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<title>XMLShrink - MLton Standard ML Compiler (SML Compiler)</title>
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XMLShrink is an optimization pass for the <a href="XML">XML</a> <a href="IntermediateLanguage">IntermediateLanguage</a>, invoked from <a href="XMLSimplify">XMLSimplify</a>. <h2 id="head-55f8ebc805e65b5b71ddafdae390e3be2bcd69af">Description</h2>
<p>
This pass performs optimizations based on a reduction system. 
</p>
<h2 id="head-8781d615fd77be9578225c40ac67b9471394cced">Implementation</h2>
<p>
<a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/mlton/xml/shrink.sig?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">shrink.sig</a> <a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/mlton/xml/shrink.fun?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">shrink.fun</a> 
</p>
<h2 id="head-35ec00231a68203708e39f0e2cc10b50c6bf62de">Details and Notes</h2>
<p>
The simplifier is based on  <a href = "References#AppelJim97">Shrinking Lambda Expressions in Linear Time</a>. 
</p>
<p>
The source program may contain functions that are only called once, or not even called at all.  Match compilation introduces many such functions.  In order to reduce the program size, speed up later phases, and improve the flow analysis, a source to source simplifier is run on <a href="XML">XML</a> after type inference and match compilation. 
</p>
<p>
The simplifier implements the reductions shown below.  The reductions eliminate unnecessary declarations (see the side constraint in the figure), applications where the function is immediate, and case statements where the test is immediate.  Declarations can be eliminated only when the expression is nonexpansive (see Section 4.7 of the <a href="DefinitionOfStandardML"> Definition</a>), which is a syntactic condition that ensures that the expression has no effects (assignments, raises, or nontermination).  The reductions on case statements do not show the other irrelevant cases that may exist.  The reductions were chosen so that they were strongly normalizing and so that they never increased tree size. 
</p>

    <ul>

    <li>
<p>
 
<pre>let x = e1 in e2
</pre> reduces to 
<pre>e2 [x -&gt; e1] 
</pre> if <tt>e1</tt> is a constant or variable or if <tt>e1</tt> is nonexpansive and <tt>x</tt> occurs zero or one time in <tt>e2</tt>
</p>
</li>
    <li class="gap">
<p>
 
<pre>(fn x =&gt; e1) e2
</pre> reduces to 
<pre>let x = e2 in e1
</pre>
</p>
</li>
    <li class="gap">
<p>
 
<pre>e1 handle e2
</pre> reduces to 
<pre>e1
</pre> if <tt>e1</tt> is nonexpansive
</p>
</li>
    <li class="gap">
<p>
 
<pre>case let d in e end of p1 =&gt; e1 ...
</pre> reduces to 
<pre>let d in case e of p1 =&gt; e1 ... end
</pre>
</p>
</li>
    <li class="gap">
<p>
 
<pre>case C e1 of C x =&gt; e2
</pre> reduces to 
<pre>let x = e1 in e2
</pre>
</p>
</li>
</ul>

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Last edited on 2006-11-02 17:33:47 by <span title="76.16.241.4"><a href="MatthewFluet">MatthewFluet</a></span>.
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