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MLton
20241230
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<h1>CommonBlock</h1>
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<p><a href="#">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>
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<h2 id="_description">Description</h2>
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<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 <code>Raise</code>, <code>Return</code>, or <code>Goto</code> of a single
global variable.</p>
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<h2 id="_implementation">Implementation</h2>
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<li>
<p><a href="https://github.com/MLton/mlton/blob/master/mlton/ssa/common-block.fun"><code>common-block.fun</code></a></p>
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<h2 id="_details_and_notes">Details and Notes</h2>
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<li>
<p>Rewrites</p>
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<pre>L_X ()
  raise (global_Y)</pre>
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<p>to</p>
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<pre>L_X ()
  L_Y' ()</pre>
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<p>and adds</p>
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<pre>L_Y' ()
  raise (global_Y)</pre>
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<p>to the <a href="SSA">SSA</a> function.</p>
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</li>
<li>
<p>Rewrites</p>
<div class="listingblock">
<div class="content">
<pre>L_X ()
  return (global_Y)</pre>
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<div class="paragraph">
<p>to</p>
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<div class="listingblock">
<div class="content">
<pre>L_X ()
  L_Y' ()</pre>
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<div class="paragraph">
<p>and adds</p>
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<div class="listingblock">
<div class="content">
<pre>L_Y' ()
  return (global_Y)</pre>
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<p>to the <a href="SSA">SSA</a> function.</p>
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</li>
<li>
<p>Rewrites</p>
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<div class="content">
<pre>L_X ()
  L_Z (global_Y)</pre>
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<div class="paragraph">
<p>to</p>
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<div class="listingblock">
<div class="content">
<pre>L_X ()
  L_Y' ()</pre>
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<div class="paragraph">
<p>and adds</p>
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<div class="listingblock">
<div class="content">
<pre>L_Y' ()
  L_Z (global_Y)</pre>
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<div class="paragraph">
<p>to the <a href="SSA">SSA</a> function.</p>
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</li>
</ul>
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<div class="paragraph">
<p>The <a href="Shrink">Shrink</a> pass rewrites all uses of <code>L_X</code> to <code>L_Y'</code> and drops <code>L_X</code>.</p>
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<div class="paragraph">
<p>For example, all uncaught <code>Overflow</code> exceptions in a <a href="SSA">SSA</a> function
share the same raising block.</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/CommonBlock.adoc">Log</a>
<a href="https://github.com/MLton/mlton/edit/master/doc/guide/src/CommonBlock.adoc">Edit</a>
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