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SimplifyTypes 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>
This pass computes a "cardinality" of each datatype, which is an abstraction of the number of values of the datatype. 
</p>

    <ul>

    <li>
<p>
 Zero means the datatype has no values (except for bottom). 
</p>
</li>
    <li>
<p>
 One means the datatype has one value (except for bottom). 
</p>
</li>
    <li>
<p>
 Many means the datatype has many values. 
</p>
</li>

    </ul>


<p>
This pass removes all datatypes whose cardinality is Zero or One and removes: 
</p>

    <ul>

    <li>
<p>
 components of tuples 
</p>
</li>
    <li>
<p>
 function args 
</p>
</li>
    <li>
<p>
 constructor args 
</p>
</li>

    </ul>


<p>
which are such datatypes. 
</p>
<p>
This pass marks constructors as one of: 
</p>

    <ul>

    <li>
<p>
 Useless: it never appears in a <tt>ConApp</tt>. 
</p>
</li>
    <li>
<p>
 Transparent: it is the only variant in its datatype and its argument type does not contain any uses of <tt>array</tt> or <tt>vector</tt>. 
</p>
</li>
    <li>
<p>
 Useful: otherwise 
</p>
</li>

    </ul>


<p>
This pass also removes Useless and Transparent constructors. 
</p>
<h2 id="head-8781d615fd77be9578225c40ac67b9471394cced">Implementation</h2>
<p>
<a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/mlton/ssa/simplify-types.sig?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">simplify-types.sig</a> <a href = "http://mlton.org/cgi-bin/viewsvn.cgi/mlton/tags/on-MLTONWIKIVERSION-release/mlton/ssa/simplify-types.fun?view=markup"><img src="moin-www.png" alt="[WWW]" height="11" width="11">simplify-types.fun</a> 
</p>
<h2 id="head-35ec00231a68203708e39f0e2cc10b50c6bf62de">Details and Notes</h2>
<p>
This pass must happen before polymorphic equality is implemented because 
</p>

    <ol type="1">

    <li>
<p>
it will make polymorphic equality faster because some types are simpler 
</p>
</li>
    <li>
<p>
it removes uses of polymorphic equality that must return true 
</p>
</li>

    </ol>


<p>
We must keep track of Transparent constructors whose argument type uses <tt>array</tt> because of datatypes like the following: 
<pre class=code>
  <B><FONT COLOR="#A020F0">datatype</FONT></B><B><FONT COLOR="#228B22"> t </FONT></B>=<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">T</FONT> <B><FONT COLOR="#A020F0">of</FONT></B> t vector
</FONT></B></PRE>
 Such a datatype has Cardinality.Many, but we cannot eliminate the datatype and replace the lhs by the rhs, i.e. we must keep the circularity around. 
</p>
<p>
Must do similar things for <tt>vectors</tt>. 
</p>
<p>
Also, to eliminate as many Transparent constructors as possible, for something like the following, 
<pre class=code>
  <B><FONT COLOR="#A020F0">datatype</FONT></B><B><FONT COLOR="#228B22"> t </FONT></B>=<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">T</FONT> <B><FONT COLOR="#A020F0">of</FONT></B> u array
       </FONT></B><B><FONT COLOR="#A020F0">and</FONT></B><B><FONT COLOR="#228B22"> u </FONT></B>=<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">U</FONT> <B><FONT COLOR="#A020F0">of</FONT></B> t vector
</FONT></B></PRE>
 we (arbitrarily) expand one of the datatypes first. The result will be something like 
<pre class=code>
  <B><FONT COLOR="#A020F0">datatype</FONT></B><B><FONT COLOR="#228B22"> u </FONT></B>=<B><FONT COLOR="#228B22"> <FONT COLOR="#B8860B">U</FONT> <B><FONT COLOR="#A020F0">of</FONT></B> u array array
</FONT></B></PRE>
 where all uses of <tt>t</tt> are replaced by <tt>u&nbsp;array</tt>. 
</p>
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<p>
<hr>
Last edited on 2006-11-02 17:39:32 by <span title="76.16.241.4"><a href="MatthewFluet">MatthewFluet</a></span>.
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