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CommonBlock 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>
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 <tt>Raise</tt>, <tt>Return</tt>, or <tt>Goto</tt> of a single global variable. 
</p>
<h2 id="head-8781d615fd77be9578225c40ac67b9471394cced">Implementation</h2>
<a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/mlton/ssa/common-block.sig?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">common-block.sig</a> <a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/mlton/ssa/common-block.fun?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">common-block.fun</a> <h2 id="head-35ec00231a68203708e39f0e2cc10b50c6bf62de">Details and Notes</h2>

    <ul>

    <li>
<p>
 Rewrites 
<pre> L_X ()
   raise (global_Y)
</pre> to 
<pre> L_X ()
   L_Y' ()
</pre> and adds 
<pre> L_Y' ()
   raise (global_Y)
</pre> to the <a href="SSA">SSA</a> function.
</p>
</li>
    <li>
<p>
 Rewrites 
<pre> L_X ()
   return (global_Y)
</pre> to 
<pre> L_X ()
   L_Y' ()
</pre> and adds 
<pre> L_Y' ()
   return (global_Y)
</pre> to the <a href="SSA">SSA</a> function.
</p>
</li>
    <li>
<p>
 Rewrites 
<pre> L_X ()
   L_Z (global_Y)
</pre> to 
<pre> L_X ()
   L_Y' ()
</pre> and adds 
<pre> L_Y' ()
   L_Z (global_Y)
</pre> to the <a href="SSA">SSA</a> function.
</p>
</li>

    </ul>


<p>
The <a href="Shrink">Shrink</a> pass rewrites all uses of <tt>L_X</tt> to <tt>L_Y'</tt> and drops <tt>L_X</tt>. 
</p>
<p>
For example, all uncaught <tt>Overflow</tt> exceptions in a <a href="SSA">SSA</a> function share the same raising block. 
</p>
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<p>
<hr>
Last edited on 2006-11-02 17:56:13 by <span title="76.16.241.4"><a href="MatthewFluet">MatthewFluet</a></span>.
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