<!DOCTYPE html>
<html lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<meta name="generator" content="AsciiDoc 8.6.10">
<title>CommonBlock</title>
<link rel="stylesheet" href="./asciidoc.css" type="text/css">
<link rel="stylesheet" href="./pygments.css" type="text/css">


<script type="text/javascript" src="./asciidoc.js"></script>
<script type="text/javascript">
/*<![CDATA[*/
asciidoc.install();
/*]]>*/
</script>
<link rel="stylesheet" href="./mlton.css" type="text/css">
</head>
<body class="article">
<div id="banner">
<div id="banner-home">
<a href="./Home">MLton 20201002</a>
</div>
</div>
<div id="header">
<h1>CommonBlock</h1>
</div>
<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>
</div>
</div>
<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>
</div>
</div>
</div>
<div id="footnotes"><hr></div>
<div id="footer">
<div id="footer-text">
</div>
<div id="footer-badges">
</div>
</div>
</body>
</html>
