Y)</pre>
</div></div>
<div class="paragraph"><p>to the <a href="SSA">SSA</a> function.</p></div>
</li>
<li>
<p>
Rewrites
</p>
<div class="listingblock">
<div class="content monospaced">
<pre>L_X ()
L_Z (global_Y)</pre>
</div></div>
<div class="paragraph"><p>to</p></div>
<div class="listingblock">
<div class="content monospaced">
<pre>L_X ()
L_Y' ()</pre>
</div></div>
<div class="paragraph"><p>and adds</p></div>
<div class="listingblock">
<div class="content monospaced">
<pre>L_Y' ()
L_Z (global_Y)</pre>
</div></div>
<div class="paragraph"><p>to the <a href="SSA">SSA</a> function.</p></div>
</li>
</ul></div>
<div class="paragraph"><p>The <a href="Shrink">Shrink</a> pass rewrites all uses of <span class="monospaced">L_X</span> to <span class="monospaced">L_Y'</span> and drops <span class="monospaced">L_X</span>.</p></div>
<div class="paragraph"><p>For example, all uncaught <span class="monospaced">Overflow</span> exceptions in a <a href="SSA">SSA</a> function
share the same raising block.</p></div>
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