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<a class = mltona href="Home">MLton 20061025</a>
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CommonSubexp
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CommonSubexp 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 instances of common subexpressions.
</p>
<h2 id="head-8781d615fd77be9578225c40ac67b9471394cced">Implementation</h2>
<a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-20061025-release/mlton/ssa/common-subexp.sig?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">common-subexp.sig</a>
<a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-20061025-release/mlton/ssa/common-subexp.fun?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">common-subexp.fun</a>
<h2 id="head-35ec00231a68203708e39f0e2cc10b50c6bf62de">Details and Notes</h2>
<p>
In addition to getting the usual sorts of things like
</p>
<ul>
<li>
<p>
<pre> (w + 0wx1) + (w + 0wx1)
</pre> rewritten to
<pre> let val w' = w + 0wx1 in w' + w' end
</pre>
</p>
</li>
</ul>
<p>
it also gets things like
</p>
<ul>
<li>
<p>
<pre> val a = Array_array n
val b = Array_length a
</pre> rewritten to
<pre> val a = Array_array n
val b = n
</pre>
</p>
</li>
</ul>
<p>
<tt>Arith</tt> transfers are handled specially. The <em>result</em> of an <tt>Arith</tt> transfer can be used in <em>common</em> <tt>Arith</tt> transfers that it dominates:
</p>
<ul>
<li>
<p>
<pre> val l = (n + m) + (n + m)
val k = (l + n) + ((l + m) handle Overflow => ((l + m)
handle Overflow => l + n))
</pre> is rewritten so that <tt>(n + m)</tt> is computed exactly once, as
</p>
</li>
</ul>
<p>
are <tt>(l + n)</tt> and <tt>(l + m)</tt>.
</p>
</div>
<p>
<hr>
Last edited on 2005-11-30 23:34:09 by <span title="ppp-71-139-183-221.dsl.snfc21.pacbell.net"><a href="StephenWeeks">StephenWeeks</a></span>.
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